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  1. #include <iostream>
  2. #include <type_traits>
  3.  
  4. template<typename CLASS>
  5. struct has_begin
  6. {
  7.     // NOTE: sig_matches() must come before fn_exists() as it is used for its
  8.     //       type.  No function bodies are needed as they are never called.
  9.  
  10.     // This matching sig results in a return type of true_type
  11. 	template<typename A_CLASS>
  12. 	static auto
  13.         sig_matches(void(A_CLASS::*)())
  14.         -> std::true_type;
  15.  
  16. 	// If the member function A_CLASS::begin exists and a sig_matches() function
  17.     // exists with the required sig, then the return type is the return type of
  18.     // sig_matches(), otherwise this function can't exist because at least one
  19.     // the types don't exist so match against fn_exists(...).
  20. 	template <typename A_CLASS>
  21. 	static auto
  22. 		fn_exists(std::nullptr_t)          
  23.         -> decltype(sig_matches<A_CLASS>(&A_CLASS::begin));
  24.  
  25. 	// Member function either doesn't exist or doesn't match against a 
  26.     // sig_matches() function.
  27. 	template<typename A_CLASS>
  28. 	static auto
  29.         fn_exists(...)
  30.         -> std::false_type;
  31.  
  32.     // Intermediate storeage of type for clarity
  33. 	typedef decltype(fn_exists<CLASS>(nullptr)) type;
  34.  
  35.     // Storing the resulting value
  36. 	static int const value = type::value;
  37. };
  38.  
  39.  
  40. struct A
  41. {
  42.     void begin()
  43.     {
  44.         std::cout << "begin() called 1" << std::endl;
  45.     }
  46. };
  47.  
  48. struct B
  49. {
  50. };
  51.  
  52. struct C
  53. {
  54.     void begin()
  55.     {
  56.         std::cout << "begin() called 1" << std::endl;
  57.     }
  58.  
  59.     void begin(float)
  60.     {
  61.         std::cout << "begin() called 2" << std::endl;
  62.     }
  63. };
  64.  
  65. template<typename T, typename...ARGs>
  66. typename std::enable_if<!!has_begin<T>::value>::type
  67.     call(ARGs...args)
  68. {
  69.     std::cout << "Found(" << has_begin<T>::value << ")" << std::endl;
  70.     T().begin(args...);
  71. }
  72.  
  73. template<typename T, typename...ARGs>
  74. typename std::enable_if<!has_begin<T>::value>::type
  75.     call(ARGs...)
  76. {
  77.     std::cout << "NOT Found(" << has_begin<T>::value << ")" << std::endl;
  78. }
  79.  
  80. int main()
  81. {
  82.     call<A>(); // A::begin() called
  83.     call<B>(); // B has no begin()
  84.     call<C>(); // C::begin() is not called.
  85.     return 0;
  86. }
  87.  
Success #stdin #stdout 0s 3096KB
comments (?)
stdin 
Standard input is empty
stdout 
Found(1)
begin() called 1
NOT Found(0)
Found(1)
begin() called 1
https://ideone.com/x40Qav
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
public

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