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25 | <div class="section"> | |
26 | <div class="titlepage"><div><div><h3 class="title"> | |
27 | <a name="boost_typetraits.reference.is_convertible"></a><a class="link" href="is_convertible.html" title="is_convertible">is_convertible</a> | |
28 | </h3></div></div></div> | |
29 | <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">From</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">To</span><span class="special">></span> | |
30 | <span class="keyword">struct</span> <span class="identifier">is_convertible</span> <span class="special">:</span> <span class="keyword">public</span> <em class="replaceable"><code><a class="link" href="integral_constant.html" title="integral_constant">true_type</a>-or-<a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code></em> <span class="special">{};</span> | |
31 | </pre> | |
32 | <p> | |
33 | <span class="bold"><strong>Inherits:</strong></span> If an imaginary rvalue of type | |
34 | <code class="computeroutput"><span class="identifier">From</span></code> is convertible to type | |
35 | <code class="computeroutput"><span class="identifier">To</span></code> then inherits from <a class="link" href="integral_constant.html" title="integral_constant">true_type</a>, | |
36 | otherwise inherits from <a class="link" href="integral_constant.html" title="integral_constant">false_type</a>. | |
37 | </p> | |
38 | <p> | |
39 | Type From must not be an incomplete type. | |
40 | </p> | |
41 | <p> | |
42 | Type To must not be an incomplete, or function type. | |
43 | </p> | |
44 | <p> | |
45 | No types are considered to be convertible to array types or abstract-class | |
46 | types. | |
47 | </p> | |
48 | <p> | |
49 | This template can not detect whether a converting-constructor is <code class="computeroutput"><span class="keyword">public</span></code> or not: if type <code class="computeroutput"><span class="identifier">To</span></code> | |
50 | has a <code class="computeroutput"><span class="keyword">private</span></code> converting constructor | |
51 | from type <code class="computeroutput"><span class="identifier">From</span></code> then instantiating | |
52 | <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special"><</span><span class="identifier">From</span><span class="special">,</span> <span class="identifier">To</span><span class="special">></span></code> | |
53 | will produce a compiler error. For this reason <code class="computeroutput"><span class="identifier">is_convertible</span></code> | |
54 | can not be used to determine whether a type has a <code class="computeroutput"><span class="keyword">public</span></code> | |
55 | copy-constructor or not. | |
56 | </p> | |
57 | <p> | |
58 | This template will also produce compiler errors if the conversion is ambiguous, | |
59 | for example: | |
60 | </p> | |
61 | <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">A</span> <span class="special">{};</span> | |
62 | <span class="keyword">struct</span> <span class="identifier">B</span> <span class="special">:</span> <span class="identifier">A</span> <span class="special">{};</span> | |
63 | <span class="keyword">struct</span> <span class="identifier">C</span> <span class="special">:</span> <span class="identifier">A</span> <span class="special">{};</span> | |
64 | <span class="keyword">struct</span> <span class="identifier">D</span> <span class="special">:</span> <span class="identifier">B</span><span class="special">,</span> <span class="identifier">C</span> <span class="special">{};</span> | |
65 | <span class="comment">// This produces a compiler error, the conversion is ambiguous:</span> | |
66 | <span class="keyword">bool</span> <span class="keyword">const</span> <span class="identifier">y</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">is_convertible</span><span class="special"><</span><span class="identifier">D</span><span class="special">*,</span><span class="identifier">A</span><span class="special">*>::</span><span class="identifier">value</span><span class="special">;</span> | |
67 | </pre> | |
68 | <p> | |
69 | <span class="bold"><strong>C++ Standard Reference:</strong></span> 4 and 8.5. | |
70 | </p> | |
71 | <p> | |
72 | <span class="bold"><strong>Compiler Compatibility:</strong></span> All current compilers | |
73 | are supported by this trait. | |
74 | </p> | |
75 | <p> | |
76 | <span class="bold"><strong>Header:</strong></span> <code class="computeroutput"> <span class="preprocessor">#include</span> | |
77 | <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">is_convertible</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code> | |
78 | 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> | |
79 | </p> | |
80 | <p> | |
81 | <span class="bold"><strong>Examples:</strong></span> | |
82 | </p> | |
83 | <div class="blockquote"><blockquote class="blockquote"><p> | |
84 | <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">></span></code> inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>. | |
85 | </p></blockquote></div> | |
86 | <div class="blockquote"><blockquote class="blockquote"><p> | |
87 | <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">,</span> <span class="keyword">double</span><span class="special">>::</span><span class="identifier">type</span></code> | |
88 | is the type <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>. | |
89 | </p></blockquote></div> | |
90 | <div class="blockquote"><blockquote class="blockquote"><p> | |
91 | <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special"><</span><span class="keyword">int</span><span class="special">*</span> <span class="keyword">const</span><span class="special">,</span> <span class="keyword">int</span><span class="special">*>::</span><span class="identifier">value</span></code> is an integral constant expression | |
92 | that evaluates to <span class="emphasis"><em>true</em></span>. | |
93 | </p></blockquote></div> | |
94 | <div class="blockquote"><blockquote class="blockquote"><p> | |
95 | <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special"><</span><span class="keyword">int</span> <span class="keyword">const</span><span class="special">*,</span> <span class="keyword">int</span><span class="special">*>::</span><span class="identifier">value</span></code> is an integral constant expression | |
96 | that evaluates to <span class="emphasis"><em>false</em></span>: the conversion would require | |
97 | a <code class="computeroutput"><span class="keyword">const_cast</span></code>. | |
98 | </p></blockquote></div> | |
99 | <div class="blockquote"><blockquote class="blockquote"><p> | |
100 | <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special"><</span><span class="keyword">int</span> <span class="keyword">const</span><span class="special">&,</span> <span class="keyword">long</span><span class="special">>::</span><span class="identifier">value</span></code> | |
101 | is an integral constant expression that evaluates to <span class="emphasis"><em>true</em></span>. | |
102 | </p></blockquote></div> | |
103 | <div class="blockquote"><blockquote class="blockquote"><p> | |
104 | <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="keyword">int</span><span class="special">>::</span><span class="identifier">value</span></code> | |
105 | is an integral constant expression that evaluates to <span class="emphasis"><em>true</em></span>. | |
106 | </p></blockquote></div> | |
107 | <div class="blockquote"><blockquote class="blockquote"><p> | |
108 | <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">T</span><span class="special">>::</span><span class="identifier">value_type</span></code> | |
109 | is the type <code class="computeroutput"><span class="keyword">bool</span></code>. | |
110 | </p></blockquote></div> | |
111 | </div> | |
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116 | Jesse Jones, Mat Marcus, Itay Maman, John Maddock, Alexander Nasonov, Thorsten | |
117 | Ottosen, Roman Perepelitsa, Robert Ramey, Jeremy Siek, Robert Stewart and Steven | |
118 | Watanabe<p> | |
119 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
120 | file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) | |
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