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  1. #include <utility>
  2. #include <type_traits>
  3.  
  4.  
  5. template<typename T>
  6. using NoRef = typename std::remove_reference<T>::type;
  7.  
  8. template<typename T>
  9. using EnableIf = typename std::enable_if<T::value, void>::type;
  10.  
  11. template<typename T, typename = void>
  12. struct is_callable_impl : std::is_function<T> {};
  13.  
  14. template<typename T>
  15. struct is_callable_impl<T, EnableIf<std::is_class<NoRef<T>>>> {
  16.     using yes = char;
  17.     using no = struct { char s[2]; };
  18.  
  19.     struct F { void operator()(); };
  20.     struct Derived : T, F { };
  21.     template<typename U, U> struct Check;
  22.  
  23.     template<typename V>
  24.     static no test(Check<void (F::*)(), &V::operator()>*);
  25.  
  26.     template<typename>
  27.     static yes test(...);
  28.  
  29.     static const bool value = sizeof(test<Derived>(0)) == sizeof(yes);
  30. };
  31.  
  32. template<typename T>
  33. struct Callable : std::integral_constant<bool, is_callable_impl<T>::value> {};
  34.  
  35. struct A {};
  36. struct B { 
  37.     void operator()(); 
  38. };
  39.  
  40. struct C { 
  41.     char operator()(int, int); 
  42. };
  43. struct D {
  44.     template<typename T, typename U>
  45.     void operator()(T, U);
  46.     float operator()(int, char, double);
  47. };
  48.  
  49. struct E {
  50.     template<typename T>
  51.     char operator()(T, double);
  52. };
  53.  
  54. int main() {
  55.     static_assert(!Callable<int>::value, "...");
  56.     static_assert(Callable<void()>::value, "...");
  57.     auto l = [] () { };
  58.     static_assert(Callable<decltype(l)>::value, "...");
  59.     static_assert(!Callable<A>::value, "...");
  60.     static_assert(Callable<B>::value, "...");
  61.     static_assert(Callable<C>::value, "...");
  62.     static_assert(Callable<D>::value, "...");
  63.     static_assert(Callable<E>::value, "...");
  64. }
Success #stdin #stdout 0s 3336KB
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stdout 
Standard output is empty
https://ideone.com/bnnsMp
language:
C++14 (gcc 8.3)
created:
12 years ago
visibility:
secret

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