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<p>
Boost.YAP is written against the C++14 and C++17 standards. It will use C++17
features if they are present, and otherwise is C++14 code.
</p>
<p>
Boost.YAP also uses Boost.Hana extensively, so only compilers that support
Hana will be usable in building Boost.YAP.
</p>
<p>
Clang versions 3.8 and later (which corresponds to XCode versions 7.3 or later),
and GCC versions 6 and later should work.
</p>
<p>
MSVC++ version 14.15 (Visual Studio 2017 version 15.8) is partially supported.
The tests all pass, but some compilation errors can be found in some of the
examples.
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<td align="right"><div class="copyright-footer">Copyright © 2018 T. Zachary Laine<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>)
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Concepts</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">
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<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>
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<hr>
<div class="spirit-nav">
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</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_yap.concepts"></a><a class="link" href="concepts.html" title="Concepts">Concepts</a>
</h2></div></div></div>
<h4>
<a name="boost_yap.concepts.h0"></a>
<span class="phrase"><a name="boost_yap.concepts.expression"></a></span><a class="link" href="concepts.html#boost_yap.concepts.expression">Expression</a>
</h4>
<p>
<a class="link" href="concepts.html#boost_yap.concepts.expression">Expression</a> is the central
concept in Boost.YAP. It must contain at least an <code class="computeroutput"><a class="link" href="../boost/yap/expr_kind.html" title="Type expr_kind">expr_kind</a></code> and a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">hana</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;&gt;</span></code>
of values.
</p>
<p>
Here is a summary of the requirements on <a class="link" href="concepts.html#boost_yap.concepts.expression">Expression</a>.
In the tables below, <code class="computeroutput"><span class="identifier">E</span></code> is a
type that models <a class="link" href="concepts.html#boost_yap.concepts.expression">Expression</a>;
<code class="computeroutput"><span class="identifier">e</span></code> is an object of type <code class="computeroutput"><span class="identifier">E</span></code>; <code class="computeroutput"><span class="identifier">Tuple</span></code>
is an instantiation of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">hana</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;&gt;</span></code>;
and <code class="computeroutput"><span class="identifier">t</span></code> is an object of type
<code class="computeroutput"><span class="identifier">Tuple</span></code>.
</p>
<div class="table">
<a name="boost_yap.concepts.expression_requirements"></a><p class="title"><b>Table 48.6. Expression Requirements</b></p>
<div class="table-contents"><table class="table" summary="Expression Requirements">
<colgroup>
<col>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Expression
</p>
</th>
<th>
<p>
Type
</p>
</th>
<th>
<p>
Description
</p>
</th>
<th>
<p>
Notes
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">E</span><span class="special">::</span><span class="identifier">kind</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><a class="link" href="../boost/yap/expr_kind.html" title="Type expr_kind">expr_kind</a></code>
</p>
</td>
<td>
<p>
The kind of expression <code class="computeroutput"><span class="identifier">E</span></code>
is.
</p>
</td>
<td>
<p>
Must be a compile-time constant.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">e</span><span class="special">.</span><span class="identifier">elements</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">Tuple</span></code>
</p>
</td>
<td>
<p>
The child expressions of <code class="computeroutput"><span class="identifier">e</span></code>.
</p>
</td>
<td>
<p>
The types of the elements must be appropriate to the kind of the
expression.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">E</span> <span class="identifier">e</span><span class="special">{</span><span class="identifier">t</span><span class="special">}</span></code>
</p>
</td>
<td>
</td>
<td>
<p>
Construction/initialization of <code class="computeroutput"><span class="identifier">e</span></code>.
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">t</span></code> must be stored
in <code class="computeroutput"><span class="identifier">e</span><span class="special">.</span><span class="identifier">elements</span></code>.
</p>
</td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><p>
As stated above, the <code class="computeroutput"><span class="identifier">elements</span></code>
data member must match the kind of the expression:
</p>
<div class="table">
<a name="boost_yap.concepts.expression_requirements0"></a><p class="title"><b>Table 48.7. Expression Requirements</b></p>
<div class="table-contents"><table class="table" summary="Expression Requirements">
<colgroup>
<col>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
<code class="computeroutput"><span class="identifier">E</span><span class="special">::</span><span class="identifier">kind</span></code>
</p>
</th>
<th>
<p>
<code class="computeroutput"><span class="identifier">hana</span><span class="special">::</span><span class="identifier">size</span><span class="special">(</span><span class="identifier">e</span><span class="special">.</span><span class="identifier">elements</span><span class="special">)</span></code>
</p>
</th>
<th>
<p>
Possible Tuple Element Types
</p>
</th>
<th>
<p>
Notes
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">expr_kind</span><span class="special">::</span><span class="identifier">expr_ref</span></code>
</p>
</td>
<td>
<p>
1
</p>
</td>
<td>
<p>
Any non-<a class="link" href="../boost/yap/expr_kind.html#boost.yap.expr_kind.expr_ref"><code class="computeroutput"><span class="identifier">expr_kind</span><span class="special">::</span><span class="identifier">expr_ref</span></code></a> <a class="link" href="concepts.html#boost_yap.concepts.expression">Expression</a>.
</p>
</td>
<td>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">expr_kind</span><span class="special">::</span><span class="identifier">terminal</span></code>
</p>
</td>
<td>
<p>
1
</p>
</td>
<td>
<p>
Any non-<a class="link" href="concepts.html#boost_yap.concepts.expression">Expression</a>.
</p>
</td>
<td>
<p>
A terminal with a <code class="computeroutput"><a class="link" href="../boost/yap/placeholder.html" title="Struct template placeholder">placeholder&lt;&gt;</a></code>
value will be treated as a placeholder.
</p>
</td>
</tr>
<tr>
<td>
<p>
Any unary operator
</p>
</td>
<td>
<p>
1
</p>
</td>
<td>
<p>
Any <a class="link" href="concepts.html#boost_yap.concepts.expression">Expression</a>.
</p>
</td>
<td>
</td>
</tr>
<tr>
<td>
<p>
Any binary operator
</p>
</td>
<td>
<p>
2
</p>
</td>
<td>
<p>
Any <a class="link" href="concepts.html#boost_yap.concepts.expression">Expression</a>.
</p>
</td>
<td>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">expr_kind</span><span class="special">::</span><span class="identifier">if_else</span></code>
</p>
</td>
<td>
<p>
3
</p>
</td>
<td>
<p>
Any <a class="link" href="concepts.html#boost_yap.concepts.expression">Expression</a>.
</p>
</td>
<td>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">expr_kind</span><span class="special">::</span><span class="identifier">call</span></code>
</p>
</td>
<td>
<p>
Any number &gt;= 1.
</p>
</td>
<td>
<p>
Any <a class="link" href="concepts.html#boost_yap.concepts.expression">Expression</a>.
</p>
</td>
<td>
</td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><h4>
<a name="boost_yap.concepts.h1"></a>
<span class="phrase"><a name="boost_yap.concepts.expressiontemplate"></a></span><a class="link" href="concepts.html#boost_yap.concepts.expressiontemplate">ExpressionTemplate</a>
</h4>
<p>
<a class="link" href="concepts.html#boost_yap.concepts.expressiontemplate">ExpressionTemplate</a>
is any template with two parameters that, when instantiated with an <code class="computeroutput"><a class="link" href="../boost/yap/expr_kind.html" title="Type expr_kind">expr_kind</a></code>
and a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">hana</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;&gt;</span></code>, results in an <a class="link" href="concepts.html#boost_yap.concepts.expression">Expression</a>.
</p>
<h4>
<a name="boost_yap.concepts.h2"></a>
<span class="phrase"><a name="boost_yap.concepts.transform"></a></span><a class="link" href="concepts.html#boost_yap.concepts.transform">Transform</a>
</h4>
<p>
<code class="computeroutput"><a class="link" href="../boost/yap/transform.html" title="Function template transform">transform()</a></code> takes a <a class="link" href="concepts.html#boost_yap.concepts.transform">Transform</a>
as its second parameter. A <a class="link" href="concepts.html#boost_yap.concepts.transform">Transform</a>
is a <a href="http://en.cppreference.com/w/cpp/concept/Callable" target="_top">Callable</a>
that takes expressions and returns values of unconstrained type. There are
two sorts of overloads <a class="link" href="concepts.html#boost_yap.concepts.transform">Transform</a>
may use: <a class="link" href="concepts.html#boost_yap.concepts.expressiontransform">ExpressionTransform</a>
and <a class="link" href="concepts.html#boost_yap.concepts.tagtransform">TagTransform</a>.
</p>
<p>
A <a class="link" href="concepts.html#boost_yap.concepts.transform">Transform</a> may have any
number of overloads, including none.
</p>
<h4>
<a name="boost_yap.concepts.h3"></a>
<span class="phrase"><a name="boost_yap.concepts.expressiontransform"></a></span><a class="link" href="concepts.html#boost_yap.concepts.expressiontransform">ExpressionTransform</a>
</h4>
<p>
<a class="link" href="concepts.html#boost_yap.concepts.expressiontransform">ExpressionTransform</a>
takes an <a class="link" href="concepts.html#boost_yap.concepts.expression">Expression</a> as
its only parameter. Here are some examples.
</p>
<p>
This one takes any <a class="link" href="concepts.html#boost_yap.concepts.expression">Expression</a>:
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">xform</span>
<span class="special">{</span>
<span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">auto</span> <span class="keyword">operator</span><span class="special">()</span> <span class="special">(</span><span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// ...</span>
<span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
This one takes any type of <a class="link" href="concepts.html#boost_yap.concepts.expression">Expression</a>
that satisfies the constraints imposed by its template parameters:
</p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr1</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr2</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Expr3</span><span class="special">&gt;</span>
<span class="keyword">decltype</span><span class="special">(</span><span class="keyword">auto</span><span class="special">)</span> <span class="identifier">xform</span> <span class="special">(</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">yap</span><span class="special">::</span><span class="identifier">expression</span><span class="special">&lt;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">yap</span><span class="special">::</span><span class="identifier">expr_kind</span><span class="special">::</span><span class="identifier">plus</span><span class="special">,</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">hana</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">yap</span><span class="special">::</span><span class="identifier">expression</span><span class="special">&lt;</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">yap</span><span class="special">::</span><span class="identifier">expr_kind</span><span class="special">::</span><span class="identifier">multiplies</span><span class="special">,</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">hana</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;</span>
<span class="identifier">Expr1</span><span class="special">,</span>
<span class="identifier">Expr2</span>
<span class="special">&gt;</span>
<span class="special">&gt;,</span>
<span class="identifier">Expr3</span>
<span class="special">&gt;</span>
<span class="special">&gt;</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span>
<span class="special">)</span> <span class="special">{</span>
<span class="comment">// ...</span>
<span class="special">}</span>
</pre>
<p>
This one takes only a specific type:
</p>
<pre class="programlisting"><span class="keyword">decltype</span><span class="special">(</span><span class="keyword">auto</span><span class="special">)</span> <span class="identifier">xform</span> <span class="special">(</span>
<span class="keyword">decltype</span><span class="special">(</span><span class="identifier">term</span><span class="special">&lt;</span><span class="identifier">number</span><span class="special">&gt;{{</span><span class="number">0.0</span><span class="special">}}</span> <span class="special">*</span> <span class="identifier">number</span><span class="special">{}</span> <span class="special">+</span> <span class="identifier">number</span><span class="special">{})</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span>
<span class="special">)</span> <span class="special">{</span>
<span class="comment">// ...</span>
<span class="special">}</span>
</pre>
<h4>
<a name="boost_yap.concepts.h4"></a>
<span class="phrase"><a name="boost_yap.concepts.tagtransform"></a></span><a class="link" href="concepts.html#boost_yap.concepts.tagtransform">TagTransform</a>
</h4>
<p>
<a class="link" href="concepts.html#boost_yap.concepts.tagtransform">TagTransform</a> takes a
tag-type as its first parameter, and the individual elements of an expression
as the remaining parameters.
</p>
<p>
Tags are named such that the tag for an expression with <code class="computeroutput"><a class="link" href="../boost/yap/expr_kind.html" title="Type expr_kind">expr_kind</a></code> <code class="computeroutput"><span class="identifier">expr_kind</span><span class="special">::</span><span class="identifier">foo</span></code> is
named <code class="computeroutput"><span class="identifier">foo_tag</span></code>. Here are some
examples.
</p>
<p>
This one takes any terminal that contains a <code class="computeroutput"><span class="identifier">user</span><span class="special">::</span><span class="identifier">number</span></code>
(or reference to such a terminal):
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">xform</span>
<span class="special">{</span>
<span class="keyword">decltype</span><span class="special">(</span><span class="keyword">auto</span><span class="special">)</span> <span class="keyword">operator</span><span class="special">()</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">yap</span><span class="special">::</span><span class="identifier">terminal_tag</span><span class="special">,</span> <span class="identifier">user</span><span class="special">::</span><span class="identifier">number</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">n</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// ...</span>
<span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
This one takes any plus expression that contains a pair of <code class="computeroutput"><span class="identifier">user</span><span class="special">::</span><span class="identifier">number</span></code>
terminals (or references to terminals):
</p>
<pre class="programlisting"><span class="keyword">decltype</span><span class="special">(</span><span class="keyword">auto</span><span class="special">)</span> <span class="identifier">xform</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">yap</span><span class="special">::</span><span class="identifier">plus_tag</span><span class="special">,</span> <span class="identifier">user</span><span class="special">::</span><span class="identifier">number</span> <span class="identifier">lhs</span><span class="special">,</span> <span class="identifier">user</span><span class="special">::</span><span class="identifier">number</span> <span class="identifier">rhs</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// ...</span>
<span class="special">}</span>
</pre>
<p>
This one takes any negation expression:
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">xform</span>
<span class="special">{</span>
<span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">Expr</span><span class="special">&gt;</span>
<span class="keyword">decltype</span><span class="special">(</span><span class="keyword">auto</span><span class="special">)</span> <span class="keyword">operator</span><span class="special">()</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">yap</span><span class="special">::</span><span class="identifier">negate_tag</span><span class="special">,</span> <span class="identifier">Expr</span> <span class="keyword">const</span> <span class="special">&amp;</span> <span class="identifier">expr</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// ...</span>
<span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
This one takes any call expression with two terminals (or references to terminals)
containing values convertible to <code class="computeroutput"><span class="keyword">double</span></code>:
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">xform</span>
<span class="special">{</span>
<span class="keyword">decltype</span><span class="special">(</span><span class="keyword">auto</span><span class="special">)</span> <span class="keyword">operator</span><span class="special">()</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">yap</span><span class="special">::</span><span class="identifier">call_tag</span><span class="special">,</span> <span class="identifier">tag_type</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">a</span><span class="special">,</span> <span class="keyword">double</span> <span class="identifier">b</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// ...</span>
<span class="special">}</span>
<span class="special">}</span>
</pre>
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<p>
Boost.YAP depends on Boost.Hana directly. It also has an optional dependency
on Boost.TypeIndex. Boost.TypeIndex is used for type printing, and is only
required if the optional <code class="computeroutput"><span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">yap</span><span class="special">/</span><span class="identifier">print</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span></code> header
is included.
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</h2></div></div></div>
<h4>
<a name="boost_yap.rationale.h0"></a>
<span class="phrase"><a name="boost_yap.rationale.decaying_values_captured_in_yap_expressions"></a></span><a class="link" href="rationale.html#boost_yap.rationale.decaying_values_captured_in_yap_expressions">Decaying
Values Captured in YAP Expressions</a>
</h4>
<p>
The main objective of Boost.YAP is to be an easy-to-use and easy-to-understand
library for using the expression template programming technique.
</p>
<p>
As such, it is very important that the way nodes in a Boost.YAP expression
tree are represented matches the way nodes in C++ builtin expression are represented.
This keeps the mental model for how to identify and manipulate parts of expression
trees consistent across C++ builtin and Boost.YAP trees.
</p>
<p>
Though this creates minor difficulties (for instance, Boost.YAP terminals cannot
contain arrays), the benefit of a consistent programming model is more important.
</p>
<h4>
<a name="boost_yap.rationale.h1"></a>
<span class="phrase"><a name="boost_yap.rationale.reference_expressions"></a></span><a class="link" href="rationale.html#boost_yap.rationale.reference_expressions">Reference
Expressions</a>
</h4>
<p>
Boost.YAP expressions can be used as subexpressions to build larger expressions.
<a class="link" href="../boost/yap/expr_kind.html#boost.yap.expr_kind.expr_ref"><code class="computeroutput"><span class="identifier">expr_kind</span><span class="special">::</span><span class="identifier">expr_ref</span></code></a>
exists because we want to be able to do this without incurring unnecessay copies
or moves. Consider <code class="computeroutput"><span class="identifier">v2</span></code> and
<code class="computeroutput"><span class="identifier">v3</span></code> in this snippet from the
Lazy Vector example. Each is a terminal that owns its value, rather than referring
to it.
</p>
<pre class="programlisting"><span class="identifier">lazy_vector</span> <span class="identifier">v2</span><span class="special">{{</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;(</span><span class="number">4</span><span class="special">,</span> <span class="number">2.0</span><span class="special">)}};</span>
<span class="identifier">lazy_vector</span> <span class="identifier">v3</span><span class="special">{{</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;(</span><span class="number">4</span><span class="special">,</span> <span class="number">3.0</span><span class="special">)}};</span>
</pre>
<p>
Now consider this expression:
</p>
<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">d1</span> <span class="special">=</span> <span class="special">(</span><span class="identifier">v2</span> <span class="special">+</span> <span class="identifier">v3</span><span class="special">)[</span><span class="number">2</span><span class="special">];</span>
</pre>
<p>
Without using reference semantics, how can we capture this expression, even
before evaluating it, without copying or moving the vectors? We cannot. We
must take references to the <code class="computeroutput"><span class="identifier">v2</span></code>
and <code class="computeroutput"><span class="identifier">v3</span></code> subexpressions to avoid
copying or moving.
</p>
<p>
This comes at a cost. Dealing with <a class="link" href="../boost/yap/expr_kind.html#boost.yap.expr_kind.expr_ref"><code class="computeroutput"><span class="identifier">expr_kind</span><span class="special">::</span><span class="identifier">expr_ref</span></code></a> expressions complicates user
code. The alternatives, silently incurring copies/moves or disallowing the
use of subexpressions to build larger expressions, are worse.
</p>
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<td align="right"><div class="copyright-footer">Copyright © 2018 T. Zachary Laine<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>)
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