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    <p>Note: These assume that the default constructor is valid for the type your checking. Not sure offhand how to get around that.</p> <p>The following seems to work if it's callable with 0 arguments. Is there something in is_function's implementation that might help to extend this to 1 or more argument callables?:</p> <pre><code>template &lt;typename T&gt; struct is_callable { // Types "yes" and "no" are guaranteed to have different sizes, // specifically sizeof(yes) == 1 and sizeof(no) == 2. typedef char yes[1]; typedef char no[2]; template &lt;typename C&gt; static yes&amp; test(decltype(C()())*); template &lt;typename&gt; static no&amp; test(...); // If the "sizeof" the result of calling test&lt;T&gt;(0) would be equal to the sizeof(yes), // the first overload worked and T has a nested type named foobar. static const bool value = sizeof(test&lt;T&gt;(0)) == sizeof(yes); }; </code></pre> <p>If you know the type of the argument (even if it's a template parameter), the following would work for 1 argument, and I imagine one could extend pretty easily from there:</p> <pre><code>template &lt;typename T, typename T2&gt; struct is_callable_1 { // Types "yes" and "no" are guaranteed to have different sizes, // specifically sizeof(yes) == 1 and sizeof(no) == 2. typedef char yes[1]; typedef char no[2]; template &lt;typename C&gt; static yes&amp; test(decltype(C()(T2()))*); template &lt;typename, typename&gt; static no&amp; test(...); // If the "sizeof" the result of calling test&lt;T&gt;(0) would be equal to the sizeof(yes), // the first overload worked and T has a nested type named foobar. static const bool value = sizeof(test&lt;T&gt;(0)) == sizeof(yes); }; </code></pre> <p><strong>Edit</strong> <a href="http://pastebin.com/i57SEY7a" rel="nofollow">here</a> is a modification that handles the case where default constructor isn't available.</p>
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    1. CORe *These assume that the default constructor is valid for the type your checking. Not sure offhand how to get around that.* Take a look at [`std::declval`](http://en.cppreference.com/w/cpp/utility/declval).
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    2. CO@jrok Thanks, I hadn't seen that one yet. in the pastebin I attached, I just used a helper struct that had the necessary conversion operator defined, but I supposed I could replace it with declval.
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