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  1. #include <iostream>
  2. #include <functional>
  3.  
  4. template <typename T> struct TagTraits;
  5.  
  6. // Generic implementation of A based on TagTraits.
  7. class A {
  8.  
  9.    template <typename Tag, typename... Args>
  10.       class Execute {
  11.          const A* a;
  12.          public:
  13.          Execute (const A* a_) : a(a_) {}
  14.          typename TagTraits<Tag>::return_type operator()(Args&&... args) const
  15.          {
  16.             return (a->*(TagTraits<Tag>::get_funtion_ptr()))(std::forward<Args>(args)...);
  17.          }
  18.       };
  19.  
  20.    public:
  21.  
  22.    virtual int foo (int) const = 0;
  23.    virtual void bar (char, double) const = 0;
  24.  
  25.    template <typename Tag, typename... Args>
  26.       typename TagTraits<Tag>::return_type execute(Args&&... args) const
  27.       {
  28.          return Execute<Tag, Args...>(this)(std::forward<Args>(args)...);
  29.       }
  30. };
  31. // tag for foo and the corresponding TagTraits
  32. struct foo_tag {};
  33.  
  34. template <> struct TagTraits<foo_tag>
  35. { 
  36.    using return_type = int;
  37.    static int (A::*get_funtion_ptr())(int) const { return &A::foo;}
  38. };
  39. // tag for bar and the corresponding TagTraits
  40. struct bar_tag {};
  41.  
  42. template <> struct TagTraits<bar_tag>
  43. {
  44.    using return_type = void;
  45.    static decltype(&A::bar) get_funtion_ptr(){ return &A::bar;}
  46. };
  47. // Derived classes of A.
  48.  
  49. class B : public A {
  50.    virtual int foo (int) const {std::cout << "B::foo() called.\n";  return 3;}
  51.    virtual void bar (char, double) const {std::cout << "B::bar() called.\n";}
  52. };
  53.  
  54. class C : public B {
  55.    virtual int foo (int) const {std::cout << "C::foo() called.\n";  return 8;}
  56.    virtual void bar (char, double) const {std::cout << "C::bar() called.\n";}
  57. };
  58. // Test B
  59. void test_B()
  60. {
  61.    A* aPtr = new B;
  62.  
  63.    int n = aPtr->foo(5);  // B::foo() called.
  64.    aPtr->bar(3, 'c');  // B::bar() called.
  65.  
  66.    n = aPtr->execute<foo_tag>(5);  // B::foo() called.
  67.    aPtr->execute<bar_tag>(3, 'c');  // B::bar() called.
  68. }
  69.  
  70. // Test C
  71. void test_C()
  72. {
  73.    A* aPtr = new C;
  74.  
  75.    int n = aPtr->foo(5);  // C::foo() called.
  76.    aPtr->bar(3, 'c');  // C::bar() called.
  77.  
  78.    n = aPtr->execute<foo_tag>(5);  // C::foo() called.
  79.    aPtr->execute<bar_tag>(3, 'c');  // C::bar() called.
  80. }
  81.  
  82. int main()
  83. {
  84.    test_B();
  85.    test_C();
  86. }
  87.  
Success #stdin #stdout 0s 3412KB
comments (?)
stdin
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Standard input is empty
stdout
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B::foo() called.
B::bar() called.
B::foo() called.
B::bar() called.
C::foo() called.
C::bar() called.
C::foo() called.
C::bar() called.
https://ideone.com/XspEyN
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
secret

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