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POtraits for testing whether func(args) is well-formed and has required return type
 

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  1. POtraits for testing whether func(args) is well-formed and has required return type
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    <p>There are a number of similar questions/answers, but I couldn't quite put those answers together to serve my purposes. I want a traits</p> <pre><code>template&lt;typename Func, typename ReturnType, typename... Args&gt; struct returns_a { static const bool value; }; </code></pre> <p>such that</p> <pre><code>returns_a&lt;F,T,Args&gt;::value </code></pre> <p>is true if <code>F(Args)</code> is well formed and returns a <code>T</code>. After some more research, I got it working as follows:</p> <pre><code>// value is true if Func(Args...) is well formed template&lt;typename Func, typename... Args&gt; class is_callable { template &lt;typename F&gt; static decltype(std::declval&lt;F&gt;()(std::declval&lt;Args&gt;()...), void(), 0) test(int); template &lt;typename&gt; static void test(...); public: static const bool value = !std::is_void&lt;decltype(test&lt;Func&gt;(0))&gt;::value; }; // return_type&lt;FunctionSignature&gt;::type is the return type of the Function template&lt;typename&gt; struct return_type {}; template&lt;typename ReturnType, typename... Args&gt; struct return_type&lt;ReturnType(Args...)&gt; { typedef ReturnType type; }; // helper class, required to use SFINAE together with variadic templates parameter // generic case: Func(Args...) is not well-defined template &lt;typename Func, typename ReturnType, typename dummy, typename... Args&gt; struct returns_a_helper { static const bool value = false; }; // Func is a function signature template &lt;typename Func, typename ReturnType, typename... Args&gt; struct returns_a_helper&lt;Func, ReturnType, typename std::enable_if&lt;std::is_function&lt;Func&gt;::value&gt;::type, Args...&gt; { static const bool value = std::is_convertible&lt;typename return_type&lt;Func&gt;::type, ReturnType&gt;::value; }; // Func(Args...) not a function call, but well-defined template &lt;typename Func, typename ReturnType, typename... Args&gt; struct returns_a_helper&lt;Func,ReturnType,typename std::enable_if&lt;is_callable&lt;Func&gt;::value &amp;&amp; !std::is_function&lt;Func&gt;::value &gt;::type, Args...&gt; { static const bool value = std::is_convertible&lt;typename std::result_of&lt;Func(Args...)&gt;::type, ReturnType&gt;::value; }; template &lt;typename Func, typename ReturnType, typename... Args&gt; struct returns_a : returns_a_helper&lt;Func, ReturnType, void, Args...&gt; {}; </code></pre> <p>which now works fine for functors and functions. Here is a simple test:</p> <pre><code>struct base { virtual bool member(int) const = 0; }; struct foo : base { bool member(int x) const { return x&amp;2; } }; struct bar { foo operator()() { return foo(); } }; foo free_function() { return foo(); } template&lt;typename T, typename Func&gt; void test(Func const&amp;func) { std::cout &lt;&lt; std::boolalpha &lt;&lt; returns_a&lt;Func,T&gt;::value &lt;&lt; std::endl; } int main() { foo x; bar m; test&lt;const base&amp;&gt;([&amp;]() { return x; }); test&lt;const base&amp;&gt;(m); test&lt;const base&amp;&gt;(free_function); return 0; } </code></pre> <p>Well, this works, but it seems a bit cumbersome. Anybody has better/more elegant/shorter solutions?</p>
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    1. CO`std::is_function<Func(Args...)>` is a wrong and unnecessary test here - it only checks if its argument is a function signature and for the free function case (where `Func == foo()`, i.e. a signature), the test is `std::is_function<foo()()>`, aka a function returning a function - [which is an error](https://ideone.com/yQn6Vk). That causes SFINAE and the primary template to be selected...
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    2. CO@Xeo okay, I now avoid that, but still cannot get it working
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