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POUsing SFINAE to check for global operator<<?
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  1. POUsing SFINAE to check for global operator<<?
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    copied!<p>I want to have several overloaded, global <code>to_string()</code> functions that take some type <code>T</code> and convert it to its string representation. For the general case, I want to be able to write:</p> <pre><code>template&lt;typename T,class OutputStringType&gt; inline typename enable_if&lt;!std::is_pointer&lt;T&gt;::value &amp;&amp; has_insertion_operator&lt;T&gt;::value, void&gt;::type to_string( T const &amp;t, OutputStringType *out ) { std::ostringstream o; o &lt;&lt; t; *out = o.str(); } </code></pre> <p>My implementation of <code>has_insertion_operator</code> so far is:</p> <pre><code>struct sfinae_base { typedef char yes[1]; typedef char no[2]; }; template&lt;typename T&gt; struct has_insertion_operator : sfinae_base { template&lt;typename U&gt; static yes&amp; test( U&amp; ); template&lt;typename U&gt; static no&amp; test(...); static std::ostream &amp;s; static T const &amp;t; static bool const value = sizeof( test( s &lt;&lt; t ) ) == sizeof( yes ); // line 48 }; </code></pre> <p>(It borrows from <a href="https://stackoverflow.com/questions/257288/">this</a> and <a href="https://stackoverflow.com/questions/4434569/">this</a>.) That seems to work. But now I want to have an overloaded version of <code>to_string</code> for types that do <em>not</em> have <code>operator&lt;&lt;</code> but <em>do</em> have their own <code>to_string()</code> <em>member</em> function, i.e.:</p> <pre><code>template&lt;class T,class OutputStringType&gt; inline typename enable_if&lt;!has_insertion_operator&lt;T&gt;::value &amp;&amp; has_to_string&lt;T,std::string (T::*)() const&gt;::value, void&gt;::type to_string( T const &amp;t, OutputStringType *out ) { *out = t.to_string(); } </code></pre> <p>The implementation of <code>has_to_string</code> is:</p> <pre><code>#define DECL_HAS_MEM_FN(FN_NAME) \ template&lt;typename T,typename S&gt; \ struct has_##FN_NAME : sfinae_base { \ template&lt;typename SignatureType,SignatureType&gt; struct type_check; \ template&lt;class U&gt; static yes&amp; test(type_check&lt;S,&amp;U::FN_NAME&gt;*); \ template&lt;class U&gt; static no&amp; test(...); \ static bool const value = sizeof( test&lt;T&gt;(0) ) == sizeof( yes ); \ } DECL_HAS_MEM_FN( to_string ); </code></pre> <p>(This part seems to work fine. It's adapted from <a href="https://stackoverflow.com/questions/257288/">this</a>.) However, when I have:</p> <pre><code>struct S { string to_string() const { return "42"; } }; int main() { string buf; S s; to_string( s, &amp;buf ); // line 104 } </code></pre> <p>I get:</p> <pre><code>foo.cpp: In instantiation of ‘const bool has_insertion_operator&lt;S&gt;::value’: foo.cpp:104: instantiated from here foo.cpp:48: error: no match for ‘operator&lt;&lt;’ in ‘has_insertion_operator&lt;S&gt;::s &lt;&lt; has_insertion_operator&lt;S&gt;::t’ </code></pre> <p>It seems like SFINAE is not happening. How do I write <code>has_insertion_operator</code> correctly such that it determines whether a global <code>operator&lt;&lt;</code> is available?</p> <p>FYI: I'm using g++ 4.2.1 (that which ships as part of Xcode on Mac OS X). Also, I'd like the code to be only standard C++03 without 3rd-party libraries, e.g., Boost.</p> <p>Thanks!</p>
 

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