1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>has_pre_increment</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. 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If <code class="computeroutput"><span class="identifier">Ret</span></code> 45 is different from the default <code class="computeroutput"><span class="identifier">dont_care</span></code> 46 type, the return value is checked to be convertible to <code class="computeroutput"><span class="identifier">Ret</span></code>. 47 Convertible to <code class="computeroutput"><span class="identifier">Ret</span></code> means 48 that the return value of the operator can be used as argument to a function 49 expecting <code class="computeroutput"><span class="identifier">Ret</span></code>: 50</p> 51<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> 52<span class="identifier">Rhs</span> <span class="identifier">rhs</span><span class="special">;</span> 53<span class="identifier">f</span><span class="special">(++</span><span class="identifier">rhs</span><span class="special">);</span> <span class="comment">// is valid if has_pre_increment<Rhs, Ret>::value==true</span> 54</pre> 55<p> 56 If <code class="computeroutput"><span class="identifier">Ret</span><span class="special">=</span><span class="keyword">void</span></code>, the return type is checked to be exactly 57 <code class="computeroutput"><span class="keyword">void</span></code>. 58 </p> 59<p> 60 <span class="bold"><strong>Header:</strong></span> <code class="computeroutput"><span class="preprocessor">#include</span> 61 <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">has_pre_increment</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code> 62 or <code class="computeroutput"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">has_operator</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code> 63 or <code class="computeroutput"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code> 64 </p> 65<p> 66 <span class="bold"><strong>Compiler Compatibility:</strong></span> Requires working 67 SFINAE (i.e. BOOST_NO_SFINAE is not set). Only a minority of rather old compilers 68 do not support this. 69 </p> 70<p> 71 <span class="bold"><strong>Examples:</strong></span> 72 </p> 73<div class="blockquote"><blockquote class="blockquote"><p> 74 <code class="computeroutput"><span class="identifier">has_pre_increment</span><span class="special"><</span><span class="identifier">Rhs</span><span class="special">,</span> <span class="identifier">Ret</span><span class="special">>::</span><span class="identifier">value_type</span></code> is the type <code class="computeroutput"><span class="keyword">bool</span></code>. 75 </p></blockquote></div> 76<div class="blockquote"><blockquote class="blockquote"><p> 77 <code class="computeroutput"><span class="identifier">has_pre_increment</span><span class="special"><</span><span class="identifier">Rhs</span><span class="special">,</span> <span class="identifier">Ret</span><span class="special">>::</span><span class="identifier">value</span></code> is a <code class="computeroutput"><span class="keyword">bool</span></code> 78 integral constant expression. 79 </p></blockquote></div> 80<div class="blockquote"><blockquote class="blockquote"><p> 81 <code class="computeroutput"><span class="identifier">has_pre_increment</span><span class="special"><</span><span class="keyword">int</span><span class="special">>::</span><span class="identifier">value</span></code> is a <code class="computeroutput"><span class="keyword">bool</span></code> 82 integral constant expression that evaluates to <code class="computeroutput"><span class="keyword">true</span></code>. 83 </p></blockquote></div> 84<div class="blockquote"><blockquote class="blockquote"><p> 85 <code class="computeroutput"><span class="identifier">has_pre_increment</span><span class="special"><</span><span class="keyword">long</span><span class="special">></span></code> 86 inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>. 87 </p></blockquote></div> 88<div class="blockquote"><blockquote class="blockquote"><p> 89 <code class="computeroutput"><span class="identifier">has_pre_increment</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">></span></code> inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>. 90 </p></blockquote></div> 91<div class="blockquote"><blockquote class="blockquote"><p> 92 <code class="computeroutput"><span class="identifier">has_pre_increment</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">long</span><span class="special">></span></code> inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>. 93 </p></blockquote></div> 94<div class="blockquote"><blockquote class="blockquote"><p> 95 <code class="computeroutput"><span class="identifier">has_pre_increment</span><span class="special"><</span><span class="keyword">double</span><span class="special">,</span> <span class="keyword">double</span><span class="special">></span></code> 96 inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>. 97 </p></blockquote></div> 98<div class="blockquote"><blockquote class="blockquote"><p> 99 <code class="computeroutput"><span class="identifier">has_pre_increment</span><span class="special"><</span><span class="keyword">double</span><span class="special">,</span> <span class="keyword">int</span><span class="special">></span></code> 100 inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>. 101 </p></blockquote></div> 102<div class="blockquote"><blockquote class="blockquote"><p> 103 <code class="computeroutput"><span class="identifier">has_pre_increment</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code> 104 inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>. 105 </p></blockquote></div> 106<div class="blockquote"><blockquote class="blockquote"><p> 107 <code class="computeroutput"><span class="identifier">has_pre_increment</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">></span></code> inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code>. 108 </p></blockquote></div> 109<div class="blockquote"><blockquote class="blockquote"><p> 110 <code class="computeroutput"><span class="identifier">has_pre_increment</span><span class="special"><</span><span class="keyword">void</span><span class="special">*></span></code> 111 inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code>. 112 </p></blockquote></div> 113<div class="blockquote"><blockquote class="blockquote"><p> 114 <code class="computeroutput"><span class="identifier">has_pre_increment</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">></span></code> inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code>. 115 </p></blockquote></div> 116<p> 117 <span class="bold"><strong>See also:</strong></span> <a class="link" href="../category/value_traits/operators.html" title="Operator Type Traits">Operator 118 Type Traits</a> 119 </p> 120<p> 121 <span class="bold"><strong>Known issues:</strong></span> 122 </p> 123<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 124<li class="listitem"> 125 This trait cannot detect whether prefix <code class="computeroutput"><span class="keyword">operator</span><span class="special">++</span></code> is public or not: if <code class="computeroutput"><span class="keyword">operator</span><span class="special">++</span></code> 126 is defined as a private member of <code class="computeroutput"><span class="identifier">Rhs</span></code> 127 then instantiating <code class="computeroutput"><span class="identifier">has_pre_increment</span><span class="special"><</span><span class="identifier">Rhs</span><span class="special">></span></code> will produce a compiler error. For 128 this reason <code class="computeroutput"><span class="identifier">has_pre_increment</span></code> 129 cannot be used to determine whether a type has a public <code class="computeroutput"><span class="keyword">operator</span><span class="special">++</span></code> 130 or not. 131<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">{</span> <span class="keyword">private</span><span class="special">:</span> <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">++();</span> <span class="special">};</span> 132<span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_pre_increment</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> 133</pre> 134 </li> 135<li class="listitem"> 136 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> 137 is convertible to <code class="computeroutput"><span class="identifier">A</span></code>. 138 In this case, the compiler will report an ambiguous overload. 139<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">{</span> <span class="special">};</span> 140<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> 141<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> 142<span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_pre_increment</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> 143<span class="identifier">boost</span><span class="special">::</span><span class="identifier">has_pre_increment</span><span class="special"><</span><span class="identifier">B</span><span class="special">>::</span><span class="identifier">value</span><span class="special">;</span> <span class="comment">// error: ambiguous overload</span> 144</pre> 145 </li> 146<li class="listitem"> 147 There is an issue when applying this trait to template classes. If <code class="computeroutput"><span class="keyword">operator</span><span class="special">++</span></code> 148 is defined but does not bind for a given template type, it is still detected 149 by the trait which returns <code class="computeroutput"><span class="keyword">true</span></code> 150 instead of <code class="computeroutput"><span class="keyword">false</span></code>. Example: 151<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_pre_increment</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 152<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span> 153 154<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> 155<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> 156 157<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> 158<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">rhs</span><span class="special">)</span> <span class="special">{</span> 159 <span class="keyword">return</span> <span class="identifier">f</span><span class="special">(</span><span class="identifier">rhs</span><span class="special">.</span><span class="identifier">data</span><span class="special">);</span> 160<span class="special">}</span> 161 162<span class="keyword">class</span> <span class="identifier">bad</span> <span class="special">{</span> <span class="special">};</span> 163<span class="keyword">class</span> <span class="identifier">good</span> <span class="special">{</span> <span class="special">};</span> 164<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="special">{</span> <span class="special">}</span> 165 166<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> <span class="special">{</span> 167 <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> 168 <span class="comment">// works fine for contains<good></span> 169 <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_pre_increment</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> 170 <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> 171 <span class="special">++</span><span class="identifier">g</span><span class="special">;</span> <span class="comment">// ok</span> 172 <span class="comment">// does not work for contains<bad></span> 173 <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_pre_increment</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> 174 <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> 175 <span class="special">++</span><span class="identifier">b</span><span class="special">;</span> <span class="comment">// compile time error</span> 176 <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> 177<span class="special">}</span> 178</pre> 179 </li> 180<li class="listitem"> 181 <code class="computeroutput"><span class="keyword">volatile</span></code> qualifier is not 182 properly handled and would lead to undefined behavior 183 </li> 184</ul></div> 185<p> 186 <span class="bold"><strong>Known issues:</strong></span> 187 </p> 188<p> 189 For modern compilers (those that support arbitrary SFINAE-expressions and 190 decltype/declval) this trait offers near perfect detection. In this situation 191 the macro <code class="computeroutput"><span class="identifier">BOOST_TT_HAS_ACCURATE_BINARY_OPERATOR_DETECTION</span></code> 192 will be defined after including <code class="literal"><boost/type_traits/has_pre_increment.hpp></code>. 193 Please note however, that detection is based on function signature only, 194 in the case that the operator is a function template then has_pre_increment 195 cannot perform introspection of the template function body to ensure that 196 the type meets all of the conceptual requirements of the actual code. 197 </p> 198<p> 199 For older compilers (<code class="computeroutput"><span class="identifier">BOOST_TT_HAS_ACCURATE_BINARY_OPERATOR_DETECTION</span></code> 200 not defined) then there are a number of issues: 201 </p> 202<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 203<li class="listitem"> 204 This trait cannot detect whether unary <code class="computeroutput"><span class="keyword">operator</span></code>++ 205 is public or not: if <code class="computeroutput"><span class="keyword">operator</span></code>++ 206 is defined as a private class member of type T then instantiating <code class="literal">has_pre_increment<T></code> 207 will produce a compiler error. For this reason <code class="literal">has_pre_increment</code> 208 cannot be used to determine whether a type has a public <code class="computeroutput"><span class="keyword">operator</span></code>++ or not. 209 </li> 210<li class="listitem"> 211 <code class="computeroutput"><span class="keyword">volatile</span></code> qualifier is not 212 properly handled and would lead to undefined behavior 213 </li> 214<li class="listitem"> 215 Capturless lambdas are not supported and will lead to a compiler error 216 if has_pre_increment is instantiated on the lambda type. 217 </li> 218<li class="listitem"> 219 Scoped enum types are not supported and will lead to a compiler error 220 if has_pre_increment is instantiated on such a type. 221 </li> 222</ul></div> 223</div> 224<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 225<td align="left"></td> 226<td align="right"><div class="copyright-footer">Copyright © 2000, 2011 Adobe Systems Inc, David Abrahams, 227 Frederic Bron, Steve Cleary, Beman Dawes, Glen Fernandes, Aleksey Gurtovoy, 228 Howard Hinnant, Jesse Jones, Mat Marcus, Itay Maman, John Maddock, Alexander 229 Nasonov, Thorsten Ottosen, Roman Perepelitsa, Robert Ramey, Jeremy Siek, Robert 230 Stewart and Steven Watanabe<p> 231 Distributed under the Boost Software License, Version 1.0. (See accompanying 232 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>) 233 </p> 234</div></td> 235</tr></table> 236<hr> 237<div class="spirit-nav"> 238<a accesskey="p" href="has_pre_decrement.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../reference.html"><img src="../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="has_right_shift.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a> 239</div> 240</body> 241</html> 242
