2840 lines
103 KiB
HTML
2840 lines
103 KiB
HTML
<!DOCTYPE html>
|
|
<html lang="en">
|
|
<head>
|
|
<meta charset="UTF-8">
|
|
<!--[if IE]><meta http-equiv="X-UA-Compatible" content="IE=edge"><![endif]-->
|
|
<meta name="viewport" content="width=device-width, initial-scale=1.0">
|
|
<meta name="generator" content="Asciidoctor 1.5.8">
|
|
<meta name="author" content="Peter Dimov">
|
|
<title>Boost.Variant2: A never valueless variant type</title>
|
|
<link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Open+Sans:300,300italic,400,400italic,600,600italic%7CNoto+Serif:400,400italic,700,700italic%7CDroid+Sans+Mono:400,700">
|
|
<style>
|
|
/* Asciidoctor default stylesheet | MIT License | http://asciidoctor.org */
|
|
/* Uncomment @import statement below to use as custom stylesheet */
|
|
/*@import "https://fonts.googleapis.com/css?family=Open+Sans:300,300italic,400,400italic,600,600italic%7CNoto+Serif:400,400italic,700,700italic%7CDroid+Sans+Mono:400,700";*/
|
|
article,aside,details,figcaption,figure,footer,header,hgroup,main,nav,section,summary{display:block}
|
|
audio,canvas,video{display:inline-block}
|
|
audio:not([controls]){display:none;height:0}
|
|
script{display:none!important}
|
|
html{font-family:sans-serif;-ms-text-size-adjust:100%;-webkit-text-size-adjust:100%}
|
|
a{background:transparent}
|
|
a:focus{outline:thin dotted}
|
|
a:active,a:hover{outline:0}
|
|
h1{font-size:2em;margin:.67em 0}
|
|
abbr[title]{border-bottom:1px dotted}
|
|
b,strong{font-weight:bold}
|
|
dfn{font-style:italic}
|
|
hr{-moz-box-sizing:content-box;box-sizing:content-box;height:0}
|
|
mark{background:#ff0;color:#000}
|
|
code,kbd,pre,samp{font-family:monospace;font-size:1em}
|
|
pre{white-space:pre-wrap}
|
|
q{quotes:"\201C" "\201D" "\2018" "\2019"}
|
|
small{font-size:80%}
|
|
sub,sup{font-size:75%;line-height:0;position:relative;vertical-align:baseline}
|
|
sup{top:-.5em}
|
|
sub{bottom:-.25em}
|
|
img{border:0}
|
|
svg:not(:root){overflow:hidden}
|
|
figure{margin:0}
|
|
fieldset{border:1px solid silver;margin:0 2px;padding:.35em .625em .75em}
|
|
legend{border:0;padding:0}
|
|
button,input,select,textarea{font-family:inherit;font-size:100%;margin:0}
|
|
button,input{line-height:normal}
|
|
button,select{text-transform:none}
|
|
button,html input[type="button"],input[type="reset"],input[type="submit"]{-webkit-appearance:button;cursor:pointer}
|
|
button[disabled],html input[disabled]{cursor:default}
|
|
input[type="checkbox"],input[type="radio"]{box-sizing:border-box;padding:0}
|
|
button::-moz-focus-inner,input::-moz-focus-inner{border:0;padding:0}
|
|
textarea{overflow:auto;vertical-align:top}
|
|
table{border-collapse:collapse;border-spacing:0}
|
|
*,*::before,*::after{-moz-box-sizing:border-box;-webkit-box-sizing:border-box;box-sizing:border-box}
|
|
html,body{font-size:100%}
|
|
body{background:#fff;color:rgba(0,0,0,.8);padding:0;margin:0;font-family:"Noto Serif","DejaVu Serif",serif;font-weight:400;font-style:normal;line-height:1;position:relative;cursor:auto;tab-size:4;-moz-osx-font-smoothing:grayscale;-webkit-font-smoothing:antialiased}
|
|
a:hover{cursor:pointer}
|
|
img,object,embed{max-width:100%;height:auto}
|
|
object,embed{height:100%}
|
|
img{-ms-interpolation-mode:bicubic}
|
|
.left{float:left!important}
|
|
.right{float:right!important}
|
|
.text-left{text-align:left!important}
|
|
.text-right{text-align:right!important}
|
|
.text-center{text-align:center!important}
|
|
.text-justify{text-align:justify!important}
|
|
.hide{display:none}
|
|
img,object,svg{display:inline-block;vertical-align:middle}
|
|
textarea{height:auto;min-height:50px}
|
|
select{width:100%}
|
|
.center{margin-left:auto;margin-right:auto}
|
|
.stretch{width:100%}
|
|
.subheader,.admonitionblock td.content>.title,.audioblock>.title,.exampleblock>.title,.imageblock>.title,.listingblock>.title,.literalblock>.title,.stemblock>.title,.openblock>.title,.paragraph>.title,.quoteblock>.title,table.tableblock>.title,.verseblock>.title,.videoblock>.title,.dlist>.title,.olist>.title,.ulist>.title,.qlist>.title,.hdlist>.title{line-height:1.45;color:#7a2518;font-weight:400;margin-top:0;margin-bottom:.25em}
|
|
div,dl,dt,dd,ul,ol,li,h1,h2,h3,#toctitle,.sidebarblock>.content>.title,h4,h5,h6,pre,form,p,blockquote,th,td{margin:0;padding:0;direction:ltr}
|
|
a{color:#2156a5;text-decoration:underline;line-height:inherit}
|
|
a:hover,a:focus{color:#1d4b8f}
|
|
a img{border:none}
|
|
p{font-family:inherit;font-weight:400;font-size:1em;line-height:1.6;margin-bottom:1.25em;text-rendering:optimizeLegibility}
|
|
p aside{font-size:.875em;line-height:1.35;font-style:italic}
|
|
h1,h2,h3,#toctitle,.sidebarblock>.content>.title,h4,h5,h6{font-family:"Open Sans","DejaVu Sans",sans-serif;font-weight:300;font-style:normal;color:#ba3925;text-rendering:optimizeLegibility;margin-top:1em;margin-bottom:.5em;line-height:1.0125em}
|
|
h1 small,h2 small,h3 small,#toctitle small,.sidebarblock>.content>.title small,h4 small,h5 small,h6 small{font-size:60%;color:#e99b8f;line-height:0}
|
|
h1{font-size:2.125em}
|
|
h2{font-size:1.6875em}
|
|
h3,#toctitle,.sidebarblock>.content>.title{font-size:1.375em}
|
|
h4,h5{font-size:1.125em}
|
|
h6{font-size:1em}
|
|
hr{border:solid #dddddf;border-width:1px 0 0;clear:both;margin:1.25em 0 1.1875em;height:0}
|
|
em,i{font-style:italic;line-height:inherit}
|
|
strong,b{font-weight:bold;line-height:inherit}
|
|
small{font-size:60%;line-height:inherit}
|
|
code{font-family:"Droid Sans Mono","DejaVu Sans Mono",monospace;font-weight:400;color:rgba(0,0,0,.9)}
|
|
ul,ol,dl{font-size:1em;line-height:1.6;margin-bottom:1.25em;list-style-position:outside;font-family:inherit}
|
|
ul,ol{margin-left:1.5em}
|
|
ul li ul,ul li ol{margin-left:1.25em;margin-bottom:0;font-size:1em}
|
|
ul.square li ul,ul.circle li ul,ul.disc li ul{list-style:inherit}
|
|
ul.square{list-style-type:square}
|
|
ul.circle{list-style-type:circle}
|
|
ul.disc{list-style-type:disc}
|
|
ol li ul,ol li ol{margin-left:1.25em;margin-bottom:0}
|
|
dl dt{margin-bottom:.3125em;font-weight:bold}
|
|
dl dd{margin-bottom:1.25em}
|
|
abbr,acronym{text-transform:uppercase;font-size:90%;color:rgba(0,0,0,.8);border-bottom:1px dotted #ddd;cursor:help}
|
|
abbr{text-transform:none}
|
|
blockquote{margin:0 0 1.25em;padding:.5625em 1.25em 0 1.1875em;border-left:1px solid #ddd}
|
|
blockquote cite{display:block;font-size:.9375em;color:rgba(0,0,0,.6)}
|
|
blockquote cite::before{content:"\2014 \0020"}
|
|
blockquote cite a,blockquote cite a:visited{color:rgba(0,0,0,.6)}
|
|
blockquote,blockquote p{line-height:1.6;color:rgba(0,0,0,.85)}
|
|
@media screen and (min-width:768px){h1,h2,h3,#toctitle,.sidebarblock>.content>.title,h4,h5,h6{line-height:1.2}
|
|
h1{font-size:2.75em}
|
|
h2{font-size:2.3125em}
|
|
h3,#toctitle,.sidebarblock>.content>.title{font-size:1.6875em}
|
|
h4{font-size:1.4375em}}
|
|
table{background:#fff;margin-bottom:1.25em;border:solid 1px #dedede}
|
|
table thead,table tfoot{background:#f7f8f7}
|
|
table thead tr th,table thead tr td,table tfoot tr th,table tfoot tr td{padding:.5em .625em .625em;font-size:inherit;color:rgba(0,0,0,.8);text-align:left}
|
|
table tr th,table tr td{padding:.5625em .625em;font-size:inherit;color:rgba(0,0,0,.8)}
|
|
table tr.even,table tr.alt,table tr:nth-of-type(even){background:#f8f8f7}
|
|
table thead tr th,table tfoot tr th,table tbody tr td,table tr td,table tfoot tr td{display:table-cell;line-height:1.6}
|
|
h1,h2,h3,#toctitle,.sidebarblock>.content>.title,h4,h5,h6{line-height:1.2;word-spacing:-.05em}
|
|
h1 strong,h2 strong,h3 strong,#toctitle strong,.sidebarblock>.content>.title strong,h4 strong,h5 strong,h6 strong{font-weight:400}
|
|
.clearfix::before,.clearfix::after,.float-group::before,.float-group::after{content:" ";display:table}
|
|
.clearfix::after,.float-group::after{clear:both}
|
|
*:not(pre)>code{font-size:.9375em;font-style:normal!important;letter-spacing:0;padding:.1em .5ex;word-spacing:-.15em;background-color:#f7f7f8;-webkit-border-radius:4px;border-radius:4px;line-height:1.45;text-rendering:optimizeSpeed;word-wrap:break-word}
|
|
*:not(pre)>code.nobreak{word-wrap:normal}
|
|
*:not(pre)>code.nowrap{white-space:nowrap}
|
|
pre,pre>code{line-height:1.45;color:rgba(0,0,0,.9);font-family:"Droid Sans Mono","DejaVu Sans Mono",monospace;font-weight:400;text-rendering:optimizeSpeed}
|
|
em em{font-style:normal}
|
|
strong strong{font-weight:400}
|
|
.keyseq{color:rgba(51,51,51,.8)}
|
|
kbd{font-family:"Droid Sans Mono","DejaVu Sans Mono",monospace;display:inline-block;color:rgba(0,0,0,.8);font-size:.65em;line-height:1.45;background-color:#f7f7f7;border:1px solid #ccc;-webkit-border-radius:3px;border-radius:3px;-webkit-box-shadow:0 1px 0 rgba(0,0,0,.2),0 0 0 .1em white inset;box-shadow:0 1px 0 rgba(0,0,0,.2),0 0 0 .1em #fff inset;margin:0 .15em;padding:.2em .5em;vertical-align:middle;position:relative;top:-.1em;white-space:nowrap}
|
|
.keyseq kbd:first-child{margin-left:0}
|
|
.keyseq kbd:last-child{margin-right:0}
|
|
.menuseq,.menuref{color:#000}
|
|
.menuseq b:not(.caret),.menuref{font-weight:inherit}
|
|
.menuseq{word-spacing:-.02em}
|
|
.menuseq b.caret{font-size:1.25em;line-height:.8}
|
|
.menuseq i.caret{font-weight:bold;text-align:center;width:.45em}
|
|
b.button::before,b.button::after{position:relative;top:-1px;font-weight:400}
|
|
b.button::before{content:"[";padding:0 3px 0 2px}
|
|
b.button::after{content:"]";padding:0 2px 0 3px}
|
|
p a>code:hover{color:rgba(0,0,0,.9)}
|
|
#header,#content,#footnotes,#footer{width:100%;margin-left:auto;margin-right:auto;margin-top:0;margin-bottom:0;max-width:62.5em;*zoom:1;position:relative;padding-left:.9375em;padding-right:.9375em}
|
|
#header::before,#header::after,#content::before,#content::after,#footnotes::before,#footnotes::after,#footer::before,#footer::after{content:" ";display:table}
|
|
#header::after,#content::after,#footnotes::after,#footer::after{clear:both}
|
|
#content{margin-top:1.25em}
|
|
#content::before{content:none}
|
|
#header>h1:first-child{color:rgba(0,0,0,.85);margin-top:2.25rem;margin-bottom:0}
|
|
#header>h1:first-child+#toc{margin-top:8px;border-top:1px solid #dddddf}
|
|
#header>h1:only-child,body.toc2 #header>h1:nth-last-child(2){border-bottom:1px solid #dddddf;padding-bottom:8px}
|
|
#header .details{border-bottom:1px solid #dddddf;line-height:1.45;padding-top:.25em;padding-bottom:.25em;padding-left:.25em;color:rgba(0,0,0,.6);display:-ms-flexbox;display:-webkit-flex;display:flex;-ms-flex-flow:row wrap;-webkit-flex-flow:row wrap;flex-flow:row wrap}
|
|
#header .details span:first-child{margin-left:-.125em}
|
|
#header .details span.email a{color:rgba(0,0,0,.85)}
|
|
#header .details br{display:none}
|
|
#header .details br+span::before{content:"\00a0\2013\00a0"}
|
|
#header .details br+span.author::before{content:"\00a0\22c5\00a0";color:rgba(0,0,0,.85)}
|
|
#header .details br+span#revremark::before{content:"\00a0|\00a0"}
|
|
#header #revnumber{text-transform:capitalize}
|
|
#header #revnumber::after{content:"\00a0"}
|
|
#content>h1:first-child:not([class]){color:rgba(0,0,0,.85);border-bottom:1px solid #dddddf;padding-bottom:8px;margin-top:0;padding-top:1rem;margin-bottom:1.25rem}
|
|
#toc{border-bottom:1px solid #e7e7e9;padding-bottom:.5em}
|
|
#toc>ul{margin-left:.125em}
|
|
#toc ul.sectlevel0>li>a{font-style:italic}
|
|
#toc ul.sectlevel0 ul.sectlevel1{margin:.5em 0}
|
|
#toc ul{font-family:"Open Sans","DejaVu Sans",sans-serif;list-style-type:none}
|
|
#toc li{line-height:1.3334;margin-top:.3334em}
|
|
#toc a{text-decoration:none}
|
|
#toc a:active{text-decoration:underline}
|
|
#toctitle{color:#7a2518;font-size:1.2em}
|
|
@media screen and (min-width:768px){#toctitle{font-size:1.375em}
|
|
body.toc2{padding-left:15em;padding-right:0}
|
|
#toc.toc2{margin-top:0!important;background-color:#f8f8f7;position:fixed;width:15em;left:0;top:0;border-right:1px solid #e7e7e9;border-top-width:0!important;border-bottom-width:0!important;z-index:1000;padding:1.25em 1em;height:100%;overflow:auto}
|
|
#toc.toc2 #toctitle{margin-top:0;margin-bottom:.8rem;font-size:1.2em}
|
|
#toc.toc2>ul{font-size:.9em;margin-bottom:0}
|
|
#toc.toc2 ul ul{margin-left:0;padding-left:1em}
|
|
#toc.toc2 ul.sectlevel0 ul.sectlevel1{padding-left:0;margin-top:.5em;margin-bottom:.5em}
|
|
body.toc2.toc-right{padding-left:0;padding-right:15em}
|
|
body.toc2.toc-right #toc.toc2{border-right-width:0;border-left:1px solid #e7e7e9;left:auto;right:0}}
|
|
@media screen and (min-width:1280px){body.toc2{padding-left:20em;padding-right:0}
|
|
#toc.toc2{width:20em}
|
|
#toc.toc2 #toctitle{font-size:1.375em}
|
|
#toc.toc2>ul{font-size:.95em}
|
|
#toc.toc2 ul ul{padding-left:1.25em}
|
|
body.toc2.toc-right{padding-left:0;padding-right:20em}}
|
|
#content #toc{border-style:solid;border-width:1px;border-color:#e0e0dc;margin-bottom:1.25em;padding:1.25em;background:#f8f8f7;-webkit-border-radius:4px;border-radius:4px}
|
|
#content #toc>:first-child{margin-top:0}
|
|
#content #toc>:last-child{margin-bottom:0}
|
|
#footer{max-width:100%;background-color:rgba(0,0,0,.8);padding:1.25em}
|
|
#footer-text{color:rgba(255,255,255,.8);line-height:1.44}
|
|
#content{margin-bottom:.625em}
|
|
.sect1{padding-bottom:.625em}
|
|
@media screen and (min-width:768px){#content{margin-bottom:1.25em}
|
|
.sect1{padding-bottom:1.25em}}
|
|
.sect1:last-child{padding-bottom:0}
|
|
.sect1+.sect1{border-top:1px solid #e7e7e9}
|
|
#content h1>a.anchor,h2>a.anchor,h3>a.anchor,#toctitle>a.anchor,.sidebarblock>.content>.title>a.anchor,h4>a.anchor,h5>a.anchor,h6>a.anchor{position:absolute;z-index:1001;width:1.5ex;margin-left:-1.5ex;display:block;text-decoration:none!important;visibility:hidden;text-align:center;font-weight:400}
|
|
#content h1>a.anchor::before,h2>a.anchor::before,h3>a.anchor::before,#toctitle>a.anchor::before,.sidebarblock>.content>.title>a.anchor::before,h4>a.anchor::before,h5>a.anchor::before,h6>a.anchor::before{content:"\00A7";font-size:.85em;display:block;padding-top:.1em}
|
|
#content h1:hover>a.anchor,#content h1>a.anchor:hover,h2:hover>a.anchor,h2>a.anchor:hover,h3:hover>a.anchor,#toctitle:hover>a.anchor,.sidebarblock>.content>.title:hover>a.anchor,h3>a.anchor:hover,#toctitle>a.anchor:hover,.sidebarblock>.content>.title>a.anchor:hover,h4:hover>a.anchor,h4>a.anchor:hover,h5:hover>a.anchor,h5>a.anchor:hover,h6:hover>a.anchor,h6>a.anchor:hover{visibility:visible}
|
|
#content h1>a.link,h2>a.link,h3>a.link,#toctitle>a.link,.sidebarblock>.content>.title>a.link,h4>a.link,h5>a.link,h6>a.link{color:#ba3925;text-decoration:none}
|
|
#content h1>a.link:hover,h2>a.link:hover,h3>a.link:hover,#toctitle>a.link:hover,.sidebarblock>.content>.title>a.link:hover,h4>a.link:hover,h5>a.link:hover,h6>a.link:hover{color:#a53221}
|
|
.audioblock,.imageblock,.literalblock,.listingblock,.stemblock,.videoblock{margin-bottom:1.25em}
|
|
.admonitionblock td.content>.title,.audioblock>.title,.exampleblock>.title,.imageblock>.title,.listingblock>.title,.literalblock>.title,.stemblock>.title,.openblock>.title,.paragraph>.title,.quoteblock>.title,table.tableblock>.title,.verseblock>.title,.videoblock>.title,.dlist>.title,.olist>.title,.ulist>.title,.qlist>.title,.hdlist>.title{text-rendering:optimizeLegibility;text-align:left;font-family:"Noto Serif","DejaVu Serif",serif;font-size:1rem;font-style:italic}
|
|
table.tableblock.fit-content>caption.title{white-space:nowrap;width:0}
|
|
.paragraph.lead>p,#preamble>.sectionbody>[class="paragraph"]:first-of-type p{font-size:1.21875em;line-height:1.6;color:rgba(0,0,0,.85)}
|
|
table.tableblock #preamble>.sectionbody>[class="paragraph"]:first-of-type p{font-size:inherit}
|
|
.admonitionblock>table{border-collapse:separate;border:0;background:none;width:100%}
|
|
.admonitionblock>table td.icon{text-align:center;width:80px}
|
|
.admonitionblock>table td.icon img{max-width:none}
|
|
.admonitionblock>table td.icon .title{font-weight:bold;font-family:"Open Sans","DejaVu Sans",sans-serif;text-transform:uppercase}
|
|
.admonitionblock>table td.content{padding-left:1.125em;padding-right:1.25em;border-left:1px solid #dddddf;color:rgba(0,0,0,.6)}
|
|
.admonitionblock>table td.content>:last-child>:last-child{margin-bottom:0}
|
|
.exampleblock>.content{border-style:solid;border-width:1px;border-color:#e6e6e6;margin-bottom:1.25em;padding:1.25em;background:#fff;-webkit-border-radius:4px;border-radius:4px}
|
|
.exampleblock>.content>:first-child{margin-top:0}
|
|
.exampleblock>.content>:last-child{margin-bottom:0}
|
|
.sidebarblock{border-style:solid;border-width:1px;border-color:#e0e0dc;margin-bottom:1.25em;padding:1.25em;background:#f8f8f7;-webkit-border-radius:4px;border-radius:4px}
|
|
.sidebarblock>:first-child{margin-top:0}
|
|
.sidebarblock>:last-child{margin-bottom:0}
|
|
.sidebarblock>.content>.title{color:#7a2518;margin-top:0;text-align:center}
|
|
.exampleblock>.content>:last-child>:last-child,.exampleblock>.content .olist>ol>li:last-child>:last-child,.exampleblock>.content .ulist>ul>li:last-child>:last-child,.exampleblock>.content .qlist>ol>li:last-child>:last-child,.sidebarblock>.content>:last-child>:last-child,.sidebarblock>.content .olist>ol>li:last-child>:last-child,.sidebarblock>.content .ulist>ul>li:last-child>:last-child,.sidebarblock>.content .qlist>ol>li:last-child>:last-child{margin-bottom:0}
|
|
.literalblock pre,.listingblock pre:not(.highlight),.listingblock pre[class="highlight"],.listingblock pre[class^="highlight "],.listingblock pre.CodeRay,.listingblock pre.prettyprint{background:#f7f7f8}
|
|
.sidebarblock .literalblock pre,.sidebarblock .listingblock pre:not(.highlight),.sidebarblock .listingblock pre[class="highlight"],.sidebarblock .listingblock pre[class^="highlight "],.sidebarblock .listingblock pre.CodeRay,.sidebarblock .listingblock pre.prettyprint{background:#f2f1f1}
|
|
.literalblock pre,.literalblock pre[class],.listingblock pre,.listingblock pre[class]{-webkit-border-radius:4px;border-radius:4px;word-wrap:break-word;overflow-x:auto;padding:1em;font-size:.8125em}
|
|
@media screen and (min-width:768px){.literalblock pre,.literalblock pre[class],.listingblock pre,.listingblock pre[class]{font-size:.90625em}}
|
|
@media screen and (min-width:1280px){.literalblock pre,.literalblock pre[class],.listingblock pre,.listingblock pre[class]{font-size:1em}}
|
|
.literalblock pre.nowrap,.literalblock pre.nowrap pre,.listingblock pre.nowrap,.listingblock pre.nowrap pre{white-space:pre;word-wrap:normal}
|
|
.literalblock.output pre{color:#f7f7f8;background-color:rgba(0,0,0,.9)}
|
|
.listingblock pre.highlightjs{padding:0}
|
|
.listingblock pre.highlightjs>code{padding:1em;-webkit-border-radius:4px;border-radius:4px}
|
|
.listingblock pre.prettyprint{border-width:0}
|
|
.listingblock>.content{position:relative}
|
|
.listingblock code[data-lang]::before{display:none;content:attr(data-lang);position:absolute;font-size:.75em;top:.425rem;right:.5rem;line-height:1;text-transform:uppercase;color:#999}
|
|
.listingblock:hover code[data-lang]::before{display:block}
|
|
.listingblock.terminal pre .command::before{content:attr(data-prompt);padding-right:.5em;color:#999}
|
|
.listingblock.terminal pre .command:not([data-prompt])::before{content:"$"}
|
|
table.pyhltable{border-collapse:separate;border:0;margin-bottom:0;background:none}
|
|
table.pyhltable td{vertical-align:top;padding-top:0;padding-bottom:0;line-height:1.45}
|
|
table.pyhltable td.code{padding-left:.75em;padding-right:0}
|
|
pre.pygments .lineno,table.pyhltable td:not(.code){color:#999;padding-left:0;padding-right:.5em;border-right:1px solid #dddddf}
|
|
pre.pygments .lineno{display:inline-block;margin-right:.25em}
|
|
table.pyhltable .linenodiv{background:none!important;padding-right:0!important}
|
|
.quoteblock{margin:0 1em 1.25em 1.5em;display:table}
|
|
.quoteblock>.title{margin-left:-1.5em;margin-bottom:.75em}
|
|
.quoteblock blockquote,.quoteblock p{color:rgba(0,0,0,.85);font-size:1.15rem;line-height:1.75;word-spacing:.1em;letter-spacing:0;font-style:italic;text-align:justify}
|
|
.quoteblock blockquote{margin:0;padding:0;border:0}
|
|
.quoteblock blockquote::before{content:"\201c";float:left;font-size:2.75em;font-weight:bold;line-height:.6em;margin-left:-.6em;color:#7a2518;text-shadow:0 1px 2px rgba(0,0,0,.1)}
|
|
.quoteblock blockquote>.paragraph:last-child p{margin-bottom:0}
|
|
.quoteblock .attribution{margin-top:.75em;margin-right:.5ex;text-align:right}
|
|
.verseblock{margin:0 1em 1.25em}
|
|
.verseblock pre{font-family:"Open Sans","DejaVu Sans",sans;font-size:1.15rem;color:rgba(0,0,0,.85);font-weight:300;text-rendering:optimizeLegibility}
|
|
.verseblock pre strong{font-weight:400}
|
|
.verseblock .attribution{margin-top:1.25rem;margin-left:.5ex}
|
|
.quoteblock .attribution,.verseblock .attribution{font-size:.9375em;line-height:1.45;font-style:italic}
|
|
.quoteblock .attribution br,.verseblock .attribution br{display:none}
|
|
.quoteblock .attribution cite,.verseblock .attribution cite{display:block;letter-spacing:-.025em;color:rgba(0,0,0,.6)}
|
|
.quoteblock.abstract blockquote::before,.quoteblock.excerpt blockquote::before,.quoteblock .quoteblock blockquote::before{display:none}
|
|
.quoteblock.abstract blockquote,.quoteblock.abstract p,.quoteblock.excerpt blockquote,.quoteblock.excerpt p,.quoteblock .quoteblock blockquote,.quoteblock .quoteblock p{line-height:1.6;word-spacing:0}
|
|
.quoteblock.abstract{margin:0 1em 1.25em;display:block}
|
|
.quoteblock.abstract>.title{margin:0 0 .375em;font-size:1.15em;text-align:center}
|
|
.quoteblock.excerpt,.quoteblock .quoteblock{margin:0 0 1.25em;padding:0 0 .25em 1em;border-left:.25em solid #dddddf}
|
|
.quoteblock.excerpt blockquote,.quoteblock.excerpt p,.quoteblock .quoteblock blockquote,.quoteblock .quoteblock p{color:inherit;font-size:1.0625rem}
|
|
.quoteblock.excerpt .attribution,.quoteblock .quoteblock .attribution{color:inherit;text-align:left;margin-right:0}
|
|
table.tableblock{max-width:100%;border-collapse:separate}
|
|
p.tableblock:last-child{margin-bottom:0}
|
|
td.tableblock>.content{margin-bottom:-1.25em}
|
|
table.tableblock,th.tableblock,td.tableblock{border:0 solid #dedede}
|
|
table.grid-all>thead>tr>.tableblock,table.grid-all>tbody>tr>.tableblock{border-width:0 1px 1px 0}
|
|
table.grid-all>tfoot>tr>.tableblock{border-width:1px 1px 0 0}
|
|
table.grid-cols>*>tr>.tableblock{border-width:0 1px 0 0}
|
|
table.grid-rows>thead>tr>.tableblock,table.grid-rows>tbody>tr>.tableblock{border-width:0 0 1px}
|
|
table.grid-rows>tfoot>tr>.tableblock{border-width:1px 0 0}
|
|
table.grid-all>*>tr>.tableblock:last-child,table.grid-cols>*>tr>.tableblock:last-child{border-right-width:0}
|
|
table.grid-all>tbody>tr:last-child>.tableblock,table.grid-all>thead:last-child>tr>.tableblock,table.grid-rows>tbody>tr:last-child>.tableblock,table.grid-rows>thead:last-child>tr>.tableblock{border-bottom-width:0}
|
|
table.frame-all{border-width:1px}
|
|
table.frame-sides{border-width:0 1px}
|
|
table.frame-topbot,table.frame-ends{border-width:1px 0}
|
|
table.stripes-all tr,table.stripes-odd tr:nth-of-type(odd){background:#f8f8f7}
|
|
table.stripes-none tr,table.stripes-odd tr:nth-of-type(even){background:none}
|
|
th.halign-left,td.halign-left{text-align:left}
|
|
th.halign-right,td.halign-right{text-align:right}
|
|
th.halign-center,td.halign-center{text-align:center}
|
|
th.valign-top,td.valign-top{vertical-align:top}
|
|
th.valign-bottom,td.valign-bottom{vertical-align:bottom}
|
|
th.valign-middle,td.valign-middle{vertical-align:middle}
|
|
table thead th,table tfoot th{font-weight:bold}
|
|
tbody tr th{display:table-cell;line-height:1.6;background:#f7f8f7}
|
|
tbody tr th,tbody tr th p,tfoot tr th,tfoot tr th p{color:rgba(0,0,0,.8);font-weight:bold}
|
|
p.tableblock>code:only-child{background:none;padding:0}
|
|
p.tableblock{font-size:1em}
|
|
td>div.verse{white-space:pre}
|
|
ol{margin-left:1.75em}
|
|
ul li ol{margin-left:1.5em}
|
|
dl dd{margin-left:1.125em}
|
|
dl dd:last-child,dl dd:last-child>:last-child{margin-bottom:0}
|
|
ol>li p,ul>li p,ul dd,ol dd,.olist .olist,.ulist .ulist,.ulist .olist,.olist .ulist{margin-bottom:.625em}
|
|
ul.checklist,ul.none,ol.none,ul.no-bullet,ol.no-bullet,ol.unnumbered,ul.unstyled,ol.unstyled{list-style-type:none}
|
|
ul.no-bullet,ol.no-bullet,ol.unnumbered{margin-left:.625em}
|
|
ul.unstyled,ol.unstyled{margin-left:0}
|
|
ul.checklist{margin-left:.625em}
|
|
ul.checklist li>p:first-child>.fa-square-o:first-child,ul.checklist li>p:first-child>.fa-check-square-o:first-child{width:1.25em;font-size:.8em;position:relative;bottom:.125em}
|
|
ul.checklist li>p:first-child>input[type="checkbox"]:first-child{margin-right:.25em}
|
|
ul.inline{display:-ms-flexbox;display:-webkit-box;display:flex;-ms-flex-flow:row wrap;-webkit-flex-flow:row wrap;flex-flow:row wrap;list-style:none;margin:0 0 .625em -1.25em}
|
|
ul.inline>li{margin-left:1.25em}
|
|
.unstyled dl dt{font-weight:400;font-style:normal}
|
|
ol.arabic{list-style-type:decimal}
|
|
ol.decimal{list-style-type:decimal-leading-zero}
|
|
ol.loweralpha{list-style-type:lower-alpha}
|
|
ol.upperalpha{list-style-type:upper-alpha}
|
|
ol.lowerroman{list-style-type:lower-roman}
|
|
ol.upperroman{list-style-type:upper-roman}
|
|
ol.lowergreek{list-style-type:lower-greek}
|
|
.hdlist>table,.colist>table{border:0;background:none}
|
|
.hdlist>table>tbody>tr,.colist>table>tbody>tr{background:none}
|
|
td.hdlist1,td.hdlist2{vertical-align:top;padding:0 .625em}
|
|
td.hdlist1{font-weight:bold;padding-bottom:1.25em}
|
|
.literalblock+.colist,.listingblock+.colist{margin-top:-.5em}
|
|
.colist td:not([class]):first-child{padding:.4em .75em 0;line-height:1;vertical-align:top}
|
|
.colist td:not([class]):first-child img{max-width:none}
|
|
.colist td:not([class]):last-child{padding:.25em 0}
|
|
.thumb,.th{line-height:0;display:inline-block;border:solid 4px #fff;-webkit-box-shadow:0 0 0 1px #ddd;box-shadow:0 0 0 1px #ddd}
|
|
.imageblock.left{margin:.25em .625em 1.25em 0}
|
|
.imageblock.right{margin:.25em 0 1.25em .625em}
|
|
.imageblock>.title{margin-bottom:0}
|
|
.imageblock.thumb,.imageblock.th{border-width:6px}
|
|
.imageblock.thumb>.title,.imageblock.th>.title{padding:0 .125em}
|
|
.image.left,.image.right{margin-top:.25em;margin-bottom:.25em;display:inline-block;line-height:0}
|
|
.image.left{margin-right:.625em}
|
|
.image.right{margin-left:.625em}
|
|
a.image{text-decoration:none;display:inline-block}
|
|
a.image object{pointer-events:none}
|
|
sup.footnote,sup.footnoteref{font-size:.875em;position:static;vertical-align:super}
|
|
sup.footnote a,sup.footnoteref a{text-decoration:none}
|
|
sup.footnote a:active,sup.footnoteref a:active{text-decoration:underline}
|
|
#footnotes{padding-top:.75em;padding-bottom:.75em;margin-bottom:.625em}
|
|
#footnotes hr{width:20%;min-width:6.25em;margin:-.25em 0 .75em;border-width:1px 0 0}
|
|
#footnotes .footnote{padding:0 .375em 0 .225em;line-height:1.3334;font-size:.875em;margin-left:1.2em;margin-bottom:.2em}
|
|
#footnotes .footnote a:first-of-type{font-weight:bold;text-decoration:none;margin-left:-1.05em}
|
|
#footnotes .footnote:last-of-type{margin-bottom:0}
|
|
#content #footnotes{margin-top:-.625em;margin-bottom:0;padding:.75em 0}
|
|
.gist .file-data>table{border:0;background:#fff;width:100%;margin-bottom:0}
|
|
.gist .file-data>table td.line-data{width:99%}
|
|
div.unbreakable{page-break-inside:avoid}
|
|
.big{font-size:larger}
|
|
.small{font-size:smaller}
|
|
.underline{text-decoration:underline}
|
|
.overline{text-decoration:overline}
|
|
.line-through{text-decoration:line-through}
|
|
.aqua{color:#00bfbf}
|
|
.aqua-background{background-color:#00fafa}
|
|
.black{color:#000}
|
|
.black-background{background-color:#000}
|
|
.blue{color:#0000bf}
|
|
.blue-background{background-color:#0000fa}
|
|
.fuchsia{color:#bf00bf}
|
|
.fuchsia-background{background-color:#fa00fa}
|
|
.gray{color:#606060}
|
|
.gray-background{background-color:#7d7d7d}
|
|
.green{color:#006000}
|
|
.green-background{background-color:#007d00}
|
|
.lime{color:#00bf00}
|
|
.lime-background{background-color:#00fa00}
|
|
.maroon{color:#600000}
|
|
.maroon-background{background-color:#7d0000}
|
|
.navy{color:#000060}
|
|
.navy-background{background-color:#00007d}
|
|
.olive{color:#606000}
|
|
.olive-background{background-color:#7d7d00}
|
|
.purple{color:#600060}
|
|
.purple-background{background-color:#7d007d}
|
|
.red{color:#bf0000}
|
|
.red-background{background-color:#fa0000}
|
|
.silver{color:#909090}
|
|
.silver-background{background-color:#bcbcbc}
|
|
.teal{color:#006060}
|
|
.teal-background{background-color:#007d7d}
|
|
.white{color:#bfbfbf}
|
|
.white-background{background-color:#fafafa}
|
|
.yellow{color:#bfbf00}
|
|
.yellow-background{background-color:#fafa00}
|
|
span.icon>.fa{cursor:default}
|
|
a span.icon>.fa{cursor:inherit}
|
|
.admonitionblock td.icon [class^="fa icon-"]{font-size:2.5em;text-shadow:1px 1px 2px rgba(0,0,0,.5);cursor:default}
|
|
.admonitionblock td.icon .icon-note::before{content:"\f05a";color:#19407c}
|
|
.admonitionblock td.icon .icon-tip::before{content:"\f0eb";text-shadow:1px 1px 2px rgba(155,155,0,.8);color:#111}
|
|
.admonitionblock td.icon .icon-warning::before{content:"\f071";color:#bf6900}
|
|
.admonitionblock td.icon .icon-caution::before{content:"\f06d";color:#bf3400}
|
|
.admonitionblock td.icon .icon-important::before{content:"\f06a";color:#bf0000}
|
|
.conum[data-value]{display:inline-block;color:#fff!important;background-color:rgba(0,0,0,.8);-webkit-border-radius:100px;border-radius:100px;text-align:center;font-size:.75em;width:1.67em;height:1.67em;line-height:1.67em;font-family:"Open Sans","DejaVu Sans",sans-serif;font-style:normal;font-weight:bold}
|
|
.conum[data-value] *{color:#fff!important}
|
|
.conum[data-value]+b{display:none}
|
|
.conum[data-value]::after{content:attr(data-value)}
|
|
pre .conum[data-value]{position:relative;top:-.125em}
|
|
b.conum *{color:inherit!important}
|
|
.conum:not([data-value]):empty{display:none}
|
|
dt,th.tableblock,td.content,div.footnote{text-rendering:optimizeLegibility}
|
|
h1,h2,p,td.content,span.alt{letter-spacing:-.01em}
|
|
p strong,td.content strong,div.footnote strong{letter-spacing:-.005em}
|
|
p,blockquote,dt,td.content,span.alt{font-size:1.0625rem}
|
|
p{margin-bottom:1.25rem}
|
|
.sidebarblock p,.sidebarblock dt,.sidebarblock td.content,p.tableblock{font-size:1em}
|
|
.exampleblock>.content{background-color:#fffef7;border-color:#e0e0dc;-webkit-box-shadow:0 1px 4px #e0e0dc;box-shadow:0 1px 4px #e0e0dc}
|
|
.print-only{display:none!important}
|
|
@page{margin:1.25cm .75cm}
|
|
@media print{*{-webkit-box-shadow:none!important;box-shadow:none!important;text-shadow:none!important}
|
|
html{font-size:80%}
|
|
a{color:inherit!important;text-decoration:underline!important}
|
|
a.bare,a[href^="#"],a[href^="mailto:"]{text-decoration:none!important}
|
|
a[href^="http:"]:not(.bare)::after,a[href^="https:"]:not(.bare)::after{content:"(" attr(href) ")";display:inline-block;font-size:.875em;padding-left:.25em}
|
|
abbr[title]::after{content:" (" attr(title) ")"}
|
|
pre,blockquote,tr,img,object,svg{page-break-inside:avoid}
|
|
thead{display:table-header-group}
|
|
svg{max-width:100%}
|
|
p,blockquote,dt,td.content{font-size:1em;orphans:3;widows:3}
|
|
h2,h3,#toctitle,.sidebarblock>.content>.title{page-break-after:avoid}
|
|
#toc,.sidebarblock,.exampleblock>.content{background:none!important}
|
|
#toc{border-bottom:1px solid #dddddf!important;padding-bottom:0!important}
|
|
body.book #header{text-align:center}
|
|
body.book #header>h1:first-child{border:0!important;margin:2.5em 0 1em}
|
|
body.book #header .details{border:0!important;display:block;padding:0!important}
|
|
body.book #header .details span:first-child{margin-left:0!important}
|
|
body.book #header .details br{display:block}
|
|
body.book #header .details br+span::before{content:none!important}
|
|
body.book #toc{border:0!important;text-align:left!important;padding:0!important;margin:0!important}
|
|
body.book #toc,body.book #preamble,body.book h1.sect0,body.book .sect1>h2{page-break-before:always}
|
|
.listingblock code[data-lang]::before{display:block}
|
|
#footer{padding:0 .9375em}
|
|
.hide-on-print{display:none!important}
|
|
.print-only{display:block!important}
|
|
.hide-for-print{display:none!important}
|
|
.show-for-print{display:inherit!important}}
|
|
@media print,amzn-kf8{#header>h1:first-child{margin-top:1.25rem}
|
|
.sect1{padding:0!important}
|
|
.sect1+.sect1{border:0}
|
|
#footer{background:none}
|
|
#footer-text{color:rgba(0,0,0,.6);font-size:.9em}}
|
|
@media amzn-kf8{#header,#content,#footnotes,#footer{padding:0}}
|
|
</style>
|
|
</head>
|
|
<body class="article toc2 toc-left">
|
|
<div id="header">
|
|
<h1>Boost.Variant2: A never valueless variant type</h1>
|
|
<div class="details">
|
|
<span id="author" class="author">Peter Dimov</span><br>
|
|
</div>
|
|
<div id="toc" class="toc2">
|
|
<div id="toctitle">Table of Contents</div>
|
|
<ul class="sectlevel1">
|
|
<li><a href="#overview">Overview</a>
|
|
<ul class="sectlevel2">
|
|
<li><a href="#overview_description">Description</a></li>
|
|
<li><a href="#overview_usage_examples">Usage Examples</a></li>
|
|
<li><a href="#overview_construction_and_assignment">Construction and Assignment</a></li>
|
|
<li><a href="#overview_inspecting_the_value">Inspecting the Value</a></li>
|
|
<li><a href="#overview_visitation">Visitation</a></li>
|
|
<li><a href="#overview_default_construction">Default Construction</a></li>
|
|
</ul>
|
|
</li>
|
|
<li><a href="#changelog">Revision History</a>
|
|
<ul class="sectlevel2">
|
|
<li><a href="#changelog_changes_in_1_76_0">Changes in 1.76.0</a></li>
|
|
<li><a href="#changelog_changes_in_1_74_0">Changes in 1.74.0</a></li>
|
|
<li><a href="#changelog_changes_in_1_73_0">Changes in 1.73.0</a></li>
|
|
<li><a href="#changelog_changes_in_1_71_0">Changes in 1.71.0</a></li>
|
|
</ul>
|
|
</li>
|
|
<li><a href="#design">Design</a>
|
|
<ul class="sectlevel2">
|
|
<li><a href="#design_features">Features</a></li>
|
|
<li><a href="#design_rationale">Rationale</a>
|
|
<ul class="sectlevel3">
|
|
<li><a href="#design_never_valueless">Never Valueless</a></li>
|
|
<li><a href="#design_strong_exception_safety">Strong Exception Safety</a></li>
|
|
</ul>
|
|
</li>
|
|
<li><a href="#design_differences_with_stdvariant">Differences with std::variant</a></li>
|
|
<li><a href="#design_differences_with_boost_variant">Differences with Boost.Variant</a></li>
|
|
</ul>
|
|
</li>
|
|
<li><a href="#implementation">Implementation</a>
|
|
<ul class="sectlevel2">
|
|
<li><a href="#implementation_dependencies">Dependencies</a></li>
|
|
<li><a href="#implementation_supported_compilers">Supported Compilers</a></li>
|
|
</ul>
|
|
</li>
|
|
<li><a href="#reference">Reference</a>
|
|
<ul class="sectlevel2">
|
|
<li><a href="#ref_boostvariant2variant_hpp"><boost/variant2/variant.hpp></a>
|
|
<ul class="sectlevel3">
|
|
<li><a href="#ref_synopsis">Synopsis</a></li>
|
|
<li><a href="#ref_variant">variant</a>
|
|
<ul class="sectlevel4">
|
|
<li><a href="#ref_constructors">Constructors</a></li>
|
|
<li><a href="#ref_destructor">Destructor</a></li>
|
|
<li><a href="#ref_assignment">Assignment</a></li>
|
|
<li><a href="#ref_modifiers">Modifiers</a></li>
|
|
<li><a href="#ref_value_status">Value Status</a></li>
|
|
<li><a href="#ref_swap">Swap</a></li>
|
|
<li><a href="#ref_converting_constructors_extension">Converting Constructors (extension)</a></li>
|
|
<li><a href="#ref_subset_extension">Subset (extension)</a></li>
|
|
</ul>
|
|
</li>
|
|
<li><a href="#ref_variant_alternative">variant_alternative</a></li>
|
|
<li><a href="#ref_holds_alternative">holds_alternative</a></li>
|
|
<li><a href="#ref_get">get</a></li>
|
|
<li><a href="#ref_get_if">get_if</a></li>
|
|
<li><a href="#ref_relational_operators">Relational Operators</a></li>
|
|
<li><a href="#ref_visit">visit</a></li>
|
|
<li><a href="#ref_swap_2">swap</a></li>
|
|
<li><a href="#ref_bad_variant_access">bad_variant_access</a></li>
|
|
</ul>
|
|
</li>
|
|
</ul>
|
|
</li>
|
|
<li><a href="#copyright">Copyright and License</a></li>
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
<div id="content">
|
|
<div class="sect1">
|
|
<h2 id="overview">Overview</h2>
|
|
<div class="sectionbody">
|
|
<div class="sect2">
|
|
<h3 id="overview_description">Description</h3>
|
|
<div class="paragraph">
|
|
<p>This library implements a type-safe discriminated/tagged union type,
|
|
<code>variant<T…​></code>, that is API-compatible with the C++17 Standard’s
|
|
<a href="http://en.cppreference.com/w/cpp/utility/variant"><code>std::variant<T…​></code></a>.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>A <code>variant<T1, T2, …​, Tn></code> variable can hold a value of any of the
|
|
types <code>T1</code>, <code>T2</code>, …​, <code>Tn</code>. For example,
|
|
<code>variant<int64_t, double, std::string></code> can hold an <code>int64_t</code> value, a
|
|
<code>double</code> value, or a <code>string</code> value.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Such a type is sometimes called a "tagged union", because it’s roughly
|
|
equivalent to</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>struct V
|
|
{
|
|
enum tag { tag_int64_t, tag_double, tag_string };
|
|
|
|
tag tag_;
|
|
|
|
union
|
|
{
|
|
int64_t i_;
|
|
double d_;
|
|
std::string s_;
|
|
};
|
|
};</code></pre>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div class="sect2">
|
|
<h3 id="overview_usage_examples">Usage Examples</h3>
|
|
<div class="paragraph">
|
|
<p>Variants can be used to represent dynamically-typed values. A configuration
|
|
file of the form</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>server.host=test.example.com
|
|
server.port=9174
|
|
cache.max_load=0.7</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>can be represented as <code>std::map<std::string, variant<int64_t, double,
|
|
std::string>></code>.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Variants can also represent polymorphism. To take a classic example, a
|
|
polymorphic collection of shapes:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>#define _USE_MATH_DEFINES
|
|
#include <iostream>
|
|
#include <vector>
|
|
#include <memory>
|
|
#include <cmath>
|
|
|
|
class Shape
|
|
{
|
|
public:
|
|
|
|
virtual ~Shape() = default;
|
|
virtual double area() const = 0;
|
|
};
|
|
|
|
class Rectangle: public Shape
|
|
{
|
|
private:
|
|
|
|
double width_, height_;
|
|
|
|
public:
|
|
|
|
Rectangle( double width, double height ):
|
|
width_( width ), height_( height ) {}
|
|
|
|
virtual double area() const { return width_ * height_; }
|
|
};
|
|
|
|
class Circle: public Shape
|
|
{
|
|
private:
|
|
|
|
double radius_;
|
|
|
|
public:
|
|
|
|
explicit Circle( double radius ): radius_( radius ) {}
|
|
virtual double area() const { return M_PI * radius_ * radius_; }
|
|
};
|
|
|
|
double total_area( std::vector<std::unique_ptr<Shape>> const & v )
|
|
{
|
|
double s = 0.0;
|
|
|
|
for( auto const& p: v )
|
|
{
|
|
s += p->area();
|
|
}
|
|
|
|
return s;
|
|
}
|
|
|
|
int main()
|
|
{
|
|
std::vector<std::unique_ptr<Shape>> v;
|
|
|
|
v.push_back( std::unique_ptr<Shape>( new Circle( 1.0 ) ) );
|
|
v.push_back( std::unique_ptr<Shape>( new Rectangle( 2.0, 3.0 ) ) );
|
|
|
|
std::cout << "Total area: " << total_area( v ) << std::endl;
|
|
}</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>can instead be represented as a collection of <code>variant<Rectangle, Circle></code>
|
|
values. This requires the possible <code>Shape</code> types be known in advance, as is
|
|
often the case. In return, we no longer need virtual functions, or to allocate
|
|
the values on the heap with <code>new Rectangle</code> and <code>new Circle</code>:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>#define _USE_MATH_DEFINES
|
|
#include <iostream>
|
|
#include <vector>
|
|
#include <cmath>
|
|
|
|
#include <boost/variant2/variant.hpp>
|
|
using namespace boost::variant2;
|
|
|
|
struct Rectangle
|
|
{
|
|
double width_, height_;
|
|
double area() const { return width_ * height_; }
|
|
};
|
|
|
|
struct Circle
|
|
{
|
|
double radius_;
|
|
double area() const { return M_PI * radius_ * radius_; }
|
|
};
|
|
|
|
double total_area( std::vector<variant<Rectangle, Circle>> const & v )
|
|
{
|
|
double s = 0.0;
|
|
|
|
for( auto const& x: v )
|
|
{
|
|
s += visit( []( auto const& y ){ return y.area(); }, x );
|
|
}
|
|
|
|
return s;
|
|
}
|
|
|
|
int main()
|
|
{
|
|
std::vector<variant<Rectangle, Circle>> v;
|
|
|
|
v.push_back( Circle{ 1.0 } );
|
|
v.push_back( Rectangle{ 2.0, 3.0 } );
|
|
|
|
std::cout << "Total area: " << total_area( v ) << std::endl;
|
|
}</code></pre>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div class="sect2">
|
|
<h3 id="overview_construction_and_assignment">Construction and Assignment</h3>
|
|
<div class="paragraph">
|
|
<p>If we look at the</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code> v.push_back( Circle{ 1.0 } );</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>line, we can deduce that <code>variant<Rectangle, Circle></code> can be (implicitly)
|
|
constructed from <code>Circle</code> (and <code>Rectangle</code>), and indeed it can. It can also
|
|
be assigned a <code>Circle</code> or a <code>Rectangle</code>:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>variant<Rectangle, Circle> v = Circle{ 1.0 }; // v holds Circle
|
|
v = Rectangle{ 2.0, 3.0 }; // v now holds Rectangle</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>If we try to construct <code>variant<int, float></code> from something that is neither
|
|
<code>int</code> nor <code>float</code>, say, <code>(short)1</code>, the behavior is "as if" the <code>variant</code> has
|
|
declared two constructors,</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>variant::variant(int x);
|
|
variant::variant(float x);</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>and the standard overload resolution rules are used to pick the one that will
|
|
be used. So <code>variant<int, float>((short)1)</code> will hold an <code>int</code>.</p>
|
|
</div>
|
|
</div>
|
|
<div class="sect2">
|
|
<h3 id="overview_inspecting_the_value">Inspecting the Value</h3>
|
|
<div class="paragraph">
|
|
<p>Putting values into a <code>variant</code> is easy, but taking them out is necessarily a
|
|
bit more convoluted. It’s not possible for <code>variant<int, float></code> to define a
|
|
member function <code>get() const</code>, because such a function will need its return
|
|
type fixed at compile time, and whether the correct return type is <code>int</code> or
|
|
<code>float</code> will only become known at run time.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>There are a few ways around that. First, there is the accessor member function</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>std::size_t variant::index() const noexcept;</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>that returns the zero-based index of the current type. For <code>variant<int,
|
|
float></code>, it will return <code>0</code> for <code>int</code> and <code>1</code> for <code>float</code>.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Once we have the index, we can use the free function <code>get<N></code> to obtain the
|
|
value. Since we’re passing the type index to <code>get</code>, it knows what to return.
|
|
<code>get<0>(v)</code> will return <code>int</code>, and <code>get<1>(v)</code> will return <code>float</code>:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>void f( variant<int, float> const& v )
|
|
{
|
|
switch( v.index() )
|
|
{
|
|
case 0:
|
|
|
|
// use get<0>(v)
|
|
break;
|
|
|
|
case 1:
|
|
|
|
// use get<1>(v)
|
|
break;
|
|
|
|
default:
|
|
|
|
assert(false); // never happens
|
|
}
|
|
}</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>If we call <code>get<0>(v)</code>, and <code>v.index()</code> is not currently <code>0</code>, an exception
|
|
(of type <code>bad_variant_access</code>) will be thrown.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>An alternative approach is to use <code>get<int>(v)</code> or <code>get<float>(v)</code>. This
|
|
works similarly.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Another alternative that avoids the possibility of <code>bad_variant_access</code> is
|
|
to use <code>get_if</code>. Instead of a reference to the contained value, it returns
|
|
a pointer to it, returning <code>nullptr</code> to indicate type mismatch. <code>get_if</code>
|
|
takes a pointer to the <code>variant</code>, so in our example we’ll use something along
|
|
the following lines:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>void f( variant<int, float> const& v )
|
|
{
|
|
if( int const * p = get_if<int>(&v) )
|
|
{
|
|
// use *p
|
|
}
|
|
else if( float const * p = get_if<float>(&v) )
|
|
{
|
|
// use *p
|
|
}
|
|
else
|
|
{
|
|
assert(false); // never happens
|
|
}
|
|
}</code></pre>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div class="sect2">
|
|
<h3 id="overview_visitation">Visitation</h3>
|
|
<div class="paragraph">
|
|
<p>Last but not least, there’s <code>visit</code>. <code>visit(f, v)</code> calls the a function object
|
|
<code>f</code> with the value contained in the <code>variant</code> <code>v</code> and returns the result. When
|
|
<code>v</code> is <code>variant<int, float></code>, it will call <code>f</code> with either an <code>int</code> or a
|
|
<code>float</code>. The function object must be prepared to accept both.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>In practice, this can be achieved by having the function take a type that can
|
|
be passed either <code>int</code> or <code>float</code>, such as <code>double</code>:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>double f( double x ) { return x; }
|
|
|
|
double g( variant<int, float> const& v )
|
|
{
|
|
return visit( f, v );
|
|
}</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>By using a function object with an overloaded <code>operator()</code>:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>struct F
|
|
{
|
|
void operator()(int x) const { /* use x */ }
|
|
void operator()(float x) const { /* use x */ }
|
|
};
|
|
|
|
void g( variant<int, float> const& v )
|
|
{
|
|
visit( F(), v );
|
|
}</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Or by using a polymorphic lambda, as we did in our <code>Circle</code>/<code>Rectangle</code>
|
|
example:</p>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>void g( variant<int, float> const& v )
|
|
{
|
|
visit( [&]( auto const& x ){ std::cout << x << std::endl; }, v );
|
|
}</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p><code>visit</code> can also take more than one <code>variant</code>. <code>visit(f, v1, v2)</code> calls
|
|
<code>f(x1, x2)</code>, where <code>x1</code> is the value contained in <code>v1</code> and <code>x2</code> is the value
|
|
in <code>v2</code>.</p>
|
|
</div>
|
|
</div>
|
|
<div class="sect2">
|
|
<h3 id="overview_default_construction">Default Construction</h3>
|
|
<div class="paragraph">
|
|
<p>The default constructor of <code>variant</code> value-initializes the first type in
|
|
the list. <code>variant<int, float>{}</code> holds <code>0</code> (of type <code>int</code>), and
|
|
<code>variant<float, int>{}</code> holds <code>0.0f</code>.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>This is usually the desired behavior. However, in cases such as
|
|
<code>variant<std::mutex, std::recursive_mutex></code>, one might legitimately wish to
|
|
avoid constructing a <code>std::mutex</code> by default. A provided type, <code>monostate</code>,
|
|
can be used as the first type in those scenarios. <code>variant<monostate,
|
|
std::mutex, std::recursive_mutex></code> will default-construct a <code>monostate</code>,
|
|
which is basically a no-op, as <code>monostate</code> is effectively an empty <code>struct</code>.</p>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div class="sect1">
|
|
<h2 id="changelog">Revision History</h2>
|
|
<div class="sectionbody">
|
|
<div class="sect2">
|
|
<h3 id="changelog_changes_in_1_76_0">Changes in 1.76.0</h3>
|
|
<div class="ulist">
|
|
<ul>
|
|
<li>
|
|
<p>Improved generated code for the double buffered case.</p>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
<div class="sect2">
|
|
<h3 id="changelog_changes_in_1_74_0">Changes in 1.74.0</h3>
|
|
<div class="ulist">
|
|
<ul>
|
|
<li>
|
|
<p>Added support for derived types in <code>visit</code></p>
|
|
</li>
|
|
<li>
|
|
<p>Improved compilation performance for many (hundreds of) alternatives.</p>
|
|
</li>
|
|
<li>
|
|
<p>Added support for <code>visit<R></code></p>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
<div class="sect2">
|
|
<h3 id="changelog_changes_in_1_73_0">Changes in 1.73.0</h3>
|
|
<div class="ulist">
|
|
<ul>
|
|
<li>
|
|
<p>Added support for <code>std::hash</code>, <code>boost::hash</code>.</p>
|
|
</li>
|
|
<li>
|
|
<p><code>variant<T…​></code> is now trivial when all types in <code>T…​</code> are trivial.
|
|
This improves performance by enabling it to be passed to, and returned
|
|
from, functions in registers.</p>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
<div class="sect2">
|
|
<h3 id="changelog_changes_in_1_71_0">Changes in 1.71.0</h3>
|
|
<div class="paragraph">
|
|
<p>After the Boost formal review, the implementation has been
|
|
changed to provide the strong exception safety guarantee,
|
|
instead of basic. <code>expected</code> has been removed.</p>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div class="sect1">
|
|
<h2 id="design">Design</h2>
|
|
<div class="sectionbody">
|
|
<div class="sect2">
|
|
<h3 id="design_features">Features</h3>
|
|
<div class="paragraph">
|
|
<p>This <code>variant</code> implementation has two distinguishing features:</p>
|
|
</div>
|
|
<div class="ulist">
|
|
<ul>
|
|
<li>
|
|
<p>It’s never "valueless", that is, <code>variant<T1, T2, …​, Tn></code> has an
|
|
invariant that it always contains a valid value of one of the types
|
|
<code>T1</code>, <code>T2</code>, …​, <code>Tn</code>.</p>
|
|
</li>
|
|
<li>
|
|
<p>It provides the strong exception safety guarantee on assignment and
|
|
<code>emplace</code>.</p>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>This is achieved with the use of double storage, unless all of the
|
|
contained types have a non-throwing move constructor.</p>
|
|
</div>
|
|
</div>
|
|
<div class="sect2">
|
|
<h3 id="design_rationale">Rationale</h3>
|
|
<div class="sect3">
|
|
<h4 id="design_never_valueless">Never Valueless</h4>
|
|
<div class="paragraph">
|
|
<p>It makes intuitive sense that <code>variant<X, Y, Z></code> can hold only values
|
|
of type <code>X</code>, type <code>Y</code>, or type <code>Z</code>, and nothing else.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>If we think of <code>variant</code> as an extension of <code>union</code>, since a <code>union</code>
|
|
has a state called "no active member", an argument can be made that a
|
|
<code>variant<X, Y, Z></code> should also have such an additional state, holding
|
|
none of <code>X</code>, <code>Y</code>, <code>Z</code>.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>This however makes <code>variant</code> less convenient in practice and less useful
|
|
as a building block. If we really need a variable that only holds <code>X</code>,
|
|
<code>Y</code>, or <code>Z</code>, the additional empty state creates complications that need
|
|
to be worked around. And in the case where we do need this additional
|
|
empty state, we can just use <code>variant<empty, X, Y, Z></code>, with a suitable
|
|
<code>struct empty {};</code>.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>From a pure design perspective, the case for no additional empty state is
|
|
solid. Implementation considerations, however, argue otherwise.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>When we replace the current value of the <code>variant</code> (of, say, type <code>X</code>) with
|
|
another (of type <code>Y</code>), since the new value needs to occupy the same storage
|
|
as the old one, we need to destroy the old <code>X</code> first, then construct a new
|
|
<code>Y</code> in its place. But since this is C++, the construction can fail with an
|
|
exception. At this point the <code>variant</code> is in the "has no active member"
|
|
state that we’ve agreed it cannot be in.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>This is a legitimate problem, and it is this problem that makes having
|
|
an empty/valueless state so appealing. We just leave the <code>variant</code> empty on
|
|
exception and we’re done.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>As explained, though, this is undesirable from a design perspective as it
|
|
makes the component less useful and less elegant.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>There are several ways around the issue. The most straightforward one is to
|
|
just disallow types whose construction can throw. Since we can always create
|
|
a temporary value first, then use the move constructor to initialize the one
|
|
in the <code>variant</code>, it’s enough to require a nonthrowing move constructor,
|
|
rather than all constructors to be nonthrowing.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Unfortunately, under at least one popular standard library implementation,
|
|
node based containers such as <code>std::list</code> and <code>std::map</code> have a potentially
|
|
throwing move constructor. Disallowing <code>variant<X, std::map<Y, Z>></code> is hardly
|
|
practical, so the exceptional case cannot be avoided.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>On exception, we could also construct some other value, leaving the <code>variant</code>
|
|
valid; but in the general case, that construction can also throw. If one of
|
|
the types has a nonthrowing default constructor, we can use it; but if not,
|
|
we can’t.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>The approach Boost.Variant takes here is to allocate a temporary copy of
|
|
the value on the heap. On exception, a pointer to that temporary copy can be
|
|
stored into the <code>variant</code>. Pointer operations don’t throw.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Another option is to use double buffering. If our <code>variant</code> occupies twice
|
|
the storage, we can construct the new value in the unused half, then, once
|
|
the construction succeeds, destroy the old value in the other half.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>When <code>std::variant</code> was standardized, none of those approaches was deemed
|
|
palatable, as all of them either introduce overhead or are too restrictive
|
|
with respect to the types a <code>variant</code> can contain. So as a compromise,
|
|
<code>std::variant</code> took a way that can (noncharitably) be described as "having
|
|
your cake and eating it too."</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Since the described exceptional situation is relatively rare, <code>std::variant</code>
|
|
has a special case, called "valueless", into which it goes on exception,
|
|
but the interface acknowledges its existence as little as possible, allowing
|
|
users to pretend that it doesn’t exist.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>This is, arguably, not that bad from a practical point of view, but it leaves
|
|
many of us wanting. Rare states that "never" occur are undertested and when
|
|
that "never" actually happens, it’s usually in the most inconvenient of times.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>This implementation does not follow <code>std::variant</code>; it statically guarantees
|
|
that <code>variant</code> is never in a valueless state. The function
|
|
<code>valueless_by_exception</code> is provided for compatibility, but it always returns
|
|
<code>false</code>.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Instead, if the contained types are such that it’s not possible to avoid an
|
|
exceptional situation when changing the contained value, double storage is
|
|
used.</p>
|
|
</div>
|
|
</div>
|
|
<div class="sect3">
|
|
<h4 id="design_strong_exception_safety">Strong Exception Safety</h4>
|
|
<div class="paragraph">
|
|
<p>The initial submission only provided the basic exception safety guarantee.
|
|
If an attempt to change the contained value (via assignment or <code>emplace</code>)
|
|
failed with an exception, and a type with a nonthrowing default constructor
|
|
existed among the alternatives, a value of that type was created into the
|
|
<code>variant</code>. The upside of this decision was that double storage was needed
|
|
less frequently.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>The reviewers were fairly united in hating it. Constructing a random type
|
|
was deemed too unpredictable and not complying with the spirit of the
|
|
basic guarantee. The default constructor of the chosen type, even if
|
|
nonthrowing, may still have undesirable side effects. Or, if not that, a
|
|
value of that type may have special significance for the surrounding code.
|
|
Therefore, some argued, the <code>variant</code> should either remain with its
|
|
old value, or transition into the new one, without synthesizing other
|
|
states.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>At the other side of the spectrum, there were those who considered double
|
|
storage unacceptable. But they considered it unacceptable in principle,
|
|
regardless of the frequency with which it was used.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>As a result, providing the strong exception safety guarantee on assignment
|
|
and <code>emplace</code> was declared an acceptance condition.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>In retrospect, this was the right decision. The reason the strong guarantee
|
|
is generally not provided is because it doesn’t compose. When <code>X</code> and <code>Y</code>
|
|
provide the basic guarantee on assignment, so does <code>struct { X x; Y y; };</code>.
|
|
Similarly, when <code>X</code> and <code>Y</code> have nonthrowing assignments, so does the
|
|
<code>struct</code>. But this doesn’t hold for the strong guarantee.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>The usual practice is to provide the basic guarantee on assignment and
|
|
let the user synthesize a "strong" assignment out of either a nonthrowing
|
|
<code>swap</code> or a nonthrowing move assignment. That is, given <code>x1</code> and <code>x2</code> of
|
|
type <code>X</code>, instead of the "basic" <code>x1 = x2;</code>, use either <code>X(x2).swap(x1);</code>
|
|
or <code>x1 = X(x2);</code>.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Nearly all types provide a nonthrowing <code>swap</code> or a nonthrowing move
|
|
assignment, so this works well. Nearly all, except <code>variant</code>, which in the
|
|
general case has neither a nonthrowing <code>swap</code> nor a nonthrowing move
|
|
assignment. If <code>variant</code> does not provide the strong guarantee itself, it’s
|
|
impossible for the user to synthesize it.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>So it should, and so it does.</p>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div class="sect2">
|
|
<h3 id="design_differences_with_stdvariant">Differences with std::variant</h3>
|
|
<div class="paragraph">
|
|
<p>The main differences between this implementation and <code>std::variant</code> are:</p>
|
|
</div>
|
|
<div class="ulist">
|
|
<ul>
|
|
<li>
|
|
<p>No valueless-by-exception state: <code>valueless_by_exception()</code> always
|
|
returns <code>false</code>.</p>
|
|
</li>
|
|
<li>
|
|
<p>Strong exception safety guarantee on assignment and <code>emplace</code>.</p>
|
|
</li>
|
|
<li>
|
|
<p><code>emplace</code> first constructs the new value and then destroys the old one;
|
|
in the single storage case, this translates to constructing a temporary
|
|
and then moving it into place.</p>
|
|
</li>
|
|
<li>
|
|
<p>A converting constructor from, e.g. <code>variant<int, float></code> to
|
|
<code>variant<float, double, int></code> is provided as an extension.</p>
|
|
</li>
|
|
<li>
|
|
<p>The reverse operation, going from <code>variant<float, double, int></code> to
|
|
<code>variant<int, float></code> is provided as the member function <code>subset<U…​></code>.
|
|
(This operation can throw if the current state of the variant cannot be
|
|
represented.)</p>
|
|
</li>
|
|
<li>
|
|
<p>The C++20 additions and changes to <code>std::variant</code> have not yet been
|
|
implemented.</p>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
<div class="sect2">
|
|
<h3 id="design_differences_with_boost_variant">Differences with Boost.Variant</h3>
|
|
<div class="paragraph">
|
|
<p>This library is API compatible with <code>std::variant</code>. As such, its interface
|
|
is different from Boost.Variant’s. For example, visitation is performed via
|
|
<code>visit</code> instead of <code>apply_visitor</code>.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Recursive variants are not supported.</p>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Double storage is used instead of temporary heap backup. This <code>variant</code> is
|
|
always "stack-based", it never allocates, and never throws <code>bad_alloc</code> on
|
|
its own.</p>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div class="sect1">
|
|
<h2 id="implementation">Implementation</h2>
|
|
<div class="sectionbody">
|
|
<div class="sect2">
|
|
<h3 id="implementation_dependencies">Dependencies</h3>
|
|
<div class="paragraph">
|
|
<p>This implementation only depends on Boost.Config and Boost.Mp11.</p>
|
|
</div>
|
|
</div>
|
|
<div class="sect2">
|
|
<h3 id="implementation_supported_compilers">Supported Compilers</h3>
|
|
<div class="ulist">
|
|
<ul>
|
|
<li>
|
|
<p>GCC 4.8 or later with <code>-std=c++11</code> or above</p>
|
|
</li>
|
|
<li>
|
|
<p>Clang 3.5 or later with <code>-std=c++11</code> or above</p>
|
|
</li>
|
|
<li>
|
|
<p>Visual Studio 2015, 2017, 2019</p>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Tested on <a href="https://travis-ci.org/boostorg/variant2/">Travis</a> and
|
|
<a href="https://ci.appveyor.com/project/pdimov/variant2-fkab9">Appveyor</a>.</p>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div class="sect1">
|
|
<h2 id="reference">Reference</h2>
|
|
<div class="sectionbody">
|
|
<div class="sect2">
|
|
<h3 id="ref_boostvariant2variant_hpp"><boost/variant2/variant.hpp></h3>
|
|
<div class="sect3">
|
|
<h4 id="ref_synopsis">Synopsis</h4>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>namespace boost {
|
|
namespace variant2 {
|
|
|
|
// in_place_type
|
|
|
|
template<class T> struct in_place_type_t {};
|
|
template<class T> constexpr in_place_type_t<T> in_place_type{};
|
|
|
|
// in_place_index
|
|
|
|
template<std::size_t I> struct in_place_index_t {};
|
|
template<std::size_t I> constexpr in_place_index_t<I> in_place_index{};
|
|
|
|
// variant
|
|
|
|
template<class... T> class variant;
|
|
|
|
// variant_size
|
|
|
|
template<class T> struct variant_size {};
|
|
|
|
template<class T> struct variant_size<T const>: variant_size<T> {};
|
|
template<class T> struct variant_size<T volatile>: variant_size<T> {};
|
|
template<class T> struct variant_size<T const volatile>: variant_size<T> {};
|
|
|
|
template<class T> struct variant_size<T&>: variant_size<T> {}; // extension
|
|
template<class T> struct variant_size<T&&>: variant_size<T> {}; // extension
|
|
|
|
template<class T>
|
|
inline constexpr size_t variant_size_v = variant_size<T>::value;
|
|
|
|
template<class... T>
|
|
struct variant_size<variant<T...>>:
|
|
std::integral_constant<std::size_t, sizeof...(T)> {};
|
|
|
|
// variant_alternative
|
|
|
|
template<size_t I, class T> struct variant_alternative {};
|
|
|
|
template<size_t I, class T> struct variant_alternative<I, T const>;
|
|
template<size_t I, class T> struct variant_alternative<I, T volatile>;
|
|
template<size_t I, class T> struct variant_alternative<I, T const volatile>;
|
|
|
|
template<size_t I, class T> struct variant_alternative<I, T&>; // extension
|
|
template<size_t I, class T> struct variant_alternative<I, T&&>; // extension
|
|
|
|
template<size_t I, class T>
|
|
using variant_alternative_t = typename variant_alternative<I, T>::type;
|
|
|
|
template<size_t I, class... T>
|
|
struct variant_alternative<I, variant<T...>>;
|
|
|
|
// variant_npos
|
|
|
|
constexpr std::size_t variant_npos = -1;
|
|
|
|
// holds_alternative
|
|
|
|
template<class U, class... T>
|
|
constexpr bool holds_alternative(const variant<T...>& v) noexcept;
|
|
|
|
// get
|
|
|
|
template<size_t I, class... T>
|
|
constexpr variant_alternative_t<I, variant<T...>>&
|
|
get(variant<T...>& v);
|
|
template<size_t I, class... T>
|
|
constexpr variant_alternative_t<I, variant<T...>>&&
|
|
get(variant<T...>&& v);
|
|
template<size_t I, class... T>
|
|
constexpr const variant_alternative_t<I, variant<T...>>&
|
|
get(const variant<T...>& v);
|
|
template<size_t I, class... T>
|
|
constexpr const variant_alternative_t<I, variant<T...>>&&
|
|
get(const variant<T...>&& v);
|
|
|
|
template<class U, class... T>
|
|
constexpr U& get(variant<T...>& v);
|
|
template<class U, class... T>
|
|
constexpr U&& get(variant<T...>&& v);
|
|
template<class U, class... T>
|
|
constexpr const U& get(const variant<T...>& v);
|
|
template<class U, class... T>
|
|
constexpr const U&& get(const variant<T...>&& v);
|
|
|
|
// get_if
|
|
|
|
template<size_t I, class... T>
|
|
constexpr add_pointer_t<variant_alternative_t<I, variant<T...>>>
|
|
get_if(variant<T...>* v) noexcept;
|
|
template<size_t I, class... T>
|
|
constexpr add_pointer_t<const variant_alternative_t<I, variant<T...>>>
|
|
get_if(const variant<T...>* v) noexcept;
|
|
|
|
template<class U, class... T>
|
|
constexpr add_pointer_t<U>
|
|
get_if(variant<T...>* v) noexcept;
|
|
template<class U, class... T>
|
|
constexpr add_pointer_t<const U>
|
|
get_if(const variant<T...>* v) noexcept;
|
|
|
|
// relational operators
|
|
|
|
template<class... T>
|
|
constexpr bool operator==(const variant<T...>& v, const variant<T...>& w);
|
|
template<class... T>
|
|
constexpr bool operator!=(const variant<T...>& v, const variant<T...>& w);
|
|
template<class... T>
|
|
constexpr bool operator<(const variant<T...>& v, const variant<T...>& w);
|
|
template<class... T>
|
|
constexpr bool operator>(const variant<T...>& v, const variant<T...>& w);
|
|
template<class... T>
|
|
constexpr bool operator<=(const variant<T...>& v, const variant<T...>& w);
|
|
template<class... T>
|
|
constexpr bool operator>=(const variant<T...>& v, const variant<T...>& w);
|
|
|
|
// visit
|
|
|
|
template<class R = /*unspecified*/, class F, class... V>
|
|
constexpr /*see below*/ visit(F&& f, V&&... v);
|
|
|
|
// monostate
|
|
|
|
struct monostate {};
|
|
|
|
constexpr bool operator==(monostate, monostate) noexcept { return true; }
|
|
constexpr bool operator!=(monostate, monostate) noexcept { return false; }
|
|
constexpr bool operator<(monostate, monostate) noexcept { return false; }
|
|
constexpr bool operator>(monostate, monostate) noexcept { return false; }
|
|
constexpr bool operator<=(monostate, monostate) noexcept { return true; }
|
|
constexpr bool operator>=(monostate, monostate) noexcept { return true; }
|
|
|
|
// swap
|
|
|
|
template<class... T>
|
|
void swap(variant<T...>& v, variant<T...>& w) noexcept( /*see below*/ );
|
|
|
|
// bad_variant_access
|
|
|
|
class bad_variant_access;
|
|
|
|
} // namespace variant2
|
|
} // namespace boost</code></pre>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div class="sect3">
|
|
<h4 id="ref_variant">variant</h4>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>namespace boost {
|
|
namespace variant2 {
|
|
|
|
template<class... T> class variant
|
|
{
|
|
public:
|
|
|
|
// constructors
|
|
|
|
constexpr variant() noexcept( /*see below*/ );
|
|
|
|
constexpr variant( variant const & r ) noexcept( /*see below*/ );
|
|
constexpr variant( variant&& r ) noexcept( /*see below*/ );
|
|
|
|
template<class U>
|
|
constexpr variant( U&& u ) noexcept( /*see below*/ );
|
|
|
|
template<class U, class... A>
|
|
constexpr explicit variant( in_place_type_t<U>, A&&... a );
|
|
template<class U, class V, class... A>
|
|
constexpr explicit variant( in_place_type_t<U>,
|
|
std::initializer_list<V> il, A&&... a );
|
|
|
|
template<size_t I, class... A>
|
|
constexpr explicit variant( in_place_index_t<I>, A&&... a );
|
|
template<size_t I, class V, class... A>
|
|
constexpr explicit variant( in_place_index_t<I>,
|
|
std::initializer_list<V> il, A&&... a );
|
|
|
|
// destructor
|
|
|
|
~variant();
|
|
|
|
// assignment
|
|
|
|
constexpr variant& operator=( variant const & r ) noexcept( /*see below*/ );
|
|
constexpr variant& operator=( variant&& r ) noexcept( /*see below*/ );
|
|
|
|
template<class U> constexpr variant& operator=( U&& u ) noexcept( /*see below*/ );
|
|
|
|
// modifiers
|
|
|
|
template<class U, class... A>
|
|
constexpr U& emplace( A&&... a );
|
|
template<class U, class V, class... A>
|
|
constexpr U& emplace( std::initializer_list<V> il, A&&... a );
|
|
|
|
template<size_t I, class... A>
|
|
constexpr variant_alternative_t<I, variant<T...>>&
|
|
emplace( A&&... a );
|
|
template<size_t I, class V, class... A>
|
|
constexpr variant_alternative_t<I, variant<T...>>&
|
|
emplace( std::initializer_list<V> il, A&&... a );
|
|
|
|
// value status
|
|
|
|
constexpr bool valueless_by_exception() const noexcept;
|
|
constexpr size_t index() const noexcept;
|
|
|
|
// swap
|
|
|
|
void swap( variant& r ) noexcept( /*see below*/ );
|
|
|
|
// converting constructors (extension)
|
|
|
|
template<class... U> variant( variant<U...> const& r )
|
|
noexcept( /*see below*/ );
|
|
|
|
template<class... U> variant( variant<U...>&& r )
|
|
noexcept( /*see below*/ );
|
|
|
|
// subset (extension)
|
|
|
|
template<class... U> constexpr variant<U...> subset() & ;
|
|
template<class... U> constexpr variant<U...> subset() && ;
|
|
template<class... U> constexpr variant<U...> subset() const& ;
|
|
template<class... U> constexpr variant<U...> subset() const&& ;
|
|
};
|
|
|
|
} // namespace variant2
|
|
} // namespace boost</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>In the descriptions that follow, let <code>i</code> be in the range <code>[0, sizeof…​(T))</code>,
|
|
and <code>Ti</code> be the <code>i</code>-th type in <code>T…​</code>.</p>
|
|
</div>
|
|
<div class="sect4">
|
|
<h5 id="ref_constructors">Constructors</h5>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>constexpr variant() noexcept( std::is_nothrow_default_constructible_v<T0> );</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Effects: </dt>
|
|
<dd>
|
|
<p>Constructs a <code>variant</code> holding a value-initialized value of
|
|
type <code>T0</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Ensures: </dt>
|
|
<dd>
|
|
<p><code>index() == 0</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Throws: </dt>
|
|
<dd>
|
|
<p>Any exception thrown by the value-initialization of <code>T0</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Remarks: </dt>
|
|
<dd>
|
|
<p>This function does not participate in overload resolution unless
|
|
<code>std::is_default_constructible_v<T0></code> is <code>true</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>constexpr variant( variant const & w )
|
|
noexcept( mp_all<std::is_nothrow_copy_constructible<T>...>::value );</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Effects: </dt>
|
|
<dd>
|
|
<p>Initializes the variant to hold the same alternative and value as
|
|
<code>w</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Throws: </dt>
|
|
<dd>
|
|
<p>Any exception thrown by the initialization of the contained value.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Remarks: </dt>
|
|
<dd>
|
|
<p>This function does not participate in overload resolution unless
|
|
<code>std::is_copy_constructible_v<Ti></code> is <code>true</code> for all <code>i</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>constexpr variant( variant&& w )
|
|
noexcept( mp_all<std::is_nothrow_move_constructible<T>...>::value );</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Effects: </dt>
|
|
<dd>
|
|
<p>Initializes the variant to hold the same alternative and value as
|
|
<code>w</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Throws: </dt>
|
|
<dd>
|
|
<p>Any exception thrown by the move-initialization of the contained
|
|
value.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Remarks: </dt>
|
|
<dd>
|
|
<p>This function does not participate in overload resolution unless
|
|
<code>std::is_move_constructible_v<Ti></code> is <code>true</code> for all <code>i</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class U> constexpr variant( U&& u ) noexcept(/*see below*/);</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="paragraph">
|
|
<p>Let <code>Tj</code> be a type that is determined as follows: build an imaginary function
|
|
<code>FUN(Ti)</code> for each alternative type <code>Ti</code>. The overload <code>FUN(Tj)</code> selected by
|
|
overload resolution for the expression <code>FUN(std::forward<U>(u))</code> defines the
|
|
alternative <code>Tj</code> which is the type of the contained value after construction.</p>
|
|
</div>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Effects: </dt>
|
|
<dd>
|
|
<p>Initializes <code>*this</code> to hold the alternative type <code>Tj</code> and
|
|
initializes the contained value from <code>std::forward<U>(u)</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Ensures: </dt>
|
|
<dd>
|
|
<p><code>holds_alternative<Tj>(*this)</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Throws: </dt>
|
|
<dd>
|
|
<p>Any exception thrown by the initialization of the contained value.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Remarks: </dt>
|
|
<dd>
|
|
<p>The expression inside <code>noexcept</code> is equivalent to
|
|
<code>std::is_nothrow_constructible_v<Tj, U></code>. This function does not participate in
|
|
overload resolution unless</p>
|
|
<div class="ulist">
|
|
<ul>
|
|
<li>
|
|
<p><code>sizeof…​(T)</code> is nonzero,</p>
|
|
</li>
|
|
<li>
|
|
<p><code>std::is_same_v<std::remove_cvref_t<U>, variant></code> is <code>false</code>,</p>
|
|
</li>
|
|
<li>
|
|
<p><code>std::remove_cvref_t<U></code> is neither a specialization of <code>in_place_type_t</code> nor a
|
|
specialization of <code>in_place_index_t</code>,</p>
|
|
</li>
|
|
<li>
|
|
<p><code>std::is_constructible_v<Tj, U></code> is <code>true</code>, and</p>
|
|
</li>
|
|
<li>
|
|
<p>the expression <code>FUN(std::forward<U>(u))</code> is well-formed.</p>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class U, class... A>
|
|
constexpr explicit variant( in_place_type_t<U>, A&&... a );</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Effects: </dt>
|
|
<dd>
|
|
<p>Initializes the contained value of type <code>U</code> with the arguments
|
|
<code>std::forward<A>(a)…​</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Ensures: </dt>
|
|
<dd>
|
|
<p><code>holds_alternative<U>(*this)</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Throws: </dt>
|
|
<dd>
|
|
<p>Any exception thrown by the initialization of the contained value.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Remarks: </dt>
|
|
<dd>
|
|
<p>This function does not participate in overload resolution unless
|
|
there is exactly one occurrence of <code>U</code> in <code>T…​</code> and
|
|
<code>std::is_constructible_v<U, A…​></code> is true.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class U, class V, class... A>
|
|
constexpr explicit variant( in_place_type_t<U>, std::initializer_list<V> il,
|
|
A&&... a );</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Effects: </dt>
|
|
<dd>
|
|
<p>Initializes the contained value of type <code>U</code> with the arguments <code>il</code>,
|
|
<code>std::forward<A>(a)…​</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Ensures: </dt>
|
|
<dd>
|
|
<p><code>holds_alternative<U>(*this)</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Throws: </dt>
|
|
<dd>
|
|
<p>Any exception thrown by the initialization of the contained value.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Remarks: </dt>
|
|
<dd>
|
|
<p>This function does not participate in overload resolution unless
|
|
there is exactly one occurrence of <code>U</code> in <code>T…​</code> and
|
|
<code>std::is_constructible_v<U, initializer_list<V>&, A…​></code> is <code>true</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<size_t I, class... A>
|
|
constexpr explicit variant( in_place_index_t<I>, A&&... a );</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Effects: </dt>
|
|
<dd>
|
|
<p>Initializes the contained value of type <code>TI</code> with the arguments
|
|
<code>std::forward<A>(a)…​</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Ensures: </dt>
|
|
<dd>
|
|
<p><code>index() == I</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Throws: </dt>
|
|
<dd>
|
|
<p>Any exception thrown by the initialization of the contained value.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Remarks: </dt>
|
|
<dd>
|
|
<p>This function does not participate in overload resolution unless
|
|
<code>I < sizeof…​(T)</code> and <code>std::is_constructible_v<TI, A…​></code> is <code>true</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<size_t I, class V, class... A>
|
|
constexpr explicit variant( in_place_index_t<I>, std::initializer_list<V> il,
|
|
A&&... a );</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Effects: </dt>
|
|
<dd>
|
|
<p>Initializes the contained value of type <code>TI</code> with the arguments
|
|
<code>il</code>, <code>std::forward<A>(a)…​</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Ensures: </dt>
|
|
<dd>
|
|
<p><code>index() == I</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Throws: </dt>
|
|
<dd>
|
|
<p>Any exception thrown by the initialization of the contained value.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Remarks: </dt>
|
|
<dd>
|
|
<p>This function does not participate in overload resolution unless
|
|
<code>I < sizeof…​(T)</code> and
|
|
<code>std::is_constructible_v<TI, initializer_list<V>&, A…​></code> is <code>true</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
<div class="sect4">
|
|
<h5 id="ref_destructor">Destructor</h5>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>~variant();</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Effects: </dt>
|
|
<dd>
|
|
<p>Destroys the currently contained value.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
<div class="sect4">
|
|
<h5 id="ref_assignment">Assignment</h5>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>constexpr variant& operator=( const variant& r )
|
|
noexcept( mp_all<std::is_nothrow_copy_constructible<T>...>::value );</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="paragraph">
|
|
<p>Let <code>j</code> be <code>r.index()</code>.</p>
|
|
</div>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Effects: </dt>
|
|
<dd>
|
|
<p><code>emplace<j>(get<j>(r))</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Returns: </dt>
|
|
<dd>
|
|
<p><code>*this</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Ensures: </dt>
|
|
<dd>
|
|
<p><code>index() == r.index()</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Remarks: </dt>
|
|
<dd>
|
|
<p>This operator does not participate in overload resolution unless
|
|
<code>std::is_copy_constructible_v<Ti> && std::is_copy_assignable_v<Ti></code> is
|
|
<code>true</code> for all <code>i</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>constexpr variant& operator=( variant&& r )
|
|
noexcept( mp_all<std::is_nothrow_move_constructible<T>...>::value );</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="paragraph">
|
|
<p>Let <code>j</code> be <code>r.index()</code>.</p>
|
|
</div>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Effects: </dt>
|
|
<dd>
|
|
<p><code>emplace<j>(get<j>(std::move(r)))</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Returns: </dt>
|
|
<dd>
|
|
<p><code>*this</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Ensures: </dt>
|
|
<dd>
|
|
<p><code>index() == r.index()</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Remarks: </dt>
|
|
<dd>
|
|
<p>This operator does not participate in overload resolution unless
|
|
<code>std::is_move_constructible_v<Ti> && std::is_move_assignable_v<Ti></code> is
|
|
<code>true</code> for all <code>i</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class U> constexpr variant& operator=( U&& u )
|
|
noexcept( /*see below*/ );</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="paragraph">
|
|
<p>Let <code>Tj</code> be a type that is determined as follows: build an imaginary function
|
|
<code>FUN(Ti)</code> for each alternative type <code>Ti</code>. The overload <code>FUN(Tj)</code> selected by
|
|
overload resolution for the expression <code>FUN(std::forward<U>(u))</code> defines the
|
|
alternative <code>Tj</code> which is the type of the contained value after construction.</p>
|
|
</div>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Effects: </dt>
|
|
<dd>
|
|
<p><code>emplace<j>(std::forward<U>(u))</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Returns: </dt>
|
|
<dd>
|
|
<p><code>*this</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Ensures: </dt>
|
|
<dd>
|
|
<p><code>index() == j</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Remarks: </dt>
|
|
<dd>
|
|
<p>The expression inside <code>noexcept</code> is <code>std::is_nothrow_constructible_v<Tj, U&&></code>.
|
|
This operator does not participate in overload resolution unless</p>
|
|
<div class="ulist">
|
|
<ul>
|
|
<li>
|
|
<p><code>std::is_same_v<std::remove_cvref_t<T>, variant></code> is <code>false</code>,</p>
|
|
</li>
|
|
<li>
|
|
<p><code>std::is_constructible_v<Tj, U&&> && std::is_assignable_v<Tj&, U&&></code> is
|
|
<code>true</code>, and</p>
|
|
</li>
|
|
<li>
|
|
<p>the expression <code>FUN(std::forward<U>(u))</code> (with <code>FUN</code> being the
|
|
above-mentioned set of imaginary functions) is well-formed.</p>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
<div class="sect4">
|
|
<h5 id="ref_modifiers">Modifiers</h5>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class U, class... A>
|
|
constexpr U& emplace( A&&... a );</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="paragraph">
|
|
<p>Let <code>I</code> be the zero-based index of <code>U</code> in <code>T…​</code>.</p>
|
|
</div>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Effects: </dt>
|
|
<dd>
|
|
<p>Equivalent to: <code>return emplace<I>(std::forward<A>(a)…​);</code></p>
|
|
</dd>
|
|
<dt class="hdlist1">Remarks: </dt>
|
|
<dd>
|
|
<p>This function shall not participate in overload resolution unless
|
|
<code>std::is_constructible_v<U, A&&…​></code> is <code>true</code> and <code>U</code> occurs exactly once
|
|
in <code>T…​</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class U, class V, class... A>
|
|
constexpr U& emplace( std::initializer_list<V> il, A&&... a );</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="paragraph">
|
|
<p>Let <code>I</code> be the zero-based index of <code>U</code> in <code>T…​</code>.</p>
|
|
</div>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Effects: </dt>
|
|
<dd>
|
|
<p>Equivalent to: <code>return emplace<I>(il, std::forward<A>(a)…​);</code></p>
|
|
</dd>
|
|
<dt class="hdlist1">Remarks: </dt>
|
|
<dd>
|
|
<p>This function shall not participate in overload resolution unless
|
|
<code>std::is_constructible_v<U, std::initializer_list<V>&, A&&…​></code> is <code>true</code>
|
|
and <code>U</code> occurs exactly once in <code>T…​</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<size_t I, class... A>
|
|
constexpr variant_alternative_t<I, variant<T...>>&
|
|
emplace( A&&... a );</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Requires: </dt>
|
|
<dd>
|
|
<p><code>I < sizeof…​(T)</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Effects: </dt>
|
|
<dd>
|
|
<p>Initializes a new contained value as if using the expression
|
|
<code>Ti(std::forward<A>(a)…​)</code>, then destroys the currently contained value.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Ensures: </dt>
|
|
<dd>
|
|
<p><code>index() == I</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Returns: </dt>
|
|
<dd>
|
|
<p>A reference to the new contained value.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Throws: </dt>
|
|
<dd>
|
|
<p>Nothing unless the initialization of the new contained value throws.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Exception Safety: </dt>
|
|
<dd>
|
|
<p>Strong. On exception, the contained value is unchanged.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Remarks: </dt>
|
|
<dd>
|
|
<p>This function shall not participate in overload resolution unless
|
|
<code>std::is_constructible_v<Ti, A&&…​></code> is <code>true</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<size_t I, class V, class... A>
|
|
constexpr variant_alternative_t<I, variant<T...>>&
|
|
emplace( std::initializer_list<V> il, A&&... a );</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Requires: </dt>
|
|
<dd>
|
|
<p><code>I < sizeof…​(T)</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Effects: </dt>
|
|
<dd>
|
|
<p>Initializes a new contained value as if using the expression
|
|
<code>Ti(il, std::forward<A>(a)…​)</code>, then destroys the currently contained value.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Ensures: </dt>
|
|
<dd>
|
|
<p><code>index() == I</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Returns: </dt>
|
|
<dd>
|
|
<p>A reference to the new contained value.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Throws: </dt>
|
|
<dd>
|
|
<p>Nothing unless the initialization of the new contained value throws.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Exception Safety: </dt>
|
|
<dd>
|
|
<p>Strong. On exception, the contained value is unchanged.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Remarks: </dt>
|
|
<dd>
|
|
<p>This function shall not participate in overload resolution unless
|
|
<code>std::is_constructible_v<Ti, std::initializer_list<V>&, A&&…​></code> is <code>true</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
<div class="sect4">
|
|
<h5 id="ref_value_status">Value Status</h5>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>constexpr bool valueless_by_exception() const noexcept;</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Returns: </dt>
|
|
<dd>
|
|
<p><code>false</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="admonitionblock note">
|
|
<table>
|
|
<tr>
|
|
<td class="icon">
|
|
<div class="title">Note</div>
|
|
</td>
|
|
<td class="content">
|
|
This function is provided purely for compatibility with <code>std::variant</code>.
|
|
</td>
|
|
</tr>
|
|
</table>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>constexpr size_t index() const noexcept;</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Returns: </dt>
|
|
<dd>
|
|
<p>The zero-based index of the active alternative.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
<div class="sect4">
|
|
<h5 id="ref_swap">Swap</h5>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>void swap( variant& r ) noexcept( mp_all<std::is_nothrow_move_constructible<T>...,
|
|
is_nothrow_swappable<T>...>::value );</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Effects: </dt>
|
|
<dd>
|
|
<div class="ulist">
|
|
<ul>
|
|
<li>
|
|
<p>If <code>index() == r.index()</code>, calls <code>swap(get<I>(*this), get<I>(r))</code>,
|
|
where <code>I</code> is <code>index()</code>.</p>
|
|
</li>
|
|
<li>
|
|
<p>Otherwise, as if
|
|
<code>variant tmp(std::move(*this)); *this = std::move(r); r = std::move(tmp);</code></p>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
<div class="sect4">
|
|
<h5 id="ref_converting_constructors_extension">Converting Constructors (extension)</h5>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class... U> variant( variant<U...> const& r )
|
|
noexcept( mp_all<std::is_nothrow_copy_constructible<U>...>::value );</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Effects: </dt>
|
|
<dd>
|
|
<p>Initializes the contained value from the contained value of <code>r</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Throws: </dt>
|
|
<dd>
|
|
<p>Any exception thrown by the initialization of the contained value.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Remarks: </dt>
|
|
<dd>
|
|
<p>This function does not participate in overload resolution unless
|
|
all types in <code>U…​</code> are in <code>T…​</code> and
|
|
<code>std::is_copy_constructible_v<Ui>::value</code> is <code>true</code> for all <code>Ui</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class... U> variant( variant<U...>&& r )
|
|
noexcept( mp_all<std::is_nothrow_move_constructible<U>...>::value );</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Effects: </dt>
|
|
<dd>
|
|
<p>Initializes the contained value from the contained value of
|
|
<code>std::move(r)</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Throws: </dt>
|
|
<dd>
|
|
<p>Any exception thrown by the initialization of the contained value.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Remarks: </dt>
|
|
<dd>
|
|
<p>This function does not participate in overload resolution unless
|
|
all types in <code>U…​</code> are in <code>T…​</code> and
|
|
<code>std::is_move_constructible_v<Ui>::value</code> is <code>true</code> for all <code>Ui</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
<div class="sect4">
|
|
<h5 id="ref_subset_extension">Subset (extension)</h5>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class... U> constexpr variant<U...> subset() & ;</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class... U> constexpr variant<U...> subset() const& ;</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Returns: </dt>
|
|
<dd>
|
|
<p>A <code>variant<U…​></code> whose contained value is copy-initialized from
|
|
the contained value of <code>*this</code> and has the same type.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Throws: </dt>
|
|
<dd>
|
|
<div class="ulist">
|
|
<ul>
|
|
<li>
|
|
<p>If the active alternative of <code>*this</code> is not among the types in <code>U…​</code>,
|
|
<code>bad_variant_access</code>.</p>
|
|
</li>
|
|
<li>
|
|
<p>Otherwise, any exception thrown by the initialization of the contained value.</p>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
</dd>
|
|
<dt class="hdlist1">Remarks: </dt>
|
|
<dd>
|
|
<p>This function does not participate in overload resolution unless
|
|
all types in <code>U…​</code> are in <code>T…​</code> and
|
|
<code>std::is_copy_constructible_v<Ui>::value</code> is <code>true</code> for all <code>Ui</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class... U> constexpr variant<U...> subset() && ;</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class... U> constexpr variant<U...> subset() const&& ;</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Returns: </dt>
|
|
<dd>
|
|
<p>A <code>variant<U…​></code> whose contained value is move-initialized from
|
|
the contained value of <code>*this</code> and has the same type.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Throws: </dt>
|
|
<dd>
|
|
<div class="ulist">
|
|
<ul>
|
|
<li>
|
|
<p>If the active alternative of <code>*this</code> is not among the types in <code>U…​</code>,
|
|
<code>bad_variant_access</code>.</p>
|
|
</li>
|
|
<li>
|
|
<p>Otherwise, any exception thrown by the initialization of the contained value.</p>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
</dd>
|
|
<dt class="hdlist1">Remarks: </dt>
|
|
<dd>
|
|
<p>This function does not participate in overload resolution unless
|
|
all types in <code>U…​</code> are in <code>T…​</code> and
|
|
<code>std::is_move_constructible_v<Ui>::value</code> is <code>true</code> for all <code>Ui</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div class="sect3">
|
|
<h4 id="ref_variant_alternative">variant_alternative</h4>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<size_t I, class T> struct variant_alternative<I, T const>;</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<size_t I, class T> struct variant_alternative<I, T volatile>;</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<size_t I, class T> struct variant_alternative<I, T const volatile>;</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<size_t I, class T> struct variant_alternative<I, T&>; // extension</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<size_t I, class T> struct variant_alternative<I, T&&>; // extension</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="openblock">
|
|
<div class="content">
|
|
<div class="paragraph">
|
|
<p>If <code>typename variant_alternative<I, T>::type</code> exists and is <code>U</code>,</p>
|
|
</div>
|
|
<div class="ulist">
|
|
<ul>
|
|
<li>
|
|
<p><code>variant_alternative<I, T const>::type</code> is <code>U const</code>;</p>
|
|
</li>
|
|
<li>
|
|
<p><code>variant_alternative<I, T volatile>::type</code> is <code>U volatile</code>;</p>
|
|
</li>
|
|
<li>
|
|
<p><code>variant_alternative<I, T const volatile>::type</code> is <code>U const volatile</code>.</p>
|
|
</li>
|
|
<li>
|
|
<p><code>variant_alternative<I, T&>::type</code> is <code>U&</code>.</p>
|
|
</li>
|
|
<li>
|
|
<p><code>variant_alternative<I, T&&>::type</code> is <code>U&&</code>.</p>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="paragraph">
|
|
<p>Otherwise, these structs have no member <code>type</code>.</p>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<size_t I, class... T>
|
|
struct variant_alternative<I, variant<T...>>;</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="paragraph">
|
|
<p>When <code>I < sizeof…​(T)</code>, the nested type <code>type</code> is an alias for the <code>I</code>-th
|
|
(zero-based) type in <code>T…​</code>. Otherwise, there is no member <code>type</code>.</p>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
<div class="sect3">
|
|
<h4 id="ref_holds_alternative">holds_alternative</h4>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class U, class... T>
|
|
constexpr bool holds_alternative(const variant<T...>& v) noexcept;</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Requires: </dt>
|
|
<dd>
|
|
<p>The type <code>U</code> occurs exactly once in <code>T…​</code>. Otherwise, the
|
|
program is ill-formed.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Returns: </dt>
|
|
<dd>
|
|
<p><code>true</code> if <code>index()</code> is equal to the zero-based index of <code>U</code>
|
|
in <code>T…​</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
<div class="sect3">
|
|
<h4 id="ref_get">get</h4>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<size_t I, class... T>
|
|
constexpr variant_alternative_t<I, variant<T...>>&
|
|
get(variant<T...>& v);</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<size_t I, class... T>
|
|
constexpr variant_alternative_t<I, variant<T...>>&&
|
|
get(variant<T...>&& v);</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<size_t I, class... T>
|
|
constexpr const variant_alternative_t<I, variant<T...>>&
|
|
get(const variant<T...>& v);</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<size_t I, class... T>
|
|
constexpr const variant_alternative_t<I, variant<T...>>&&
|
|
get(const variant<T...>&& v);</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Effects: </dt>
|
|
<dd>
|
|
<p>If <code>v.index()</code> is <code>I</code>, returns a reference to the object stored in
|
|
the variant. Otherwise, throws <code>bad_variant_access</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Remarks: </dt>
|
|
<dd>
|
|
<p>These functions do not participate in overload resolution
|
|
unless <code>I</code> < <code>sizeof…​(T)</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class U, class... T>
|
|
constexpr U& get(variant<T...>& v);</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class U, class... T>
|
|
constexpr U&& get(variant<T...>&& v);</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class U, class... T>
|
|
constexpr const U& get(const variant<T...>& v);</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class U, class... T>
|
|
constexpr const U&& get(const variant<T...>&& v);</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Requires: </dt>
|
|
<dd>
|
|
<p>The type <code>U</code> occurs exactly once in <code>T…​</code>. Otherwise, the
|
|
program is ill-formed.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Effects: </dt>
|
|
<dd>
|
|
<p>If <code>v</code> holds a value of type <code>U</code>, returns a reference to that value.
|
|
Otherwise, throws <code>bad_variant_access</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
<div class="sect3">
|
|
<h4 id="ref_get_if">get_if</h4>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<size_t I, class... T>
|
|
constexpr add_pointer_t<variant_alternative_t<I, variant<T...>>>
|
|
get_if(variant<T...>* v) noexcept;</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<size_t I, class... T>
|
|
constexpr add_pointer_t<const variant_alternative_t<I, variant<T...>>>
|
|
get_if(const variant<T...>* v) noexcept;</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Effects: </dt>
|
|
<dd>
|
|
<p>A pointer to the value stored in the variant, if
|
|
<code>v != nullptr && v->index() == I</code>. Otherwise, <code>nullptr</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Remarks: </dt>
|
|
<dd>
|
|
<p>These functions do not participate in overload resolution
|
|
unless <code>I</code> < <code>sizeof…​(T)</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class U, class... T>
|
|
constexpr add_pointer_t<U>
|
|
get_if(variant<T...>* v) noexcept;</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class U, class... T>
|
|
constexpr add_pointer_t<const U>
|
|
get_if(const variant<T...>* v) noexcept;</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Requires: </dt>
|
|
<dd>
|
|
<p>The type <code>U</code> occurs exactly once in <code>T…​</code>. Otherwise, the
|
|
program is ill-formed.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Effects: </dt>
|
|
<dd>
|
|
<p>Equivalent to: <code>return get_if<I>(v);</code> with <code>I</code> being
|
|
the zero-based index of <code>U</code> in <code>T…​</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
<div class="sect3">
|
|
<h4 id="ref_relational_operators">Relational Operators</h4>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class... T>
|
|
constexpr bool operator==(const variant<T...>& v, const variant<T...>& w);</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Returns: </dt>
|
|
<dd>
|
|
<p><code>v.index() == w.index() && get<I>(v) == get<I>(w)</code>, where <code>I</code>
|
|
is <code>v.index()</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class... T>
|
|
constexpr bool operator!=(const variant<T...>& v, const variant<T...>& w);</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Returns: </dt>
|
|
<dd>
|
|
<p><code>!(v == w)</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class... T>
|
|
constexpr bool operator<(const variant<T...>& v, const variant<T...>& w);</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Returns: </dt>
|
|
<dd>
|
|
<p><code>v.index() < w.index() || (v.index() == w.index() && get<I>(v) < get<I>(w))</code>,
|
|
where <code>I</code> is <code>v.index()</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class... T>
|
|
constexpr bool operator>(const variant<T...>& v, const variant<T...>& w);</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Returns: </dt>
|
|
<dd>
|
|
<p><code>w < v</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class... T>
|
|
constexpr bool operator<=(const variant<T...>& v, const variant<T...>& w);</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Returns: </dt>
|
|
<dd>
|
|
<p><code>v.index() < w.index() || (v.index() == w.index() && get<I>(v) <= get<I>(w))</code>,
|
|
where <code>I</code> is <code>v.index()</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class... T>
|
|
constexpr bool operator>=(const variant<T...>& v, const variant<T...>& w);</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Returns: </dt>
|
|
<dd>
|
|
<p><code>w <= v</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
<div class="sect3">
|
|
<h4 id="ref_visit">visit</h4>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class R = /*unspecified*/, class F, class... V>
|
|
constexpr /*see below*/ visit(F&& f, V&&... v);</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Returns: </dt>
|
|
<dd>
|
|
<p><code>std::forward<F>(f)(get<I>(std::forward<V>(v))…​)</code>, where
|
|
<code>I…​</code> is <code>v.index()…​</code>.</p>
|
|
</dd>
|
|
<dt class="hdlist1">Remarks: </dt>
|
|
<dd>
|
|
<p>If <code>R</code> is given explicitly, as in <code>visit<int></code>, the return
|
|
type is <code>R</code>. Otherwise, it’s deduced from <code>F</code>. All possible applications
|
|
of <code>F</code> to the variant alternatives must have the same return type for
|
|
this deduction to succeed.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
<div class="sect3">
|
|
<h4 id="ref_swap_2">swap</h4>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>template<class... T>
|
|
void swap(variant<T...>& v, variant<T...>& w) noexcept( /*see below*/ );</code></pre>
|
|
</div>
|
|
</div>
|
|
<div class="ulist none">
|
|
<ul class="none">
|
|
<li>
|
|
<p></p>
|
|
<div class="dlist">
|
|
<dl>
|
|
<dt class="hdlist1">Effects: </dt>
|
|
<dd>
|
|
<p>Equivalent to <code>v.swap(w)</code>.</p>
|
|
</dd>
|
|
</dl>
|
|
</div>
|
|
</li>
|
|
</ul>
|
|
</div>
|
|
</div>
|
|
<div class="sect3">
|
|
<h4 id="ref_bad_variant_access">bad_variant_access</h4>
|
|
<div class="listingblock">
|
|
<div class="content">
|
|
<pre class="highlight"><code>class bad_variant_access: public std::exception
|
|
{
|
|
public:
|
|
|
|
bad_variant_access() noexcept = default;
|
|
|
|
char const * what() const noexcept
|
|
{
|
|
return "bad_variant_access";
|
|
}
|
|
};</code></pre>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div class="sect1">
|
|
<h2 id="copyright">Copyright and License</h2>
|
|
<div class="sectionbody">
|
|
<div class="paragraph">
|
|
<p>This documentation is copyright 2018, 2019 Peter Dimov and is distributed under
|
|
the <a href="http://www.boost.org/LICENSE_1_0.txt">Boost Software License, Version 1.0</a>.</p>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div id="footer">
|
|
<div id="footer-text">
|
|
Last updated 2021-04-13 16:23:13 UTC
|
|
</div>
|
|
</div>
|
|
<style>
|
|
|
|
*:not(pre)>code { background: none; color: #600000; }
|
|
:not(pre):not([class^=L])>code { background: none; color: #600000; }
|
|
|
|
</style>
|
|
</body>
|
|
</html> |