1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Operator Type Traits</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="../value_traits.html" title="Type Traits that Describe the Properties of a Type"> 9<link rel="prev" href="relate.html" title="Relationships Between Two Types"> 10<link rel="next" href="../transform.html" title="Type Traits that Transform One Type to Another"> 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="relate.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../value_traits.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="../transform.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h4 class="title"> 27<a name="boost_typetraits.category.value_traits.operators"></a><a class="link" href="operators.html" title="Operator Type Traits">Operator 28 Type Traits</a> 29</h4></div></div></div> 30<h6> 31<a name="boost_typetraits.category.value_traits.operators.h0"></a> 32 <span class="phrase"><a name="boost_typetraits.category.value_traits.operators.introduction"></a></span><a class="link" href="operators.html#boost_typetraits.category.value_traits.operators.introduction">Introduction</a> 33 </h6> 34<p> 35 These traits are all <span class="emphasis"><em>value traits</em></span> inheriting from 36 <a class="link" href="../../reference/integral_constant.html" title="integral_constant">integral_constant</a> 37 and providing a simple <code class="computeroutput"><span class="keyword">true</span></code> 38 or <code class="computeroutput"><span class="keyword">false</span></code> boolean <code class="computeroutput"><span class="identifier">value</span></code> which reflects the fact that given 39 types can or cannot be used with given operators. 40 </p> 41<p> 42 For example, <code class="computeroutput"><span class="identifier">has_plus</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="identifier">value</span></code> 43 is a <code class="computeroutput"><span class="keyword">bool</span></code> which value is 44 <code class="computeroutput"><span class="keyword">true</span></code> because it is possible 45 to add a <code class="computeroutput"><span class="keyword">double</span></code> to an <code class="computeroutput"><span class="keyword">int</span></code> like in the following code: 46</p> 47<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span> 48<span class="keyword">double</span> <span class="identifier">d</span><span class="special">;</span> 49<span class="identifier">i</span><span class="special">+</span><span class="identifier">d</span><span class="special">;</span> 50</pre> 51<p> 52 It is also possible to know if the result of the operator can be used as 53 function argument of a given type. For example, <code class="computeroutput"><span class="identifier">has_plus</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">float</span><span class="special">>::</span><span class="identifier">value</span></code> 54 is <code class="computeroutput"><span class="keyword">true</span></code> because it is possible 55 to add a <code class="computeroutput"><span class="keyword">double</span></code> to an <code class="computeroutput"><span class="keyword">int</span></code> and the result (<code class="computeroutput"><span class="keyword">double</span></code>) 56 can be converted to a <code class="computeroutput"><span class="keyword">float</span></code> 57 argument like in the following code: 58</p> 59<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">f</span><span class="special">(</span><span class="keyword">float</span><span class="special">)</span> <span class="special">{</span> <span class="special">};</span> 60<span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span> 61<span class="keyword">double</span> <span class="identifier">d</span><span class="special">;</span> 62<span class="identifier">f</span><span class="special">(</span><span class="identifier">i</span><span class="special">+</span><span class="identifier">d</span><span class="special">);</span> 63</pre> 64<p> 65 </p> 66<h6> 67<a name="boost_typetraits.category.value_traits.operators.h1"></a> 68 <span class="phrase"><a name="boost_typetraits.category.value_traits.operators.example_of_application"></a></span><a class="link" href="operators.html#boost_typetraits.category.value_traits.operators.example_of_application">Example 69 of application</a> 70 </h6> 71<p> 72 These traits can be useful to optimize the code for types supporting given 73 operations. For example a function <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">advance</span></code> 74 that increases an iterator of a given number of steps could be implemented 75 as follows: 76 </p> 77<p> 78</p> 79<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> 80 81<span class="keyword">namespace</span> <span class="identifier">detail</span> <span class="special">{</span> 82<span class="keyword">template</span> <span class="special"><</span> <span class="keyword">class</span> <span class="identifier">Iterator</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Distance</span><span class="special">,</span> <span class="keyword">bool</span> <span class="identifier">has_plus_assign</span> <span class="special">></span> 83<span class="keyword">struct</span> <span class="identifier">advance_impl</span><span class="special">;</span> 84 85<span class="comment">// this is used if += exists (efficient)</span> 86<span class="keyword">template</span> <span class="special"><</span> <span class="keyword">class</span> <span class="identifier">Iterator</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Distance</span> <span class="special">></span> 87<span class="keyword">struct</span> <span class="identifier">advance_impl</span><span class="special"><</span><span class="identifier">Iterator</span><span class="special">,</span> <span class="identifier">Distance</span><span class="special">,</span> <span class="keyword">true</span><span class="special">></span> <span class="special">{</span> 88 <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Iterator</span> <span class="special">&</span><span class="identifier">i</span><span class="special">,</span> <span class="identifier">Distance</span> <span class="identifier">n</span><span class="special">)</span> <span class="special">{</span> 89 <span class="identifier">i</span><span class="special">+=</span><span class="identifier">n</span><span class="special">;</span> 90 <span class="special">}</span> 91<span class="special">};</span> 92 93<span class="comment">// this is use if += does not exists (less efficient but cannot do better)</span> 94<span class="keyword">template</span> <span class="special"><</span> <span class="keyword">class</span> <span class="identifier">Iterator</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Distance</span> <span class="special">></span> 95<span class="keyword">struct</span> <span class="identifier">advance_impl</span><span class="special"><</span><span class="identifier">Iterator</span><span class="special">,</span> <span class="identifier">Distance</span><span class="special">,</span> <span class="keyword">false</span><span class="special">></span> <span class="special">{</span> 96 <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span><span class="identifier">Iterator</span> <span class="special">&</span><span class="identifier">i</span><span class="special">,</span> <span class="identifier">Distance</span> <span class="identifier">n</span><span class="special">)</span> <span class="special">{</span> 97 <span class="keyword">if</span> <span class="special">(</span><span class="identifier">n</span><span class="special">></span><span class="number">0</span><span class="special">)</span> <span class="special">{</span> 98 <span class="keyword">while</span> <span class="special">(</span><span class="identifier">n</span><span class="special">--)</span> <span class="special">++</span><span class="identifier">i</span><span class="special">;</span> 99 <span class="special">}</span> <span class="keyword">else</span> <span class="special">{</span> 100 <span class="keyword">while</span> <span class="special">(</span><span class="identifier">n</span><span class="special">++)</span> <span class="special">--</span><span class="identifier">i</span><span class="special">;</span> 101 <span class="special">}</span> 102 <span class="special">}</span> 103<span class="special">};</span> 104<span class="special">}</span> <span class="comment">// namespace detail</span> 105 106<span class="keyword">template</span> <span class="special"><</span> <span class="keyword">class</span> <span class="identifier">Iterator</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Distance</span> <span class="special">></span> 107<span class="keyword">inline</span> <span class="keyword">void</span> <span class="identifier">advance</span><span class="special">(</span><span class="identifier">Iterator</span> <span class="special">&</span><span class="identifier">i</span><span class="special">,</span> <span class="identifier">Distance</span> <span class="identifier">n</span><span class="special">)</span> <span class="special">{</span> 108 <span class="identifier">detail</span><span class="special">::</span><span class="identifier">advance_impl</span><span class="special"><</span> <span class="identifier">Iterator</span><span class="special">,</span> <span class="identifier">Distance</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">Iterator</span><span class="special">>::</span><span class="identifier">value</span> <span class="special">>()(</span><span class="identifier">i</span><span class="special">,</span> <span class="identifier">n</span><span class="special">);</span> 109<span class="special">}</span> 110</pre> 111<p> 112 </p> 113<p> 114 Then the compiler chooses the most efficient implementation according to 115 the type's ability to perform <code class="computeroutput"><span class="special">+=</span></code> 116 operation: 117 </p> 118<p> 119</p> 120<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> 121 122<span class="keyword">class</span> <span class="identifier">with</span> <span class="special">{</span> 123 <span class="keyword">int</span> <span class="identifier">m_i</span><span class="special">;</span> 124 <span class="keyword">public</span><span class="special">:</span> 125 <span class="identifier">with</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span><span class="special">=</span><span class="number">0</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">m_i</span><span class="special">(</span><span class="identifier">i</span><span class="special">)</span> <span class="special">{</span> <span class="special">}</span> 126 <span class="identifier">with</span> <span class="special">&</span><span class="keyword">operator</span><span class="special">+=(</span><span class="keyword">int</span> <span class="identifier">rhs</span><span class="special">)</span> <span class="special">{</span> <span class="identifier">m_i</span><span class="special">+=</span><span class="identifier">rhs</span><span class="special">;</span> <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span> <span class="special">}</span> 127 <span class="keyword">operator</span> <span class="keyword">int</span> <span class="keyword">const</span> <span class="special">()</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">m_i</span><span class="special">;</span> <span class="special">}</span> 128<span class="special">};</span> 129 130<span class="keyword">class</span> <span class="identifier">without</span> <span class="special">{</span> 131 <span class="keyword">int</span> <span class="identifier">m_i</span><span class="special">;</span> 132 <span class="keyword">public</span><span class="special">:</span> 133 <span class="identifier">without</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span><span class="special">=</span><span class="number">0</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">m_i</span><span class="special">(</span><span class="identifier">i</span><span class="special">)</span> <span class="special">{</span> <span class="special">}</span> 134 <span class="identifier">without</span> <span class="special">&</span><span class="keyword">operator</span><span class="special">++()</span> <span class="special">{</span> <span class="special">++</span><span class="identifier">m_i</span><span class="special">;</span> <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span> <span class="special">}</span> 135 <span class="identifier">without</span> <span class="special">&</span><span class="keyword">operator</span><span class="special">--()</span> <span class="special">{</span> <span class="special">--</span><span class="identifier">m_i</span><span class="special">;</span> <span class="keyword">return</span> <span class="special">*</span><span class="keyword">this</span><span class="special">;</span> <span class="special">}</span> 136 <span class="keyword">operator</span> <span class="keyword">int</span> <span class="keyword">const</span> <span class="special">()</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">m_i</span><span class="special">;</span> <span class="special">}</span> 137<span class="special">};</span> 138 139<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> <span class="special">{</span> 140 <span class="identifier">with</span> <span class="identifier">i</span><span class="special">=</span><span class="number">0</span><span class="special">;</span> 141 <span class="identifier">advance</span><span class="special">(</span><span class="identifier">i</span><span class="special">,</span> <span class="number">10</span><span class="special">);</span> <span class="comment">// uses +=</span> 142 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special"><<</span><span class="string">"with: "</span><span class="special"><<</span><span class="identifier">i</span><span class="special"><<</span><span class="char">'\n'</span><span class="special">;</span> 143 <span class="identifier">without</span> <span class="identifier">j</span><span class="special">=</span><span class="number">0</span><span class="special">;</span> 144 <span class="identifier">advance</span><span class="special">(</span><span class="identifier">j</span><span class="special">,</span> <span class="number">10</span><span class="special">);</span> <span class="comment">// uses ++</span> 145 <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span><span class="special"><<</span><span class="string">"without: "</span><span class="special"><<</span><span class="identifier">j</span><span class="special"><<</span><span class="char">'\n'</span><span class="special">;</span> 146 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 147<span class="special">}</span> 148</pre> 149<p> 150 </p> 151<h6> 152<a name="boost_typetraits.category.value_traits.operators.h2"></a> 153 <span class="phrase"><a name="boost_typetraits.category.value_traits.operators.description"></a></span><a class="link" href="operators.html#boost_typetraits.category.value_traits.operators.description">Description</a> 154 </h6> 155<p> 156 The syntax is the following: 157</p> 158<pre class="programlisting"><span class="keyword">template</span> <code class="computeroutput"><span class="special"><</span> <span class="keyword">class</span> <span class="identifier">Rhs</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></code> <span class="identifier">has_op</span><span class="special">;</span> <span class="comment">// prefix operator</span> 159<span class="keyword">template</span> <code class="computeroutput"><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">Ret</span><span class="special">=</span><span class="identifier">dont_care</span> <span class="special">></span></code> <span class="identifier">has_op</span><span class="special">;</span> <span class="comment">// postfix operator</span> 160<span class="keyword">template</span> <code class="computeroutput"><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></code> <span class="identifier">has_op</span><span class="special">;</span> <span class="comment">// binary operator</span> 161</pre> 162<p> 163 where: 164 </p> 165<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 166<li class="listitem"> 167 op represents the operator name 168 </li> 169<li class="listitem"> 170 <code class="computeroutput"><span class="identifier">Lhs</span></code> is the type used 171 at the left hand side of <code class="computeroutput"><span class="keyword">operator</span> 172 <span class="identifier">op</span></code>, 173 </li> 174<li class="listitem"> 175 <code class="computeroutput"><span class="identifier">Rhs</span></code> is the type used 176 at the right hand side of <code class="computeroutput"><span class="keyword">operator</span> 177 <span class="identifier">op</span></code>, 178 </li> 179<li class="listitem"> 180 <code class="computeroutput"><span class="identifier">Ret</span></code> is the type for 181 which we want to know if the result of <code class="computeroutput"><span class="keyword">operator</span> 182 <span class="identifier">op</span></code> can be converted to. 183 </li> 184</ul></div> 185<p> 186 The default behaviour (<code class="computeroutput"><span class="identifier">Ret</span><span class="special">=</span><span class="identifier">dont_care</span></code>) 187 is to not check for the return value of the operator. 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>, the return value is checked 188 to be convertible to <code class="computeroutput"><span class="identifier">Ret</span></code>. 189 Convertible to <code class="computeroutput"><span class="identifier">Ret</span></code> means 190 that the return value can be used as argument to a function expecting 191 <code class="computeroutput"><span class="identifier">Ret</span></code>: 192</p> 193<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> 194<span class="identifier">Lhs</span> <span class="identifier">lhs</span><span class="special">;</span> 195<span class="identifier">Rhs</span> <span class="identifier">rhs</span><span class="special">;</span> 196<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<Lhs, Rhs, Ret>::value==true</span> 197</pre> 198<p> 199 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 200 <code class="computeroutput"><span class="keyword">void</span></code>. 201 </p> 202<p> 203 The following tables give the list of supported binary, prefix and postfix 204 operators. 205 </p> 206<div class="table"> 207<a name="boost_typetraits.category.value_traits.operators.supported_prefix_operators"></a><p class="title"><b>Table 1.4. Supported prefix operators</b></p> 208<div class="table-contents"><table class="table" summary="Supported prefix operators"> 209<colgroup> 210<col> 211<col> 212</colgroup> 213<thead><tr> 214<th> 215 <p> 216 prefix operator 217 </p> 218 </th> 219<th> 220 <p> 221 trait name 222 </p> 223 </th> 224</tr></thead> 225<tbody> 226<tr> 227<td> 228 <p> 229 <code class="computeroutput"><span class="special">!</span></code> 230 </p> 231 </td> 232<td> 233 <p> 234 <a class="link" href="../../reference/has_logical_not.html" title="has_logical_not"><code class="computeroutput"><span class="identifier">has_logical_not</span></code></a> <code class="computeroutput"><span class="special"><</span> <span class="keyword">class</span> 235 <span class="identifier">Rhs</span><span class="special">,</span> 236 <span class="keyword">class</span> <span class="identifier">Ret</span><span class="special">=</span><span class="identifier">dont_care</span> 237 <span class="special">></span></code> 238 </p> 239 </td> 240</tr> 241<tr> 242<td> 243 <p> 244 <code class="computeroutput"><span class="special">+</span></code> 245 </p> 246 </td> 247<td> 248 <p> 249 <a class="link" href="../../reference/has_unary_plus.html" title="has_unary_plus"><code class="computeroutput"><span class="identifier">has_unary_plus</span></code></a> 250 </p> 251 </td> 252</tr> 253<tr> 254<td> 255 <p> 256 <code class="computeroutput"><span class="special">-</span></code> 257 </p> 258 </td> 259<td> 260 <p> 261 <a class="link" href="../../reference/has_unary_minus.html" title="has_unary_minus"><code class="computeroutput"><span class="identifier">has_unary_minus</span></code></a> and 262 <a class="link" href="../../reference/has_negate.html" title="has_negate"><code class="computeroutput"><span class="identifier">has_negate</span></code></a> 263 </p> 264 </td> 265</tr> 266<tr> 267<td> 268 <p> 269 <code class="computeroutput"><span class="special">~</span></code> 270 </p> 271 </td> 272<td> 273 <p> 274 <a class="link" href="../../reference/has_complement.html" title="has_complement"><code class="computeroutput"><span class="identifier">has_complement</span></code></a> 275 </p> 276 </td> 277</tr> 278<tr> 279<td> 280 <p> 281 <code class="computeroutput"><span class="special">*</span></code> 282 </p> 283 </td> 284<td> 285 <p> 286 <a class="link" href="../../reference/has_dereference.html" title="has_dereference"><code class="computeroutput"><span class="identifier">has_dereference</span></code></a> 287 </p> 288 </td> 289</tr> 290<tr> 291<td> 292 <p> 293 <code class="computeroutput"><span class="special">++</span></code> 294 </p> 295 </td> 296<td> 297 <p> 298 <a class="link" href="../../reference/has_pre_increment.html" title="has_pre_increment"><code class="computeroutput"><span class="identifier">has_pre_increment</span></code></a> 299 </p> 300 </td> 301</tr> 302<tr> 303<td> 304 <p> 305 <code class="computeroutput"><span class="special">--</span></code> 306 </p> 307 </td> 308<td> 309 <p> 310 <a class="link" href="../../reference/has_pre_decrement.html" title="has_pre_decrement"><code class="computeroutput"><span class="identifier">has_pre_decrement</span></code></a> 311 </p> 312 </td> 313</tr> 314</tbody> 315</table></div> 316</div> 317<br class="table-break"><div class="table"> 318<a name="boost_typetraits.category.value_traits.operators.supported_postfix_operators"></a><p class="title"><b>Table 1.5. Supported postfix operators</b></p> 319<div class="table-contents"><table class="table" summary="Supported postfix operators"> 320<colgroup> 321<col> 322<col> 323</colgroup> 324<thead><tr> 325<th> 326 <p> 327 postfix operator 328 </p> 329 </th> 330<th> 331 <p> 332 trait name 333 </p> 334 </th> 335</tr></thead> 336<tbody> 337<tr> 338<td> 339 <p> 340 <code class="computeroutput"><span class="special">++</span></code> 341 </p> 342 </td> 343<td> 344 <p> 345 <a class="link" href="../../reference/has_post_increment.html" title="has_post_increment"><code class="computeroutput"><span class="identifier">has_post_increment</span></code></a> 346 <code class="computeroutput"><span class="special"><</span> <span class="keyword">class</span> 347 <span class="identifier">Lhs</span><span class="special">,</span> 348 <span class="keyword">class</span> <span class="identifier">Ret</span><span class="special">=</span><span class="identifier">dont_care</span> 349 <span class="special">></span></code> 350 </p> 351 </td> 352</tr> 353<tr> 354<td> 355 <p> 356 <code class="computeroutput"><span class="special">--</span></code> 357 </p> 358 </td> 359<td> 360 <p> 361 <a class="link" href="../../reference/has_post_decrement.html" title="has_post_decrement"><code class="computeroutput"><span class="identifier">has_post_decrement</span></code></a> 362 </p> 363 </td> 364</tr> 365</tbody> 366</table></div> 367</div> 368<br class="table-break"><div class="table"> 369<a name="boost_typetraits.category.value_traits.operators.supported_binary_operators"></a><p class="title"><b>Table 1.6. Supported binary operators</b></p> 370<div class="table-contents"><table class="table" summary="Supported binary operators"> 371<colgroup> 372<col> 373<col> 374</colgroup> 375<thead><tr> 376<th> 377 <p> 378 binary operator 379 </p> 380 </th> 381<th> 382 <p> 383 trait name 384 </p> 385 </th> 386</tr></thead> 387<tbody> 388<tr> 389<td> 390 <p> 391 <code class="computeroutput"><span class="special">+</span></code> 392 </p> 393 </td> 394<td> 395 <p> 396 <a class="link" href="../../reference/has_plus.html" title="has_plus"><code class="computeroutput"><span class="identifier">has_plus</span></code></a> <code class="computeroutput"><span class="special"><</span> <span class="keyword">class</span> 397 <span class="identifier">Lhs</span><span class="special">,</span> 398 <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> 399 <span class="identifier">Ret</span><span class="special">=</span><span class="identifier">dont_care</span> <span class="special">></span></code> 400 </p> 401 </td> 402</tr> 403<tr> 404<td> 405 <p> 406 <code class="computeroutput"><span class="special">-</span></code> 407 </p> 408 </td> 409<td> 410 <p> 411 <a class="link" href="../../reference/has_minus.html" title="has_minus"><code class="computeroutput"><span class="identifier">has_minus</span></code></a> 412 </p> 413 </td> 414</tr> 415<tr> 416<td> 417 <p> 418 <code class="computeroutput"><span class="special">*</span></code> 419 </p> 420 </td> 421<td> 422 <p> 423 <a class="link" href="../../reference/has_multiplies.html" title="has_multiplies"><code class="computeroutput"><span class="identifier">has_multiplies</span></code></a> 424 </p> 425 </td> 426</tr> 427<tr> 428<td> 429 <p> 430 <code class="computeroutput"><span class="special">/</span></code> 431 </p> 432 </td> 433<td> 434 <p> 435 <a class="link" href="../../reference/has_divides.html" title="has_divides"><code class="computeroutput"><span class="identifier">has_divides</span></code></a> 436 </p> 437 </td> 438</tr> 439<tr> 440<td> 441 <p> 442 <code class="computeroutput"><span class="special">%</span></code> 443 </p> 444 </td> 445<td> 446 <p> 447 <a class="link" href="../../reference/has_modulus.html" title="has_modulus"><code class="computeroutput"><span class="identifier">has_modulus</span></code></a> 448 </p> 449 </td> 450</tr> 451<tr> 452<td> 453 <p> 454 <code class="computeroutput"><span class="special">+=</span></code> 455 </p> 456 </td> 457<td> 458 <p> 459 <a class="link" href="../../reference/has_plus_assign.html" title="has_plus_assign"><code class="computeroutput"><span class="identifier">has_plus_assign</span></code></a> 460 </p> 461 </td> 462</tr> 463<tr> 464<td> 465 <p> 466 <code class="computeroutput"><span class="special">-=</span></code> 467 </p> 468 </td> 469<td> 470 <p> 471 <a class="link" href="../../reference/has_minus_assign.html" title="has_minus_assign"><code class="computeroutput"><span class="identifier">has_minus_assign</span></code></a> 472 </p> 473 </td> 474</tr> 475<tr> 476<td> 477 <p> 478 <code class="computeroutput"><span class="special">*=</span></code> 479 </p> 480 </td> 481<td> 482 <p> 483 <a class="link" href="../../reference/has_multiplies_assign.html" title="has_multiplies_assign"><code class="computeroutput"><span class="identifier">has_multiplies_assign</span></code></a> 484 </p> 485 </td> 486</tr> 487<tr> 488<td> 489 <p> 490 <code class="computeroutput"><span class="special">/=</span></code> 491 </p> 492 </td> 493<td> 494 <p> 495 <a class="link" href="../../reference/has_divides_assign.html" title="has_divides_assign"><code class="computeroutput"><span class="identifier">has_divides_assign</span></code></a> 496 </p> 497 </td> 498</tr> 499<tr> 500<td> 501 <p> 502 <code class="computeroutput"><span class="special">%=</span></code> 503 </p> 504 </td> 505<td> 506 <p> 507 <a class="link" href="../../reference/has_modulus_assign.html" title="has_modulus_assign"><code class="computeroutput"><span class="identifier">has_modulus_assign</span></code></a> 508 </p> 509 </td> 510</tr> 511<tr> 512<td> 513 <p> 514 <code class="computeroutput"><span class="special">&</span></code> 515 </p> 516 </td> 517<td> 518 <p> 519 <a class="link" href="../../reference/has_bit_and.html" title="has_bit_and"><code class="computeroutput"><span class="identifier">has_bit_and</span></code></a> 520 </p> 521 </td> 522</tr> 523<tr> 524<td> 525 <p> 526 <code class="computeroutput"><span class="special">|</span></code> 527 </p> 528 </td> 529<td> 530 <p> 531 <a class="link" href="../../reference/has_bit_or.html" title="has_bit_or"><code class="computeroutput"><span class="identifier">has_bit_or</span></code></a> 532 </p> 533 </td> 534</tr> 535<tr> 536<td> 537 <p> 538 <code class="computeroutput"><span class="special">^</span></code> 539 </p> 540 </td> 541<td> 542 <p> 543 <a class="link" href="../../reference/has_bit_xor.html" title="has_bit_xor"><code class="computeroutput"><span class="identifier">has_bit_xor</span></code></a> 544 </p> 545 </td> 546</tr> 547<tr> 548<td> 549 <p> 550 <code class="computeroutput"><span class="special">&=</span></code> 551 </p> 552 </td> 553<td> 554 <p> 555 <a class="link" href="../../reference/has_bit_and_assign.html" title="has_bit_and_assign"><code class="computeroutput"><span class="identifier">has_bit_and_assign</span></code></a> 556 </p> 557 </td> 558</tr> 559<tr> 560<td> 561 <p> 562 <code class="computeroutput"><span class="special">|=</span></code> 563 </p> 564 </td> 565<td> 566 <p> 567 <a class="link" href="../../reference/has_bit_or_assign.html" title="has_bit_or_assign"><code class="computeroutput"><span class="identifier">has_bit_or_assign</span></code></a> 568 </p> 569 </td> 570</tr> 571<tr> 572<td> 573 <p> 574 <code class="computeroutput"><span class="special">^=</span></code> 575 </p> 576 </td> 577<td> 578 <p> 579 <a class="link" href="../../reference/has_bit_xor_assign.html" title="has_bit_xor_assign"><code class="computeroutput"><span class="identifier">has_bit_xor_assign</span></code></a> 580 </p> 581 </td> 582</tr> 583<tr> 584<td> 585 <p> 586 <code class="computeroutput"><span class="special"><<</span></code> 587 </p> 588 </td> 589<td> 590 <p> 591 <a class="link" href="../../reference/has_left_shift.html" title="has_left_shift"><code class="computeroutput"><span class="identifier">has_left_shift</span></code></a> 592 </p> 593 </td> 594</tr> 595<tr> 596<td> 597 <p> 598 <code class="computeroutput"><span class="special">>></span></code> 599 </p> 600 </td> 601<td> 602 <p> 603 <a class="link" href="../../reference/has_right_shift.html" title="has_right_shift"><code class="computeroutput"><span class="identifier">has_right_shift</span></code></a> 604 </p> 605 </td> 606</tr> 607<tr> 608<td> 609 <p> 610 <code class="computeroutput"><span class="special"><<=</span></code> 611 </p> 612 </td> 613<td> 614 <p> 615 <a class="link" href="../../reference/has_left_shift_assign.html" title="has_left_shift_assign"><code class="computeroutput"><span class="identifier">has_left_shift_assign</span></code></a> 616 </p> 617 </td> 618</tr> 619<tr> 620<td> 621 <p> 622 <code class="computeroutput"><span class="special">>>=</span></code> 623 </p> 624 </td> 625<td> 626 <p> 627 <a class="link" href="../../reference/has_right_shift_assign.html" title="has_right_shift_assign"><code class="computeroutput"><span class="identifier">has_right_shift_assign</span></code></a> 628 </p> 629 </td> 630</tr> 631<tr> 632<td> 633 <p> 634 <code class="computeroutput"><span class="special">==</span></code> 635 </p> 636 </td> 637<td> 638 <p> 639 <a class="link" href="../../reference/has_equal_to.html" title="has_equal_to"><code class="computeroutput"><span class="identifier">has_equal_to</span></code></a> 640 </p> 641 </td> 642</tr> 643<tr> 644<td> 645 <p> 646 <code class="computeroutput"><span class="special">!=</span></code> 647 </p> 648 </td> 649<td> 650 <p> 651 <a class="link" href="../../reference/has_not_equal_to.html" title="has_not_equal_to"><code class="computeroutput"><span class="identifier">has_not_equal_to</span></code></a> 652 </p> 653 </td> 654</tr> 655<tr> 656<td> 657 <p> 658 <code class="computeroutput"><span class="special"><</span></code> 659 </p> 660 </td> 661<td> 662 <p> 663 <a class="link" href="../../reference/has_less.html" title="has_less"><code class="computeroutput"><span class="identifier">has_less</span></code></a> 664 </p> 665 </td> 666</tr> 667<tr> 668<td> 669 <p> 670 <code class="computeroutput"><span class="special"><=</span></code> 671 </p> 672 </td> 673<td> 674 <p> 675 <a class="link" href="../../reference/has_less_equal.html" title="has_less_equal"><code class="computeroutput"><span class="identifier">has_less_equal</span></code></a> 676 </p> 677 </td> 678</tr> 679<tr> 680<td> 681 <p> 682 <code class="computeroutput"><span class="special">></span></code> 683 </p> 684 </td> 685<td> 686 <p> 687 <a class="link" href="../../reference/has_greater.html" title="has_greater"><code class="computeroutput"><span class="identifier">has_greater</span></code></a> 688 </p> 689 </td> 690</tr> 691<tr> 692<td> 693 <p> 694 <code class="computeroutput"><span class="special">>=</span></code> 695 </p> 696 </td> 697<td> 698 <p> 699 <a class="link" href="../../reference/has_greater_equal.html" title="has_greater_equal"><code class="computeroutput"><span class="identifier">has_greater_equal</span></code></a> 700 </p> 701 </td> 702</tr> 703<tr> 704<td> 705 <p> 706 <code class="computeroutput"><span class="special">&&</span></code> 707 </p> 708 </td> 709<td> 710 <p> 711 <a class="link" href="../../reference/has_logical_and.html" title="has_logical_and"><code class="computeroutput"><span class="identifier">has_logical_and</span></code></a> 712 </p> 713 </td> 714</tr> 715<tr> 716<td> 717 <p> 718 <code class="computeroutput"><span class="special">||</span></code> 719 </p> 720 </td> 721<td> 722 <p> 723 <a class="link" href="../../reference/has_logical_or.html" title="has_logical_or"><code class="computeroutput"><span class="identifier">has_logical_or</span></code></a> 724 </p> 725 </td> 726</tr> 727</tbody> 728</table></div> 729</div> 730<br class="table-break"><p> 731 The following operators are not supported because they could not be implemented 732 using the same technique: <code class="computeroutput"><span class="keyword">operator</span><span class="special">=</span></code>, <code class="computeroutput"><span class="keyword">operator</span><span class="special">-></span></code>, <code class="computeroutput"><span class="keyword">operator</span><span class="special">&</span></code>, <code class="computeroutput"><span class="keyword">operator</span><span class="special">[]</span></code>, <code class="computeroutput"><span class="keyword">operator</span><span class="special">,</span></code>, <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>, <code class="computeroutput"><span class="keyword">operator</span> 733 <span class="keyword">new</span></code>. 734 </p> 735<h6> 736<a name="boost_typetraits.category.value_traits.operators.h3"></a> 737 <span class="phrase"><a name="boost_typetraits.category.value_traits.operators.cv_qualifiers_and_references"></a></span><a class="link" href="operators.html#boost_typetraits.category.value_traits.operators.cv_qualifiers_and_references">cv 738 qualifiers and references</a> 739 </h6> 740<p> 741 A reference sign <code class="computeroutput"><span class="special">&</span></code> in 742 the operator argument is ignored so that <code class="computeroutput"><span class="identifier">has_plus</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="special">>::</span><span class="identifier">value</span><span class="special">==</span><span class="identifier">has_plus</span><span class="special"><</span> 743 <span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span> <span class="special">>::</span><span class="identifier">value</span></code>. This has been chosen because if 744 the following code works (does not work): 745</p> 746<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span> 747<span class="keyword">double</span> <span class="identifier">d</span><span class="special">;</span> 748<span class="identifier">i</span><span class="special">+</span><span class="identifier">d</span><span class="special">;</span> 749</pre> 750<p> 751 the following code also works (does not work): 752</p> 753<pre class="programlisting"><span class="keyword">int</span> <span class="special">&</span><span class="identifier">ir</span><span class="special">=</span><span class="identifier">i</span><span class="special">;</span> 754<span class="keyword">double</span> <span class="special">&</span><span class="identifier">dr</span><span class="special">=</span><span class="identifier">d</span><span class="special">;</span> 755<span class="identifier">ir</span><span class="special">+</span><span class="identifier">dr</span><span class="special">;</span> 756</pre> 757<p> 758 </p> 759<p> 760 It was not possible to handle properly the <code class="computeroutput"><span class="keyword">volatile</span></code> 761 qualifier so that any construct using this qualifier has undefined behavior. 762 </p> 763<p> 764 As a help, the following tables give the necessary conditions over each 765 trait template argument for the trait <code class="computeroutput"><span class="identifier">value</span></code> 766 to be <code class="computeroutput"><span class="keyword">true</span></code>. They are non sufficient 767 conditions because the conditions must be <code class="computeroutput"><span class="keyword">true</span></code> 768 for all arguments and return type for <code class="computeroutput"><span class="identifier">value</span></code> 769 to be <code class="computeroutput"><span class="keyword">true</span></code>. 770 </p> 771<div class="table"> 772<a name="boost_typetraits.category.value_traits.operators.necessary_and_non_sufficient_condition_on_operator_argument_for_value_to_be_true"></a><p class="title"><b>Table 1.7. necessary and non sufficient condition on operator argument for 773 value to be true</b></p> 774<div class="table-contents"><table class="table" summary="necessary and non sufficient condition on operator argument for 775 value to be true"> 776<colgroup> 777<col> 778<col> 779<col> 780<col> 781</colgroup> 782<thead><tr> 783<th> 784 <p> 785 operator declaration 786 </p> 787 </th> 788<th> 789 <p> 790 <code class="computeroutput"><span class="identifier">has_op</span><span class="special"><</span> 791 <span class="keyword">void</span> <span class="special">></span></code> 792 </p> 793 </th> 794<th> 795 <p> 796 <code class="computeroutput"><span class="identifier">has_op</span><span class="special"><</span> 797 <span class="identifier">Arg</span> <span class="special">></span></code> 798 and <code class="computeroutput"><span class="identifier">has_op</span><span class="special"><</span> 799 <span class="identifier">Arg</span><span class="special">&</span> 800 <span class="special">></span></code> 801 </p> 802 </th> 803<th> 804 <p> 805 <code class="computeroutput"><span class="identifier">has_op</span><span class="special"><</span> 806 <span class="identifier">Arg</span> <span class="keyword">const</span> 807 <span class="special">></span></code> and <code class="computeroutput"><span class="identifier">has_op</span><span class="special"><</span> 808 <span class="identifier">Arg</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">></span></code> 809 </p> 810 </th> 811</tr></thead> 812<tbody> 813<tr> 814<td> 815 <p> 816 <code class="computeroutput"><span class="keyword">operator</span></code>@<code class="computeroutput"><span class="special">(</span><span class="identifier">Arg</span><span class="special">)</span></code> 817 </p> 818 </td> 819<td> 820 <p> 821 false 822 </p> 823 </td> 824<td> 825 <p> 826 true 827 </p> 828 </td> 829<td> 830 <p> 831 true 832 </p> 833 </td> 834</tr> 835<tr> 836<td> 837 <p> 838 <code class="computeroutput"><span class="keyword">operator</span></code>@<code class="computeroutput"><span class="special">(</span><span class="identifier">Arg</span> 839 <span class="keyword">const</span><span class="special">)</span></code> 840 </p> 841 </td> 842<td> 843 <p> 844 false 845 </p> 846 </td> 847<td> 848 <p> 849 true 850 </p> 851 </td> 852<td> 853 <p> 854 true 855 </p> 856 </td> 857</tr> 858<tr> 859<td> 860 <p> 861 <code class="computeroutput"><span class="keyword">operator</span></code>@<code class="computeroutput"><span class="special">(</span><span class="identifier">Arg</span> 862 <span class="special">&)</span></code> 863 </p> 864 </td> 865<td> 866 <p> 867 false 868 </p> 869 </td> 870<td> 871 <p> 872 true 873 </p> 874 </td> 875<td> 876 <p> 877 false 878 </p> 879 </td> 880</tr> 881<tr> 882<td> 883 <p> 884 <code class="computeroutput"><span class="keyword">operator</span></code>@<code class="computeroutput"><span class="special">(</span><span class="identifier">Arg</span> 885 <span class="keyword">const</span> <span class="special">&)</span></code> 886 </p> 887 </td> 888<td> 889 <p> 890 false 891 </p> 892 </td> 893<td> 894 <p> 895 true 896 </p> 897 </td> 898<td> 899 <p> 900 true 901 </p> 902 </td> 903</tr> 904</tbody> 905</table></div> 906</div> 907<br class="table-break"><div class="table"> 908<a name="boost_typetraits.category.value_traits.operators.necessary_and_non_sufficient_condition_on_operator_return_type_for_value_to_be_true"></a><p class="title"><b>Table 1.8. necessary and non sufficient condition on operator return type for 909 value to be true</b></p> 910<div class="table-contents"><table class="table" summary="necessary and non sufficient condition on operator return type for 911 value to be true"> 912<colgroup> 913<col> 914<col> 915<col> 916<col> 917<col> 918<col> 919</colgroup> 920<thead><tr> 921<th> 922 <p> 923 operator declaration 924 </p> 925 </th> 926<th> 927 <p> 928 <code class="computeroutput"><span class="identifier">has_op</span><span class="special"><</span> 929 <span class="special">...,</span> <span class="keyword">void</span> 930 <span class="special">></span></code> 931 </p> 932 </th> 933<th> 934 <p> 935 <code class="computeroutput"><span class="identifier">has_op</span><span class="special"><</span> 936 <span class="special">...,</span> <span class="identifier">Ret</span> 937 <span class="special">></span></code> 938 </p> 939 </th> 940<th> 941 <p> 942 <code class="computeroutput"><span class="identifier">has_op</span><span class="special"><</span> 943 <span class="special">...,</span> <span class="identifier">Ret</span> 944 <span class="keyword">const</span> <span class="special">></span></code> 945 </p> 946 </th> 947<th> 948 <p> 949 <code class="computeroutput"><span class="identifier">has_op</span><span class="special"><</span> 950 <span class="special">...,</span> <span class="identifier">Ret</span> 951 <span class="special">&</span> <span class="special">></span></code> 952 </p> 953 </th> 954<th> 955 <p> 956 <code class="computeroutput"><span class="identifier">has_op</span><span class="special"><</span> 957 <span class="special">...,</span> <span class="identifier">Ret</span> 958 <span class="keyword">const</span> <span class="special">&</span> 959 <span class="special">></span></code> 960 </p> 961 </th> 962</tr></thead> 963<tbody> 964<tr> 965<td> 966 <p> 967 <code class="computeroutput"><span class="keyword">void</span> <span class="keyword">operator</span></code>@<code class="computeroutput"><span class="special">(...)</span></code> 968 </p> 969 </td> 970<td> 971 <p> 972 true 973 </p> 974 </td> 975<td> 976 <p> 977 false 978 </p> 979 </td> 980<td> 981 <p> 982 false 983 </p> 984 </td> 985<td> 986 <p> 987 false 988 </p> 989 </td> 990<td> 991 <p> 992 false 993 </p> 994 </td> 995</tr> 996<tr> 997<td> 998 <p> 999 <code class="computeroutput"><span class="identifier">Ret</span> <span class="keyword">operator</span></code>@<code class="computeroutput"><span class="special">(...)</span></code> 1000 </p> 1001 </td> 1002<td> 1003 <p> 1004 false 1005 </p> 1006 </td> 1007<td> 1008 <p> 1009 true 1010 </p> 1011 </td> 1012<td> 1013 <p> 1014 true 1015 </p> 1016 </td> 1017<td> 1018 <p> 1019 false 1020 </p> 1021 </td> 1022<td> 1023 <p> 1024 true 1025 </p> 1026 </td> 1027</tr> 1028<tr> 1029<td> 1030 <p> 1031 <code class="computeroutput"><span class="identifier">Ret</span> <span class="keyword">const</span> 1032 <span class="keyword">operator</span></code>@<code class="computeroutput"><span class="special">(...)</span></code> 1033 </p> 1034 </td> 1035<td> 1036 <p> 1037 false 1038 </p> 1039 </td> 1040<td> 1041 <p> 1042 true 1043 </p> 1044 </td> 1045<td> 1046 <p> 1047 true 1048 </p> 1049 </td> 1050<td> 1051 <p> 1052 false 1053 </p> 1054 </td> 1055<td> 1056 <p> 1057 true 1058 </p> 1059 </td> 1060</tr> 1061<tr> 1062<td> 1063 <p> 1064 <code class="computeroutput"><span class="identifier">Ret</span> <span class="special">&</span> 1065 <span class="keyword">operator</span></code>@<code class="computeroutput"><span class="special">(...)</span></code> 1066 </p> 1067 </td> 1068<td> 1069 <p> 1070 false 1071 </p> 1072 </td> 1073<td> 1074 <p> 1075 true 1076 </p> 1077 </td> 1078<td> 1079 <p> 1080 true 1081 </p> 1082 </td> 1083<td> 1084 <p> 1085 true 1086 </p> 1087 </td> 1088<td> 1089 <p> 1090 true 1091 </p> 1092 </td> 1093</tr> 1094<tr> 1095<td> 1096 <p> 1097 <code class="computeroutput"><span class="identifier">Ret</span> <span class="keyword">const</span> 1098 <span class="special">&</span> <span class="keyword">operator</span></code>@<code class="computeroutput"><span class="special">(...)</span></code> 1099 </p> 1100 </td> 1101<td> 1102 <p> 1103 false 1104 </p> 1105 </td> 1106<td> 1107 <p> 1108 true 1109 </p> 1110 </td> 1111<td> 1112 <p> 1113 true 1114 </p> 1115 </td> 1116<td> 1117 <p> 1118 false 1119 </p> 1120 </td> 1121<td> 1122 <p> 1123 true 1124 </p> 1125 </td> 1126</tr> 1127</tbody> 1128</table></div> 1129</div> 1130<br class="table-break"><h6> 1131<a name="boost_typetraits.category.value_traits.operators.h4"></a> 1132 <span class="phrase"><a name="boost_typetraits.category.value_traits.operators.implementation"></a></span><a class="link" href="operators.html#boost_typetraits.category.value_traits.operators.implementation">Implementation</a> 1133 </h6> 1134<p> 1135 The implementation consists in only header files. The following headers 1136 should included first: 1137</p> 1138<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_operator</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></pre> 1139<p> 1140 or 1141</p> 1142<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_op</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></pre> 1143<p> 1144 where <code class="literal">op</code> is the textual name chosen for the wanted operator. 1145 The first method includes all operator traits. 1146 </p> 1147<p> 1148 All traits are implemented the same way using preprocessor macros to avoid 1149 code duplication. The main files are in <code class="literal">boost/type_traits/detail</code>: 1150 <code class="literal">has_binary_operator.hpp</code>, <code class="literal">has_prefix_operator.hpp</code> 1151 and <code class="literal">has_postfix_operator.hpp</code>. The example of prefix 1152 <code class="computeroutput"><span class="keyword">operator</span><span class="special">-</span></code> 1153 is presented below: 1154 </p> 1155<p> 1156</p> 1157<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> 1158<span class="keyword">namespace</span> <span class="identifier">detail</span> <span class="special">{</span> 1159 1160<span class="comment">// This namespace ensures that argument-dependent name lookup does not mess things up.</span> 1161<span class="keyword">namespace</span> <span class="identifier">has_unary_minus_impl</span> <span class="special">{</span> 1162 1163<span class="comment">// 1. a function to have an instance of type T without requiring T to be default</span> 1164<span class="comment">// constructible</span> 1165<span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> <span class="identifier">T</span> <span class="special">&</span><span class="identifier">make</span><span class="special">();</span> 1166 1167 1168<span class="comment">// 2. we provide our operator definition for types that do not have one already</span> 1169 1170<span class="comment">// a type returned from operator- when no such operator is</span> 1171<span class="comment">// found in the type's own namespace (our own operator is used) so that we have</span> 1172<span class="comment">// a means to know that our operator was used</span> 1173<span class="keyword">struct</span> <span class="identifier">no_operator</span> <span class="special">{</span> <span class="special">};</span> 1174 1175<span class="comment">// this class allows implicit conversions and makes the following operator</span> 1176<span class="comment">// definition less-preferred than any other such operators that might be found</span> 1177<span class="comment">// via argument-dependent name lookup</span> 1178<span class="keyword">struct</span> <span class="identifier">any</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="identifier">any</span><span class="special">(</span><span class="identifier">T</span> <span class="keyword">const</span><span class="special">&);</span> <span class="special">};</span> 1179 1180<span class="comment">// when operator- is not available, this one is used</span> 1181<span class="identifier">no_operator</span> <span class="keyword">operator</span><span class="special">-(</span><span class="keyword">const</span> <span class="identifier">any</span><span class="special">&);</span> 1182 1183 1184<span class="comment">// 3. checks if the operator returns void or not</span> 1185<span class="comment">// conditions: Rhs!=void</span> 1186 1187<span class="comment">// we first redefine "operator," so that we have no compilation error if</span> 1188<span class="comment">// operator- returns void and we can use the return type of</span> 1189<span class="comment">// (-rhs, returns_void_t()) to deduce if operator- returns void or not:</span> 1190<span class="comment">// - operator- returns void -> (-rhs, returns_void_t()) returns returns_void_t</span> 1191<span class="comment">// - operator- returns !=void -> (-rhs, returns_void_t()) returns int</span> 1192<span class="keyword">struct</span> <span class="identifier">returns_void_t</span> <span class="special">{</span> <span class="special">};</span> 1193<span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">int</span> <span class="keyword">operator</span><span class="special">,(</span><span class="keyword">const</span> <span class="identifier">T</span><span class="special">&,</span> <span class="identifier">returns_void_t</span><span class="special">);</span> 1194<span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">int</span> <span class="keyword">operator</span><span class="special">,(</span><span class="keyword">const</span> <span class="keyword">volatile</span> <span class="identifier">T</span><span class="special">&,</span> <span class="identifier">returns_void_t</span><span class="special">);</span> 1195 1196<span class="comment">// this intermediate trait has member value of type bool:</span> 1197<span class="comment">// - value==true -> operator- returns void</span> 1198<span class="comment">// - value==false -> operator- does not return void</span> 1199<span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span> <span class="special">></span> 1200<span class="keyword">struct</span> <span class="identifier">operator_returns_void</span> <span class="special">{</span> 1201 <span class="comment">// overloads of function returns_void make the difference</span> 1202 <span class="comment">// yes_type and no_type have different size by construction</span> 1203 <span class="keyword">static</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">yes_type</span> <span class="identifier">returns_void</span><span class="special">(</span><span class="identifier">returns_void_t</span><span class="special">);</span> 1204 <span class="keyword">static</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">no_type</span> <span class="identifier">returns_void</span><span class="special">(</span><span class="keyword">int</span><span class="special">);</span> 1205 <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> <span class="keyword">sizeof</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">yes_type</span><span class="special">)==</span><span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">returns_void</span><span class="special">((-</span><span class="identifier">make</span><span class="special"><</span><span class="identifier">Rhs</span><span class="special">>(),</span><span class="identifier">returns_void_t</span><span class="special">())));</span> 1206<span class="special">};</span> 1207 1208 1209<span class="comment">// 4. checks if the return type is Ret or Ret==dont_care</span> 1210<span class="comment">// conditions: Rhs!=void</span> 1211 1212<span class="keyword">struct</span> <span class="identifier">dont_care</span> <span class="special">{</span> <span class="special">};</span> 1213 1214<span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Ret</span><span class="special">,</span> <span class="keyword">bool</span> <span class="identifier">Returns_void</span> <span class="special">></span> 1215<span class="keyword">struct</span> <span class="identifier">operator_returns_Ret</span><span class="special">;</span> 1216 1217<span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span> <span class="special">></span> 1218<span class="keyword">struct</span> <span class="identifier">operator_returns_Ret</span> <span class="special"><</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="identifier">dont_care</span><span class="special">,</span> <span class="keyword">true</span> <span class="special">></span> <span class="special">{</span> 1219 <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> <span class="keyword">true</span><span class="special">;</span> 1220<span class="special">};</span> 1221 1222<span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span> <span class="special">></span> 1223<span class="keyword">struct</span> <span class="identifier">operator_returns_Ret</span> <span class="special"><</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="identifier">dont_care</span><span class="special">,</span> <span class="keyword">false</span> <span class="special">></span> <span class="special">{</span> 1224 <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> <span class="keyword">true</span><span class="special">;</span> 1225<span class="special">};</span> 1226 1227<span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span> <span class="special">></span> 1228<span class="keyword">struct</span> <span class="identifier">operator_returns_Ret</span> <span class="special"><</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="keyword">void</span><span class="special">,</span> <span class="keyword">true</span> <span class="special">></span> <span class="special">{</span> 1229 <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> <span class="keyword">true</span><span class="special">;</span> 1230<span class="special">};</span> 1231 1232<span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span> <span class="special">></span> 1233<span class="keyword">struct</span> <span class="identifier">operator_returns_Ret</span> <span class="special"><</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="keyword">void</span><span class="special">,</span> <span class="keyword">false</span> <span class="special">></span> <span class="special">{</span> 1234 <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> <span class="keyword">false</span><span class="special">;</span> 1235<span class="special">};</span> 1236 1237<span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Ret</span> <span class="special">></span> 1238<span class="keyword">struct</span> <span class="identifier">operator_returns_Ret</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="keyword">true</span> <span class="special">></span> <span class="special">{</span> 1239 <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> <span class="keyword">false</span><span class="special">;</span> 1240<span class="special">};</span> 1241 1242<span class="comment">// otherwise checks if it is convertible to Ret using the sizeof trick</span> 1243<span class="comment">// based on overload resolution</span> 1244<span class="comment">// condition: Ret!=void and Ret!=dont_care and the operator does not return void</span> 1245<span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Ret</span> <span class="special">></span> 1246<span class="keyword">struct</span> <span class="identifier">operator_returns_Ret</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="keyword">false</span> <span class="special">></span> <span class="special">{</span> 1247 <span class="keyword">static</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">yes_type</span> <span class="identifier">is_convertible_to_Ret</span><span class="special">(</span><span class="identifier">Ret</span><span class="special">);</span> <span class="comment">// this version is preferred for types convertible to Ret</span> 1248 <span class="keyword">static</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">no_type</span> <span class="identifier">is_convertible_to_Ret</span><span class="special">(...);</span> <span class="comment">// this version is used otherwise</span> 1249 1250 <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> <span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">is_convertible_to_Ret</span><span class="special">(-</span><span class="identifier">make</span><span class="special"><</span><span class="identifier">Rhs</span><span class="special">>()))==</span><span class="keyword">sizeof</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">yes_type</span><span class="special">);</span> 1251<span class="special">};</span> 1252 1253 1254<span class="comment">// 5. checks for operator existence</span> 1255<span class="comment">// condition: Rhs!=void</span> 1256 1257<span class="comment">// checks if our definition of operator- is used or an other</span> 1258<span class="comment">// existing one;</span> 1259<span class="comment">// this is done with redefinition of "operator," that returns no_operator or has_operator</span> 1260<span class="keyword">struct</span> <span class="identifier">has_operator</span> <span class="special">{</span> <span class="special">};</span> 1261<span class="identifier">no_operator</span> <span class="keyword">operator</span><span class="special">,(</span><span class="identifier">no_operator</span><span class="special">,</span> <span class="identifier">has_operator</span><span class="special">);</span> 1262 1263<span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span> <span class="special">></span> 1264<span class="keyword">struct</span> <span class="identifier">operator_exists</span> <span class="special">{</span> 1265 <span class="keyword">static</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">yes_type</span> <span class="identifier">check</span><span class="special">(</span><span class="identifier">has_operator</span><span class="special">);</span> <span class="comment">// this version is preferred when operator exists</span> 1266 <span class="keyword">static</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">no_type</span> <span class="identifier">check</span><span class="special">(</span><span class="identifier">no_operator</span><span class="special">);</span> <span class="comment">// this version is used otherwise</span> 1267 1268 <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> <span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">check</span><span class="special">(((-</span><span class="identifier">make</span><span class="special"><</span><span class="identifier">Rhs</span><span class="special">>()),</span><span class="identifier">make</span><span class="special"><</span><span class="identifier">has_operator</span><span class="special">>())))==</span><span class="keyword">sizeof</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">yes_type</span><span class="special">);</span> 1269<span class="special">};</span> 1270 1271 1272<span class="comment">// 6. main trait: to avoid any compilation error, this class behaves</span> 1273<span class="comment">// differently when operator-(Rhs) is forbidden by the standard.</span> 1274<span class="comment">// Forbidden_if is a bool that is:</span> 1275<span class="comment">// - true when the operator-(Rhs) is forbidden by the standard</span> 1276<span class="comment">// (would yield compilation error if used)</span> 1277<span class="comment">// - false otherwise</span> 1278<span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Ret</span><span class="special">,</span> <span class="keyword">bool</span> <span class="identifier">Forbidden_if</span> <span class="special">></span> 1279<span class="keyword">struct</span> <span class="identifier">trait_impl1</span><span class="special">;</span> 1280 1281<span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Ret</span> <span class="special">></span> 1282<span class="keyword">struct</span> <span class="identifier">trait_impl1</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="keyword">true</span> <span class="special">></span> <span class="special">{</span> 1283 <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> <span class="keyword">false</span><span class="special">;</span> 1284<span class="special">};</span> 1285 1286<span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Ret</span> <span class="special">></span> 1287<span class="keyword">struct</span> <span class="identifier">trait_impl1</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="keyword">false</span> <span class="special">></span> <span class="special">{</span> 1288 <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> 1289 <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">ice_and</span><span class="special"><</span> 1290 <span class="identifier">operator_exists</span> <span class="special"><</span> <span class="identifier">Rhs</span> <span class="special">>::</span><span class="identifier">value</span><span class="special">,</span> 1291 <span class="identifier">operator_returns_Ret</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">operator_returns_void</span> <span class="special"><</span> <span class="identifier">Rhs</span> <span class="special">>::</span><span class="identifier">value</span> <span class="special">>::</span><span class="identifier">value</span> 1292 <span class="special">>::</span><span class="identifier">value</span> 1293 <span class="special">;</span> 1294<span class="special">};</span> 1295 1296<span class="comment">// specialization needs to be declared for the special void case</span> 1297<span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Ret</span> <span class="special">></span> 1298<span class="keyword">struct</span> <span class="identifier">trait_impl1</span> <span class="special"><</span> <span class="keyword">void</span><span class="special">,</span> <span class="identifier">Ret</span><span class="special">,</span> <span class="keyword">false</span> <span class="special">></span> <span class="special">{</span> 1299 <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> <span class="keyword">false</span><span class="special">;</span> 1300<span class="special">};</span> 1301 1302<span class="comment">// defines some typedef for convenience</span> 1303<span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Ret</span> <span class="special">></span> 1304<span class="keyword">struct</span> <span class="identifier">trait_impl</span> <span class="special">{</span> 1305 <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_reference</span><span class="special"><</span><span class="identifier">Rhs</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">Rhs_noref</span><span class="special">;</span> 1306 <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_cv</span><span class="special"><</span><span class="identifier">Rhs_noref</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">Rhs_nocv</span><span class="special">;</span> 1307 <span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_cv</span><span class="special"><</span> <span class="keyword">typename</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_reference</span><span class="special"><</span> <span class="keyword">typename</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">remove_pointer</span><span class="special"><</span><span class="identifier">Rhs_noref</span><span class="special">>::</span><span class="identifier">type</span> <span class="special">>::</span><span class="identifier">type</span> <span class="special">>::</span><span class="identifier">type</span> <span class="identifier">Rhs_noptr</span><span class="special">;</span> 1308 <span class="keyword">static</span> <span class="keyword">const</span> <span class="keyword">bool</span> <span class="identifier">value</span> <span class="special">=</span> <span class="identifier">trait_impl1</span> <span class="special"><</span> <span class="identifier">Rhs_noref</span><span class="special">,</span> <span class="identifier">Ret</span><span class="special">,</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_pointer</span><span class="special"><</span> <span class="identifier">Rhs_noref</span> <span class="special">>::</span><span class="identifier">value</span> <span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> 1309<span class="special">};</span> 1310 1311<span class="special">}</span> <span class="comment">// namespace impl</span> 1312<span class="special">}</span> <span class="comment">// namespace detail</span> 1313 1314<span class="comment">// this is the accessible definition of the trait to end user</span> 1315<span class="keyword">template</span> <span class="special"><</span> <span class="keyword">typename</span> <span class="identifier">Rhs</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Ret</span><span class="special">=::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">detail</span><span class="special">::</span><span class="identifier">has_unary_minus_impl</span><span class="special">::</span><span class="identifier">dont_care</span> <span class="special">></span> 1316<span class="keyword">struct</span> <span class="identifier">has_unary_minus</span> <span class="special">:</span> <span class="special">::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">integral_constant</span><span class="special"><</span><span class="keyword">bool</span><span class="special">,(::</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">detail</span><span class="special">::</span><span class="identifier">has_unary_minus_impl</span><span class="special">::</span><span class="identifier">trait_impl</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><span class="special">)></span> <span class="special">{</span> <span class="special">};</span> 1317 1318<span class="special">}</span> <span class="comment">// namespace boost</span> 1319</pre> 1320<p> 1321 </p> 1322<h6> 1323<a name="boost_typetraits.category.value_traits.operators.h5"></a> 1324 <span class="phrase"><a name="boost_typetraits.category.value_traits.operators.limitation"></a></span><a class="link" href="operators.html#boost_typetraits.category.value_traits.operators.limitation">Limitation</a> 1325 </h6> 1326<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"> 1327 Requires a compiler with working SFINAE. 1328 </li></ul></div> 1329<h6> 1330<a name="boost_typetraits.category.value_traits.operators.h6"></a> 1331 <span class="phrase"><a name="boost_typetraits.category.value_traits.operators.known_issues"></a></span><a class="link" href="operators.html#boost_typetraits.category.value_traits.operators.known_issues">Known 1332 issues</a> 1333 </h6> 1334<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 1335<li class="listitem"> 1336 These traits cannot detect whether the operators are public or not: 1337 if an operator is defined as a private member of type <code class="computeroutput"><span class="identifier">T</span></code> then the instantiation of the corresponding 1338 trait will produce a compiler error. For this reason these traits cannot 1339 be used to determine whether a type has a public operator or not. 1340<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="identifier">A</span> <span class="keyword">operator</span><span class="special">-();</span> <span class="special">};</span> 1341<span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_unary_minus</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-() is private</span> 1342</pre> 1343 </li> 1344<li class="listitem"> 1345 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> 1346 is convertible to <code class="computeroutput"><span class="identifier">A</span></code>. 1347 In this case, the compiler will report an ambiguous overload because 1348 both the existing operator and the one we provide (with argument of 1349 type <code class="computeroutput"><span class="identifier">any</span></code>) need type 1350 conversion, so that none is preferred. 1351<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">{</span> <span class="special">};</span> 1352<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> 1353<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> 1354<span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_unary_minus</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> 1355<span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_unary_minus</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 between</span> 1356 <span class="comment">// operator-(const any&) and</span> 1357 <span class="comment">// operator-(const A&)</span> 1358 <span class="comment">// both need type conversion</span> 1359</pre> 1360<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">B</span> <span class="special">{</span> <span class="special">};</span> 1361<span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">{</span> <span class="identifier">A</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">B</span><span class="special">&)</span> <span class="special">{</span> <span class="special">}</span> <span class="special">};</span> 1362<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> 1363<span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_unary_minus</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> 1364<span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_unary_minus</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 between</span> 1365 <span class="comment">// operator-(const any&) and</span> 1366 <span class="comment">// operator-(const A&)</span> 1367 <span class="comment">// both need type conversion</span> 1368</pre> 1369 </li> 1370<li class="listitem"> 1371 There is an issue when applying these traits to template classes. If 1372 the operator is defined but does not bind for a given template type, 1373 it is still detected by the trait which returns <code class="computeroutput"><span class="keyword">true</span></code> 1374 instead of <code class="computeroutput"><span class="keyword">false</span></code>. This 1375 applies in particular to the containers of the standard library and 1376 <code class="computeroutput"><span class="keyword">operator</span><span class="special">==</span></code>. 1377 Example: 1378<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_equal_to</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 1379<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> 1380 1381<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> 1382<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> 1383 1384<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> 1385<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> 1386 <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> 1387<span class="special">}</span> 1388 1389<span class="keyword">class</span> <span class="identifier">bad</span> <span class="special">{</span> <span class="special">};</span> 1390<span class="keyword">class</span> <span class="identifier">good</span> <span class="special">{</span> <span class="special">};</span> 1391<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> 1392 1393<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> <span class="special">{</span> 1394 <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> 1395 <span class="comment">// works fine for contains<good></span> 1396 <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_equal_to</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> 1397 <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> 1398 <span class="identifier">g</span><span class="special">==</span><span class="identifier">g</span><span class="special">;</span> <span class="comment">// ok</span> 1399 <span class="comment">// does not work for contains<bad></span> 1400 <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_equal_to</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> 1401 <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> 1402 <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> 1403 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 1404<span class="special">}</span> 1405</pre> 1406 </li> 1407<li class="listitem"> 1408 <code class="computeroutput"><span class="keyword">volatile</span></code> qualifier is 1409 not properly handled and would lead to undefined behavior 1410 </li> 1411</ul></div> 1412<h6> 1413<a name="boost_typetraits.category.value_traits.operators.h7"></a> 1414 <span class="phrase"><a name="boost_typetraits.category.value_traits.operators.acknowledgments"></a></span><a class="link" href="operators.html#boost_typetraits.category.value_traits.operators.acknowledgments">Acknowledgments</a> 1415 </h6> 1416<p> 1417 Frederic Bron is very thankful to numerous people from the boost mailing 1418 list for their kind help and patience. In particular, the following persons 1419 have been very helpful for the implementation: Edward Diener, Eric Niebler, 1420 Jeffrey Lee Hellrung (Jr.), Robert Stewart, Roman Perepelitsa, Steven Watanabe, 1421 Vicente Botet. 1422 </p> 1423</div> 1424<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 1425<td align="left"></td> 1426<td align="right"><div class="copyright-footer">Copyright © 2000, 2011 Adobe Systems Inc, David Abrahams, 1427 Frederic Bron, Steve Cleary, Beman Dawes, Glen Fernandes, Aleksey Gurtovoy, 1428 Howard Hinnant, Jesse Jones, Mat Marcus, Itay Maman, John Maddock, Alexander 1429 Nasonov, Thorsten Ottosen, Roman Perepelitsa, Robert Ramey, Jeremy Siek, Robert 1430 Stewart and Steven Watanabe<p> 1431 Distributed under the Boost Software License, Version 1.0. (See accompanying 1432 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>) 1433 </p> 1434</div></td> 1435</tr></table> 1436<hr> 1437<div class="spirit-nav"> 1438<a accesskey="p" href="relate.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../value_traits.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="../transform.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> 1439</div> 1440</body> 1441</html> 1442
