1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>has_plus_assign</title> 5<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../index.html" title="Chapter 1. Boost.TypeTraits"> 8<link rel="up" href="../reference.html" title="Alphabetical Reference"> 9<link rel="prev" href="has_plus.html" title="has_plus"> 10<link rel="next" href="has_post_decrement.html" title="has_post_decrement"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="has_plus.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../reference.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="has_post_decrement.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h3 class="title"> 27<a name="boost_typetraits.reference.has_plus_assign"></a><a class="link" href="has_plus_assign.html" title="has_plus_assign">has_plus_assign</a> 28</h3></div></div></div> 29<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">Lhs</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Rhs</span><span class="special">=</span><span class="identifier">Lhs</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Ret</span><span class="special">=</span><span class="identifier">dont_care</span><span class="special">></span> 30<span class="keyword">struct</span> <span class="identifier">has_plus_assign</span> <span class="special">:</span> <span class="keyword">public</span> <em class="replaceable"><code><a class="link" href="integral_constant.html" title="integral_constant">true_type</a>-or-<a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code></em> <span class="special">{};</span> 31</pre> 32<p> 33 <span class="bold"><strong>Inherits:</strong></span> If (i) <code class="computeroutput"><span class="identifier">lhs</span></code> 34 of type <code class="computeroutput"><span class="identifier">Lhs</span></code> and <code class="computeroutput"><span class="identifier">rhs</span></code> of type <code class="computeroutput"><span class="identifier">Rhs</span></code> 35 can be used in expression <code class="computeroutput"><span class="identifier">lhs</span><span class="special">+=</span><span class="identifier">rhs</span></code>, 36 and (ii) <code class="computeroutput"><span class="identifier">Ret</span><span class="special">=</span><span class="identifier">dont_care</span></code> or the result of expression 37 <code class="computeroutput"><span class="identifier">lhs</span><span class="special">+=</span><span class="identifier">rhs</span></code> is convertible to <code class="computeroutput"><span class="identifier">Ret</span></code> 38 then inherits from <a class="link" href="integral_constant.html" title="integral_constant">true_type</a>, 39 otherwise inherits from <a class="link" href="integral_constant.html" title="integral_constant">false_type</a>. 40 </p> 41<p> 42 The default behaviour (<code class="computeroutput"><span class="identifier">Ret</span><span class="special">=</span><span class="identifier">dont_care</span></code>) 43 is to not check for the return value of binary <code class="computeroutput"><span class="keyword">operator</span><span class="special">+=</span></code>. If <code class="computeroutput"><span class="identifier">Ret</span></code> 44 is different from the default <code class="computeroutput"><span class="identifier">dont_care</span></code> 45 type, the return value is checked to be convertible to <code class="computeroutput"><span class="identifier">Ret</span></code>. 46 Convertible to <code class="computeroutput"><span class="identifier">Ret</span></code> means 47 that the return value of the operator can be used as argument to a function 48 expecting <code class="computeroutput"><span class="identifier">Ret</span></code>: 49</p> 50<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">Ret</span><span class="special">);</span> 51<span class="identifier">Lhs</span> <span class="identifier">lhs</span><span class="special">;</span> 52<span class="identifier">Rhs</span> <span class="identifier">rhs</span><span class="special">;</span> 53<span class="identifier">f</span><span class="special">(</span><span class="identifier">lhs</span><span class="special">+=</span><span class="identifier">rhs</span><span class="special">);</span> <span class="comment">// is valid if has_plus_assign<Lhs, Rhs, Ret>::value==true</span> 54</pre> 55<p> 56 If <code class="computeroutput"><span class="identifier">Ret</span><span class="special">=</span><span class="keyword">void</span></code>, the return type is checked to be exactly 57 <code class="computeroutput"><span class="keyword">void</span></code>. 58 </p> 59<p> 60 <span class="bold"><strong>Header:</strong></span> <code class="computeroutput"><span class="preprocessor">#include</span> 61 <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">has_plus_assign</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code> 62 or <code class="computeroutput"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">has_operator</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code> 63 or <code class="computeroutput"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code> 64 </p> 65<p> 66 <span class="bold"><strong>Compiler Compatibility:</strong></span> Requires working 67 SFINAE (i.e. BOOST_NO_SFINAE is not set). Only a minority of rather old compilers 68 do not support this. 69 </p> 70<p> 71 <span class="bold"><strong>Examples:</strong></span> 72 </p> 73<div class="blockquote"><blockquote class="blockquote"><p> 74 <code class="computeroutput"><span class="identifier">has_plus_assign</span><span class="special"><</span><span class="identifier">Lhs</span><span class="special">,</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="identifier">Ret</span><span class="special">>::</span><span class="identifier">value_type</span></code> is the type <code class="computeroutput"><span class="keyword">bool</span></code>. 75 </p></blockquote></div> 76<div class="blockquote"><blockquote class="blockquote"><p> 77 <code class="computeroutput"><span class="identifier">has_plus_assign</span><span class="special"><</span><span class="identifier">Lhs</span><span class="special">,</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="identifier">Ret</span><span class="special">>::</span><span class="identifier">value</span></code> is a <code class="computeroutput"><span class="keyword">bool</span></code> 78 integral constant expression. 79 </p></blockquote></div> 80<div class="blockquote"><blockquote class="blockquote"><p> 81 <code class="computeroutput"><span class="identifier">has_plus_assign</span><span class="special"><</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">value</span></code> is a <code class="computeroutput"><span class="keyword">bool</span></code> 82 integral constant expression that evaluates to <code class="computeroutput"><span class="keyword">true</span></code>. 83 </p></blockquote></div> 84<div class="blockquote"><blockquote class="blockquote"><p> 85 <code class="computeroutput"><span class="identifier">has_plus_assign</span><span class="special"><</span><span class="keyword">long</span><span class="special">></span></code> 86 inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>. 87 </p></blockquote></div> 88<div class="blockquote"><blockquote class="blockquote"><p> 89 <code class="computeroutput"><span class="identifier">has_plus_assign</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">></span></code> 90 inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>. 91 </p></blockquote></div> 92<div class="blockquote"><blockquote class="blockquote"><p> 93 <code class="computeroutput"><span class="identifier">has_plus_assign</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">,</span> <span class="keyword">long</span><span class="special">></span></code> 94 inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>. 95 </p></blockquote></div> 96<div class="blockquote"><blockquote class="blockquote"><p> 97 <code class="computeroutput"><span class="identifier">has_plus_assign</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">,</span> <span class="keyword">double</span><span class="special">></span></code> inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>. 98 </p></blockquote></div> 99<div class="blockquote"><blockquote class="blockquote"><p> 100 <code class="computeroutput"><span class="identifier">has_plus_assign</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">,</span> <span class="keyword">int</span><span class="special">></span></code> 101 inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>. 102 </p></blockquote></div> 103<div class="blockquote"><blockquote class="blockquote"><p> 104 <code class="computeroutput"><span class="identifier">has_plus_assign</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">>::</span><span class="identifier">value</span></code> inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code>. 105 </p></blockquote></div> 106<div class="blockquote"><blockquote class="blockquote"><p> 107 <code class="computeroutput"><span class="identifier">has_plus_assign</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">></span></code> inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code>. 108 </p></blockquote></div> 109<p> 110 <span class="bold"><strong>See also:</strong></span> <a class="link" href="../category/value_traits/operators.html" title="Operator Type Traits">Operator 111 Type Traits</a> 112 </p> 113<p> 114 <span class="bold"><strong>Known issues:</strong></span> 115 </p> 116<p> 117 For modern compilers (those that support arbitrary SFINAE-expressions and 118 decltype/declval) this trait offers near perfect detection. In this situation 119 the macro <code class="computeroutput"><span class="identifier">BOOST_TT_HAS_ACCURATE_BINARY_OPERATOR_DETECTION</span></code> 120 will be defined after including <code class="literal"><boost/type_traits/has_plus_assign.hpp></code>. 121 Please note however, that detection is based on function signature only, 122 in the case that the operator is a function template then has_plus_assign 123 cannot perform introspection of the template function body to ensure that 124 the type meets all of the conceptual requirements of the actual code. 125 </p> 126<p> 127 Example: 128 </p> 129<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span>has_plus_assign<span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 130<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> 131 132<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> 133<span class="keyword">struct</span> <span class="identifier">contains</span> 134<span class="special">{</span> 135 <span class="identifier">T</span> <span class="identifier">data</span><span class="special">;</span> 136 <span class="identifier">contains</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">d</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">data</span><span class="special">(</span><span class="identifier">d</span><span class="special">)</span> <span class="special">{}</span> 137<span class="special">};</span> 138 139<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> 140contains<T>& <span class="keyword">operator</span> += <span class="special">(</span>const <span class="identifier">contains</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">&</span><span class="identifier">lhs</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">contains</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">&</span><span class="identifier">rhs</span><span class="special">)</span> <span class="special">{</span> 141 <span class="keyword">return</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">lhs</span><span class="special">.</span><span class="identifier">data</span><span class="special">,</span> <span class="identifier">rhs</span><span class="special">.</span><span class="identifier">data</span><span class="special">);</span> 142<span class="special">}</span> 143 144<span class="keyword">class</span> <span class="identifier">bad</span> <span class="special">{</span> <span class="special">};</span> 145<span class="keyword">class</span> <span class="identifier">good</span> <span class="special">{</span> <span class="special">};</span> 146<span class="identifier">good</span> <span class="identifier">f</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">good</span><span class="special">&,</span> <span class="keyword">const</span> <span class="identifier">good</span><span class="special">&)</span> <span class="special">{</span> <span class="keyword">return</span> <span class="comment">/*something*/</span><span class="special">;</span> <span class="special">}</span> 147 148<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> <span class="special">{</span> 149 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special"><<</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">boolalpha</span><span class="special">;</span> 150 <span class="comment">// works fine for contains<good></span> 151 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special"><<</span><span class="identifier">boost</span><span class="special">::</span>has_plus_assign<span class="special"><</span> <span class="identifier">contains</span><span class="special"><</span> <span class="identifier">good</span> <span class="special">></span> <span class="special">>::</span><span class="identifier">value</span><span class="special"><<</span><span class="char">'\n'</span><span class="special">;</span> <span class="comment">// true</span> 152 <span class="identifier">contains</span><span class="special"><</span><span class="identifier">good</span><span class="special">></span> <span class="identifier">g</span><span class="special">;</span> 153 <span class="identifier">g</span><span class="special">&</span><span class="identifier">g</span><span class="special">;</span> <span class="comment">// ok</span> 154 <span class="comment">// does not work for contains<bad></span> 155 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special"><<</span><span class="identifier">boost</span><span class="special">::</span>has_plus_assign<span class="special"><</span> <span class="identifier">contains</span><span class="special"><</span> <span class="identifier">bad</span> <span class="special">></span> <span class="special">>::</span><span class="identifier">value</span><span class="special"><<</span><span class="char">'\n'</span><span class="special">;</span> <span class="comment">// true, should be false</span> 156 <span class="identifier">contains</span><span class="special"><</span><span class="identifier">bad</span><span class="special">></span> <span class="identifier">b</span><span class="special">;</span> 157 <span class="identifier">b</span><span class="special">&</span><span class="identifier">b</span><span class="special">;</span> <span class="comment">// compile time error</span> 158 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 159<span class="special">}</span> 160</pre> 161<p> 162 For older compilers (<code class="computeroutput"><span class="identifier">BOOST_TT_HAS_ACCURATE_BINARY_OPERATOR_DETECTION</span></code> 163 not defined) then there are a number of issues: 164 </p> 165<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"> 166 This trait cannot detect whether binary <code class="computeroutput"><span class="keyword">operator</span></code>+= 167 is public or not: if <code class="computeroutput"><span class="keyword">operator</span></code>+= 168 is defined as a private member of <code class="computeroutput"><span class="identifier">Lhs</span></code> 169 then instantiating <code class="literal">has_plus_assign<Lhs></code> will 170 produce a compiler error. For this reason <code class="literal">has_plus_assign</code> 171 cannot be used to determine whether a type has a public <code class="computeroutput"><span class="keyword">operator</span></code>+= or not. 172 </li></ul></div> 173<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">{</span> <span class="keyword">private</span><span class="special">:</span> A& <span class="keyword">operator</span> += <span class="special">(</span><span class="keyword">const</span> <span class="identifier">A</span><span class="special">&);</span> <span class="special">};</span> 174<span class="identifier">boost</span><span class="special">::</span>has_plus_assign<span class="special"><</span><span class="identifier">A</span><span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// error: A::operator += (const A&) is private</span> 175</pre> 176<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"> 177 There is an issue if the operator exists only for type <code class="computeroutput"><span class="identifier">A</span></code> and <code class="computeroutput"><span class="identifier">B</span></code> 178 is convertible to <code class="computeroutput"><span class="identifier">A</span></code>. 179 In this case, the compiler will report an ambiguous overload. 180 </li></ul></div> 181<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">{</span> <span class="special">};</span> 182A& <span class="keyword">operator</span> += <span class="special">(</span>const <span class="identifier">A</span><span class="special">&,</span> <span class="keyword">const</span> <span class="identifier">A</span><span class="special">&);</span> 183<span class="keyword">struct</span> <span class="identifier">B</span> <span class="special">{</span> <span class="keyword">operator</span> <span class="identifier">A</span><span class="special">();</span> <span class="special">};</span> 184<span class="identifier">boost</span><span class="special">::</span>has_plus_assign<span class="special"><</span><span class="identifier">A</span><span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// this is fine</span> 185<span class="identifier">boost</span><span class="special">::</span>has_plus_assign<span class="special"><</span><span class="identifier">B</span><span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// error: ambiguous overload</span> 186</pre> 187<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"> 188 <code class="computeroutput"><span class="keyword">volatile</span></code> qualifier is not 189 properly handled and would lead to undefined behavior 190 </li></ul></div> 191</div> 192<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 193<td align="left"></td> 194<td align="right"><div class="copyright-footer">Copyright © 2000, 2011 Adobe Systems Inc, David Abrahams, 195 Frederic Bron, Steve Cleary, Beman Dawes, Glen Fernandes, Aleksey Gurtovoy, 196 Howard Hinnant, Jesse Jones, Mat Marcus, Itay Maman, John Maddock, Alexander 197 Nasonov, Thorsten Ottosen, Roman Perepelitsa, Robert Ramey, Jeremy Siek, Robert 198 Stewart and Steven Watanabe<p> 199 Distributed under the Boost Software License, Version 1.0. (See accompanying 200 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>) 201 </p> 202</div></td> 203</tr></table> 204<hr> 205<div class="spirit-nav"> 206<a accesskey="p" href="has_plus.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../reference.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="has_post_decrement.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 207</div> 208</body> 209</html> 210
