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  1. #include <iostream>
  2. using namespace std;
  3.  
  4. /// function traits for member functions 
  5. // gives you the class_type, return_type and arguments tuple for any member function (const and not const)
  6.   template<typename T>
  7.   struct func_traits{
  8.   	static const bool is_const_call = false;
  9.     typedef void class_type;
  10.     typedef void return_type;
  11.     typedef void args_tuple;
  12.   };
  13.   // specialization for non const call member functions
  14.   template < typename TClass ,typename TRet, typename ... TArgs>
  15.   struct func_traits<TRet(TClass::*)(TArgs...)>
  16.   {
  17.     static const bool is_const_call = false;
  18.     typedef TClass class_type;
  19.     typedef TRet return_type;
  20.     typedef std::tuple<TArgs...> args_tuple;
  21.   };
  22.   // specialization for const call member function
  23.   template < typename TClass, typename TRet, typename ... TArgs>
  24.   struct func_traits<TRet(TClass::*)(TArgs...)const>
  25.   {
  26.     static const bool is_const_call = true;
  27.     typedef TClass class_type;
  28.     typedef TRet return_type;
  29.     typedef std::tuple<TArgs...> args_tuple;
  30.   };
  31.  
  32. // checks wether a single call operator exists by sfinae ideom
  33.   template<typename T>
  34.   struct is_callable{
  35.  
  36.   private:
  37. 	// if the argument type is deducable this function is used
  38.     template<typename TX = T>
  39.     static char check(typename func_traits<decltype(&TX::operator())>::class_type);
  40.     // else this function is used
  41.     static double check(...);
  42.     static T t;
  43.     // if first function is used the call operator exists and the size is sizeof(char) instead of sizeof(double)
  44.     enum{ _value = sizeof(check(t)) == sizeof(char) };
  45.   public:
  46.     const static bool value = _value;
  47.   };
  48.  
  49.  
  50. int main() {
  51. 	// B is a functor class, A not callable
  52.   class B{ public:void operator()(int i)const{} };
  53.   class A{};
  54.   // this order works  in MSVC 2013
  55. // static_assert(is_callable<B>::value == true, "should be true");
  56. // static_assert(is_callable<A>::value == false, "should be false");
  57.  
  58. 	// this order does not work in MSVC 2013
  59. 	static_assert(is_callable<A>::value == false, "should be false");
  60. 	static_assert(is_callable<B>::value == true, "should be true");
  61.  
  62. 	// 
  63. 	return 0;
  64. }
  65.  
  66.  
Success #stdin #stdout 0s 3292KB
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https://ideone.com/mpXcI9
language:
C++14 (gcc 8.3)
created:
9 years ago
visibility:
public

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