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  1. #include <iostream>
  2.  
  3. class A { };
  4. class B { };
  5.  
  6. class Executor {
  7. public:
  8. 	// Callback base class including placeholders for operators that
  9. 	// are not overridden.
  10. 	class Callback {
  11. 	public:
  12. 		virtual ~Callback() { }
  13.  
  14. 		virtual void operator()(A const &) {
  15. 			std::cout << "Placeholder A" << std::endl;
  16. 		}
  17. 		virtual void operator()(B const &) {
  18. 			std::cout << "Placeholder B" << std::endl;
  19. 		}
  20. 	};
  21.  
  22. 	// Template method that can take a Callback directly, or it can
  23. 	// take a lambda and will wrap it as a Callback.
  24. 	template <typename T_Callback>
  25. 	void execute(T_Callback &&callback);
  26. private:
  27. 	// This method finds As and Bs to call the callback with.
  28. 	// It is not a template method and can be located in a source
  29. 	// (rather than header) file.
  30. 	void run(Callback &callback) {
  31. 		A a;
  32. 		B b;
  33. 		callback(a);
  34. 		callback(b);
  35. 	}
  36.  
  37. 	// Callback class for wrapping a lambda taking A.
  38. 	template <typename T_Callback>
  39. 	class CallbackFunctionA : public Callback {
  40. 	public:
  41. 		CallbackFunctionA(T_Callback &callback)
  42. 		: callback_(callback) { }
  43.  
  44. 		// Define operator taking A.
  45. 		virtual void operator()(A const &a) {
  46. 			callback_(a);
  47. 		}
  48. 		// Use other placeholder operators.
  49. 		using Callback::operator();
  50. 	private:
  51. 		T_Callback &callback_;
  52. 	};
  53.  
  54. 	// Callback class for wrapping a lambda taking B.
  55. 	template <typename T_Callback>
  56. 	class CallbackFunctionB : public Callback {
  57. 	public:
  58. 		CallbackFunctionB(T_Callback &callback)
  59. 		: callback_(callback) { }
  60.  
  61. 		// Define operator taking B.
  62. 		virtual void operator()(B const &b) {
  63. 			callback_(b);
  64. 		}
  65. 		// Use other placeholder operators.
  66. 		using Callback::operator();
  67. 	private:
  68. 		T_Callback &callback_;
  69. 	};
  70.  
  71. 	// Determine type in which to wrap T_Callback.
  72. 	template <typename T_Callback>
  73. 	class CallbackType {
  74. 	private:
  75. 		template <typename T>
  76. 		static CallbackFunctionA<T> testlambda(void (T::*op)(A const &) const);
  77. 		template <typename T>
  78. 		static CallbackFunctionB<T> testlambda(void (T::*op)(B const &) const);
  79.  
  80. 		template <typename T>
  81. 		static decltype(testlambda<T>(&T::operator())) testany(int);
  82. 		template <typename T>
  83. 		static T &testany(...);
  84. 	public:
  85. 		typedef decltype(testany<T_Callback>(0)) type;
  86. 	};
  87. };
  88.  
  89. template <typename T_Callback>
  90. void Executor::execute(T_Callback &&callback) {
  91. 	// Determine callback class to use and instantiate it.
  92. 	typename CallbackType<T_Callback>::type cb = callback;
  93. 	run(cb);
  94. }
  95.  
  96. int main() {
  97. 	Executor executor;
  98.  
  99. 	// Execute executor given a functor taking either A or B.
  100. 	struct : Executor::Callback {
  101. 		virtual void operator()(A const &) {
  102. 			std::cout << "Functor A" << std::endl;
  103. 		}
  104. 		virtual void operator()(B const &) {
  105. 			std::cout << "Functor B" << std::endl;
  106. 		}
  107. 	} callback;
  108. 	executor.execute(callback);
  109.  
  110. 	// Execute executor given a lambda taking an A.
  111. 	executor.execute(
  112. 		[](A const &) {
  113. 			std::cout << "Lambda A" << std::endl;
  114. 		}
  115. 	);
  116.  
  117. 	// Execute executor given a lambda taking a B.
  118. 	executor.execute(
  119. 		[](B const &) {
  120. 			std::cout << "Lambda B" << std::endl;
  121. 		}
  122. 	);
  123. }
  124.  
Success #stdin #stdout 0s 3344KB
comments (?)
stdin
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Standard input is empty
stdout
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Functor A
Functor B
Lambda A
Placeholder B
Placeholder A
Lambda B
https://ideone.com/ZWBHAM
language:
C++14 (gcc 8.3)
created:
9 years ago
visibility:
public

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