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POErrors using decltype() and SFINAE
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  1. POErrors using decltype() and SFINAE
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    copied!<p>In response to .. some other question somewhere, I wrote this code.</p> <pre><code>struct no_type{}; template&lt;typename T&gt; struct has_apply { static decltype(T().apply&lt;0u&gt;(double())) func( T* ptr ); static no_type func( ... ); static const bool result = !std::is_same&lt;no_type, decltype(func(nullptr))&gt;::value; }; class A { public: template&lt; unsigned n &gt; void apply( const double&amp; ); }; class B { }; int main() { std::cout &lt;&lt; std::boolalpha &lt;&lt; has_apply&lt; A &gt;::result &lt;&lt; '\n'; std::cout &lt;&lt; std::boolalpha &lt;&lt; has_apply&lt; B &gt;::result &lt;&lt; '\n'; std::cin.get(); return( 0 ); } </code></pre> <p>Now it seems to me that result should be true if T offers a non-static member function "apply" that accepts a double rvalue and a template parameter literal, and false otherwise. However, the example given actually fails to compile for class B, when compiling <code>has_apply&lt;B&gt;</code>. Shouldn't the fact that the substitution of T failed in the decltype statement mean that it simply calls the other func? Isn't that kind of the point of SFINAE?</p> <p>Solved in the most ridiculous, pointless fashion ever:</p> <pre><code>struct no_type{}; template&lt;typename T&gt; struct has_apply { template&lt;typename U&gt; static decltype(U().apply&lt;0u&gt;(double())) func( U* ); template&lt;typename U&gt; static no_type func( ... ); static const bool result = !std::is_same&lt;no_type, decltype(func&lt;T&gt;(nullptr))&gt;::value; }; class A { public: template&lt; unsigned n &gt; void apply( const double&amp; ); }; class B { }; int main() { std::cout &lt;&lt; std::boolalpha &lt;&lt; has_apply&lt; A &gt;::result &lt;&lt; '\n'; std::cout &lt;&lt; std::boolalpha &lt;&lt; has_apply&lt; B &gt;::result &lt;&lt; '\n'; std::cin.get(); return( 0 ); } </code></pre>
 

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