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| <div class="titlepage"><div><div><h3 class="title"> |
| <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> |
| </h3></div></div></div> |
| <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> |
| <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> |
| </pre> |
| <p> |
| <span class="bold"><strong>Inherits:</strong></span> If (i) <code class="computeroutput"><span class="identifier">lhs</span></code> |
| 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> |
| can be used in expression <code class="computeroutput"><span class="identifier">lhs</span><span class="special">+=</span><span class="identifier">rhs</span></code>, |
| 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 |
| <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> |
| then inherits from <a class="link" href="integral_constant.html" title="integral_constant">true_type</a>, |
| otherwise inherits from <a class="link" href="integral_constant.html" title="integral_constant">false_type</a>. |
| </p> |
| <p> |
| The default behaviour (<code class="computeroutput"><span class="identifier">Ret</span><span class="special">=</span><span class="identifier">dont_care</span></code>) |
| 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> |
| is different from the default <code class="computeroutput"><span class="identifier">dont_care</span></code> |
| type, the return value is checked to be convertible to <code class="computeroutput"><span class="identifier">Ret</span></code>. |
| Convertible to <code class="computeroutput"><span class="identifier">Ret</span></code> means |
| that the return value of the operator can be used as argument to a function |
| expecting <code class="computeroutput"><span class="identifier">Ret</span></code>: |
| </p> |
| <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> |
| <span class="identifier">Lhs</span> <span class="identifier">lhs</span><span class="special">;</span> |
| <span class="identifier">Rhs</span> <span class="identifier">rhs</span><span class="special">;</span> |
| <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> |
| </pre> |
| <p> |
| 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 |
| <code class="computeroutput"><span class="keyword">void</span></code>. |
| </p> |
| <p> |
| <span class="bold"><strong>Header:</strong></span> <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_plus_assign</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code> |
| 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> |
| 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> |
| </p> |
| <p> |
| <span class="bold"><strong>Examples:</strong></span> |
| </p> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <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>. |
| </p></blockquote></div> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <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> |
| integral constant expression. |
| </p></blockquote></div> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <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> |
| integral constant expression that evaluates to <code class="computeroutput"><span class="keyword">true</span></code>. |
| </p></blockquote></div> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <code class="computeroutput"><span class="identifier">has_plus_assign</span><span class="special"><</span><span class="keyword">long</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>. |
| </p></blockquote></div> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <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> |
| inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>. |
| </p></blockquote></div> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <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> |
| inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>. |
| </p></blockquote></div> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <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>. |
| </p></blockquote></div> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <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> |
| inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>. |
| </p></blockquote></div> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <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>. |
| </p></blockquote></div> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <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>. |
| </p></blockquote></div> |
| <p> |
| <span class="bold"><strong>See also:</strong></span> <a class="link" href="../category/value_traits/operators.html" title="Operator Type Traits">Operator |
| Type Traits</a> |
| </p> |
| <p> |
| <span class="bold"><strong>Limitation:</strong></span> |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"> |
| Requires a compiler with working SFINAE. |
| </li></ul></div> |
| <p> |
| <span class="bold"><strong>Known issues:</strong></span> |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> |
| <li class="listitem"> |
| This trait cannot detect whether binary <code class="computeroutput"><span class="keyword">operator</span><span class="special">+=</span></code> is public or not: if <code class="computeroutput"><span class="keyword">operator</span><span class="special">+=</span></code> |
| is defined as a private member of <code class="computeroutput"><span class="identifier">Lhs</span></code> |
| then instantiating <code class="computeroutput"><span class="identifier">has_plus_assign</span><span class="special"><</span><span class="identifier">Lhs</span><span class="special">></span></code> will produce a compiler error. For |
| this reason <code class="computeroutput"><span class="identifier">has_plus_assign</span></code> |
| cannot be used to determine whether a type has a public <code class="computeroutput"><span class="keyword">operator</span><span class="special">+=</span></code> |
| or not. |
| <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> <span class="keyword">void</span> <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> |
| <span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_plus_assign</span><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> |
| </pre> |
| </li> |
| <li class="listitem"> |
| 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> |
| is convertible to <code class="computeroutput"><span class="identifier">A</span></code>. |
| In this case, the compiler will report an ambiguous overload. |
| <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">{</span> <span class="special">};</span> |
| <span class="keyword">void</span> <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="keyword">const</span> <span class="identifier">A</span><span class="special">&);</span> |
| <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> |
| <span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_plus_assign</span><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> |
| <span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_plus_assign</span><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> |
| </pre> |
| </li> |
| <li class="listitem"> |
| There is an issue when applying this trait to template classes. If <code class="computeroutput"><span class="keyword">operator</span><span class="special">+=</span></code> |
| is defined but does not bind for a given template type, it is still detected |
| by the trait which returns <code class="computeroutput"><span class="keyword">true</span></code> |
| instead of <code class="computeroutput"><span class="keyword">false</span></code>. Example: |
| <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><span class="identifier">has_plus_assign</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">struct</span> <span class="identifier">contains</span> <span class="special">{</span> <span class="identifier">T</span> <span class="identifier">data</span><span class="special">;</span> <span class="special">};</span> |
| |
| <span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> |
| <span class="keyword">bool</span> <span class="keyword">operator</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">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> |
| <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> |
| <span class="special">}</span> |
| |
| <span class="keyword">class</span> <span class="identifier">bad</span> <span class="special">{</span> <span class="special">};</span> |
| <span class="keyword">class</span> <span class="identifier">good</span> <span class="special">{</span> <span class="special">};</span> |
| <span class="keyword">bool</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="special">}</span> |
| |
| <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> <span class="special">{</span> |
| <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> |
| <span class="comment">// works fine for contains<good></span> |
| <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><span class="identifier">has_plus_assign</span><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> |
| <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> |
| <span class="identifier">g</span><span class="special">+=</span><span class="identifier">g</span><span class="special">;</span> <span class="comment">// ok</span> |
| <span class="comment">// does not work for contains<bad></span> |
| <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><span class="identifier">has_plus_assign</span><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> |
| <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> |
| <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> |
| <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> |
| <span class="special">}</span> |
| </pre> |
| </li> |
| <li class="listitem"> |
| <code class="computeroutput"><span class="keyword">volatile</span></code> qualifier is not |
| properly handled and would lead to undefined behavior |
| </li> |
| </ul></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">Copyright © 2000, 2011 Adobe Systems Inc, David Abrahams, |
| Frederic Bron, Steve Cleary, Beman Dawes, Aleksey Gurtovoy, Howard Hinnant, |
| Jesse Jones, Mat Marcus, Itay Maman, John Maddock, Alexander Nasonov, Thorsten |
| Ottosen, Roman Perepelitsa, Robert Ramey, Jeremy Siek, Robert Stewart and Steven |
| Watanabe<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></td> |
| </tr></table> |
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