generic-library/boost
1
0
Fork 0
Code Issues Pull Requests Releases Wiki Activity
main
boost/doc/html/typeof.html
Edouard DUPIN d0115b733d [DEV] add v1.76.0
2021-10-05 21:37:46 +02:00

188 lines
15 KiB
HTML
Raw Permalink Blame History

This file contains invisible Unicode characters

This file contains invisible Unicode characters that are indistinguishable to humans but may be processed differently by a computer. If you think that this is intentional, you can safely ignore this warning. Use the Escape button to reveal them.

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Chapter 43. Boost.Typeof</title>
<link rel="stylesheet" href="../../doc/src/boostbook.css" type="text/css">
<meta name="generator" content="DocBook XSL Stylesheets V1.79.1">
<link rel="home" href="index.html" title="The Boost C++ Libraries BoostBook Documentation Subset">
<link rel="up" href="libraries.html" title="Part I. The Boost C++ Libraries (BoostBook Subset)">
<link rel="prev" href="boost_typeindex/acknowledgements.html" title="Acknowledgements">
<link rel="next" href="typeof/tuto.html" title="Tutorial">
</head>
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
<table cellpadding="2" width="100%"><tr>
<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../boost.png"></td>
<td align="center"><a href="../../index.html">Home</a></td>
<td align="center"><a href="../../libs/libraries.htm">Libraries</a></td>
<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
<td align="center"><a href="../../more/index.htm">More</a></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="boost_typeindex/acknowledgements.html"><img src="../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="libraries.html"><img src="../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="index.html"><img src="../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="typeof/tuto.html"><img src="../../doc/src/images/next.png" alt="Next"></a>
</div>
<div class="chapter">
<div class="titlepage"><div>
<div><h2 class="title">
<a name="typeof"></a>Chapter 43. Boost.Typeof</h2></div>
<div><div class="author"><h3 class="author">
<span class="firstname">Arkadiy</span> <span class="surname">Vertleyb</span>
</h3></div></div>
<div><div class="author"><h3 class="author">
<span class="firstname">Peder</span> <span class="surname">Holt</span>
</h3></div></div>
<div><p class="copyright">Copyright © 2004, 2005 Arkadiy Vertleyb, Peder Holt</p></div>
<div><div class="legalnotice">
<a name="typeof.legal"></a><p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">
http://www.boost.org/LICENSE_1_0.txt </a>)
</p>
</div></div>
</div></div>
<div class="toc">
<p><b>Table of Contents</b></p>
<dl class="toc">
<dt><span class="section"><a href="typeof.html#typeof.moti">Motivation</a></span></dt>
<dt><span class="section"><a href="typeof/tuto.html">Tutorial</a></span></dt>
<dt><span class="section"><a href="typeof/refe.html">Reference</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="typeof/refe.html#typeof.auto">AUTO, AUTO_TPL</a></span></dt>
<dt><span class="section"><a href="typeof/refe.html#typeof.compl">COMPLIANT</a></span></dt>
<dt><span class="section"><a href="typeof/refe.html#typeof.incr">INCREMENT_REGISTRATION_GROUP</a></span></dt>
<dt><span class="section"><a href="typeof/refe.html#typeof.inte">INTEGRAL</a></span></dt>
<dt><span class="section"><a href="typeof/refe.html#typeof.limit_func">LIMIT_FUNCTION_ARITY</a></span></dt>
<dt><span class="section"><a href="typeof/refe.html#typeof.messages">MESSAGES</a></span></dt>
<dt><span class="section"><a href="typeof/refe.html#typeof.limit_size">LIMIT_SIZE</a></span></dt>
<dt><span class="section"><a href="typeof/refe.html#typeof.regtype">REGISTER_TYPE</a></span></dt>
<dt><span class="section"><a href="typeof/refe.html#typeof.regtemp">REGISTER_TEMPLATE</a></span></dt>
<dt><span class="section"><a href="typeof/refe.html#typeof.temp">TEMPLATE</a></span></dt>
<dt><span class="section"><a href="typeof/refe.html#typeof.typo">TYPEOF, TYPEOF_TPL</a></span></dt>
<dt><span class="section"><a href="typeof/refe.html#typeof.typn">TYPEOF_NESTED_TYPEDEF, TYPEOF_NESTED_TYPEDEF_TPL</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="typeof/other.html">Other considerations and tips</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="typeof/other.html#typeof.natem">Native typeof support and emulation</a></span></dt>
<dt><span class="section"><a href="typeof/other.html#typeof.parties">The three participating parties</a></span></dt>
<dt><span class="section"><a href="typeof/other.html#typeof.features">Supported features</a></span></dt>
<dt><span class="section"><a href="typeof/other.html#typeof.what">What needs to be registered?</a></span></dt>
<dt><span class="section"><a href="typeof/other.html#typeof.limi">Limitations</a></span></dt>
</dl></dd>
<dt><span class="section"><a href="typeof/cont.html">Contributed By:</a></span></dt>
<dt><span class="section"><a href="typeof/ackn.html">Acknowledgements</a></span></dt>
</dl>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="typeof.moti"></a>Motivation</h2></div></div></div>
<p>
Today many template libraries supply object generators to simplify object creation
by utilizing the C++ template argument deduction facility. Consider <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span></code>.
In order to instantiate this class template and create a temporary object of
this instantiation, one has to supply template parameters, as well as parameters
to the constructor:
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">&gt;(</span><span class="number">5</span><span class="special">,</span> <span class="number">3.14159</span><span class="special">);</span>
</pre>
<p>
To avoid this duplication, STL supplies the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">make_pair</span></code>
object generator. When it is used, the types of template parameters are deduced
from supplied function arguments:
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">make_pair</span><span class="special">(</span><span class="number">5</span><span class="special">,</span> <span class="number">3.14159</span><span class="special">);</span>
</pre>
<p>
For the temporary objects it is enough. However, when a named object needs
to be allocated, the problem appears again:
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">&gt;</span> <span class="identifier">p</span><span class="special">(</span><span class="number">5</span><span class="special">,</span> <span class="number">3.14159</span><span class="special">);</span>
</pre>
<p>
The object generator no longer helps:
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">&gt;</span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">make_pair</span><span class="special">(</span><span class="number">5</span><span class="special">,</span> <span class="number">3.14159</span><span class="special">);</span>
</pre>
<p>
It would be nice to deduce the type of the object (on the left) from the expression
it is initialized with (on the right), but the current C++ syntax does not
allow for this.
</p>
<p>
The above example demonstrates the essence of the problem but does not demonstrate
its scale. Many libraries, especially expression template libraries, create
objects of really complex types, and go a long way to hide this complexity
behind object generators. Consider a nit Boost.Lambda functor:
</p>
<pre class="programlisting"><span class="identifier">_1</span> <span class="special">&gt;</span> <span class="number">15</span> <span class="special">&amp;&amp;</span> <span class="identifier">_2</span> <span class="special">&lt;</span> <span class="number">20</span>
</pre>
<p>
If one wanted to allocate a named copy of such an innocently looking functor,
she would have to specify something like this:
</p>
<pre class="programlisting"><span class="identifier">lambda_functor</span><span class="special">&lt;</span>
<span class="identifier">lambda_functor_base</span><span class="special">&lt;</span>
<span class="identifier">logical_action</span><span class="special">&lt;</span><span class="identifier">and_action</span><span class="special">&gt;,</span>
<span class="identifier">tuple</span><span class="special">&lt;</span>
<span class="identifier">lambda_functor</span><span class="special">&lt;</span>
<span class="identifier">lambda_functor_base</span><span class="special">&lt;</span>
<span class="identifier">relational_action</span><span class="special">&lt;</span><span class="identifier">greater_action</span><span class="special">&gt;,</span>
<span class="identifier">tuple</span><span class="special">&lt;</span>
<span class="identifier">lambda_functor</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">1</span><span class="special">&gt;</span> <span class="special">&gt;,</span>
<span class="keyword">int</span> <span class="keyword">const</span>
<span class="special">&gt;</span>
<span class="special">&gt;</span>
<span class="special">&gt;,</span>
<span class="identifier">lambda_functor</span><span class="special">&lt;</span>
<span class="identifier">lambda_functor_base</span><span class="special">&lt;</span>
<span class="identifier">relational_action</span><span class="special">&lt;</span><span class="identifier">less_action</span><span class="special">&gt;,</span>
<span class="identifier">tuple</span><span class="special">&lt;</span>
<span class="identifier">lambda_functor</span><span class="special">&lt;</span><span class="identifier">placeholder</span><span class="special">&lt;</span><span class="number">2</span><span class="special">&gt;</span> <span class="special">&gt;,</span>
<span class="keyword">int</span> <span class="keyword">const</span>
<span class="special">&gt;</span>
<span class="special">&gt;</span>
<span class="special">&gt;</span>
<span class="special">&gt;</span>
<span class="special">&gt;</span>
<span class="special">&gt;</span>
<span class="identifier">f</span> <span class="special">=</span> <span class="identifier">_1</span> <span class="special">&gt;</span> <span class="number">15</span> <span class="special">&amp;&amp;</span> <span class="identifier">_2</span> <span class="special">&lt;</span> <span class="number">20</span><span class="special">;</span>
</pre>
<p>
Not exactly elegant. To solve this problem (as well as some other problems),
the C++ standard committee is considering a few additions to the standard language,
such as <code class="computeroutput"><span class="identifier">typeof</span><span class="special">/</span><span class="keyword">decltype</span></code> and <code class="computeroutput"><span class="keyword">auto</span></code>
(see <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1607.pdf" target="_top">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1607.pdf</a>).
</p>
<p>
The <code class="computeroutput"><span class="identifier">typeof</span></code> operator (or <code class="computeroutput"><span class="keyword">decltype</span></code>, which is a slightly different flavor
of <code class="computeroutput"><span class="identifier">typeof</span></code>) allows one to determine
the type of an expression at compile time. Using <code class="computeroutput"><span class="identifier">typeof</span></code>,
the above example can be simplified drastically:
</p>
<pre class="programlisting"><span class="identifier">typeof</span><span class="special">(</span><span class="identifier">_1</span> <span class="special">&gt;</span> <span class="number">15</span> <span class="special">&amp;&amp;</span> <span class="identifier">_2</span> <span class="special">&lt;</span> <span class="number">20</span><span class="special">)</span> <span class="identifier">f</span> <span class="special">=</span> <span class="identifier">_1</span> <span class="special">&gt;</span> <span class="number">15</span> <span class="special">&amp;&amp;</span> <span class="identifier">_2</span> <span class="special">&lt;</span> <span class="number">20</span><span class="special">;</span>
</pre>
<p>
Much better, but some duplication still exists. The <code class="computeroutput"><span class="keyword">auto</span></code>
type solves the rest of the problem:
</p>
<pre class="programlisting"><span class="keyword">auto</span> <span class="identifier">f</span> <span class="special">=</span> <span class="identifier">_1</span> <span class="special">&gt;</span> <span class="number">15</span> <span class="special">&amp;&amp;</span> <span class="identifier">_2</span> <span class="special">&lt;</span> <span class="number">20</span><span class="special">;</span>
</pre>
<p>
The purpose of the Boost.Typeof library is to provide a library-based solution,
which could be used until the language-based facility is added to the Standard
and becomes widely available.
</p>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer"></div></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="boost_typeindex/acknowledgements.html"><img src="../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="libraries.html"><img src="../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="index.html"><img src="../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="typeof/tuto.html"><img src="../../doc/src/images/next.png" alt="Next"></a>
</div>
</body>
</html>
Reference in New Issue View Git Blame Copy Permalink