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POMacro compiles with GCC but not with VS11
 

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  1. POMacro compiles with GCC but not with VS11
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    <p>I have written the following code to assist restricting my template functions to certain types, with meaningful error message when other types were used. I had the idea from a different question in stackoverflow, on which I still can't comment because I am new here.</p> <p>The macro compiles flawlessly under linux GCC but not with Visual Studio 2012.</p> <pre><code>#include &lt;string&gt; #include &lt;iostream&gt; #include &lt;vector&gt; #include &lt;cassert&gt; #include &lt;type_traits&gt; #define ISALLOWED(DerivedT) (std::is_same&lt;T, DerivedT&gt;::value)||(std::is_base_of&lt;T,DerivedT&gt;::value) #define FE_1(WHAT, X) WHAT(X) #define FE_2(WHAT, X, ...) WHAT(X) || FE_1(WHAT, __VA_ARGS__) #define FE_3(WHAT, X, ...) WHAT(X) || FE_2(WHAT, __VA_ARGS__) #define FE_4(WHAT, X, ...) WHAT(X) || FE_3(WHAT, __VA_ARGS__) #define FE_5(WHAT, X, ...) WHAT(X) || FE_4(WHAT, __VA_ARGS__) #define FE_6(WHAT, X, ...) WHAT(X) || FE_5(WHAT, __VA_ARGS__) //... repeat as needed #define GET_MACRO(_1,_2,_3,_4,_5,_6,NAME,...) NAME #define FOR_EACH(action,...) \ GET_MACRO(__VA_ARGS__,FE_6,FE_5,FE_4,FE_3,FE_2,FE_1)(action,__VA_ARGS__) // this is where you need to add types #define ASSERTIOTYPES \ static_assert(FOR_EACH(ISALLOWED,\ int,double,std::string\ ),"Type not defined for this template."); template&lt;class T&gt; std::ostream &amp; operator&lt;&lt;(std::ostream &amp;os, const std::vector&lt;T&gt; &amp; v) { ASSERTIOTYPES; os &lt;&lt; "["; for (size_t i = 0; i &lt; v.size(); i++) { os &lt;&lt; v[i]; if (i != v.size() - 1) os &lt;&lt; ", "; } os &lt;&lt; "]"; return os; } </code></pre> <p>The error message is: error C2977: 'std::is_same' : too many template arguments</p> <p>And it appears only when I define ASSERTIOTYPES with more than one type, but when it is defined with only one type, for example:</p> <pre><code>#define ASSERTIOTYPES \ static_assert(FOR_EACH(ISALLOWED,\ int\ ),"Type not defined for this template."); </code></pre> <p>... the code compiles normally.</p> <p>Any idea how to solve this?</p>
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    1. COPeople who do advanced preprocessor stuff consider the VS preprocessor seriously broken. I'd suggest you use [Boost.Preprocessor](http://www.boost.org/doc/libs/1_55_0/libs/preprocessor/doc/index.html) istead of handwritten macros if possible - they have the pecularities covered.
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    2. COComma-separated token sequences are expanded too late, resulting in incorrect argument-parameter matching during macro replacement. This problem is especially acute with common uses of `__VA_ARGS__` (like this one). You can use indirection to work around the buggy behavior. See e.g. the example I showed [in an answer to another question](http://stackoverflow.com/a/10234230/151292). That said, Yakk's solution is the right solution for this; much cleaner than mucking with macros.
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      1. USJames McNellis
 

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