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  1. #include <iostream>
  2. using namespace std;
  3.  
  4. class A
  5. {
  6. public:
  7. 	bool operator==(const A& a) const
  8. 	{
  9. 		return equals(a);
  10. 	}
  11. protected:
  12. 	virtual bool equals(const A& a) const = 0;
  13. };
  14.  
  15. template<class T>
  16. class A_ : public A
  17. {
  18. protected:
  19. 	virtual bool equals(const A& a) const
  20. 	{
  21. 		const T* other = dynamic_cast<const T*>(&a);
  22. 		return other != nullptr && static_cast<const T&>(*this) == *other;
  23. 	}
  24. private:
  25. 	bool operator==(const A_& a) const	// force derived classes to implement their own operator==
  26. 	{
  27. 		return false;
  28. 	}
  29. };
  30.  
  31. class B : public A_<B>
  32. {
  33. public:
  34. 	B(int i) : id(i) {}
  35. 	bool operator==(const B& other) const
  36. 	{
  37. 		return id == other.id;
  38. 	}
  39. private:
  40. 	int id;
  41. };
  42.  
  43. class C : public A_<C>
  44. {
  45. public:
  46. 	C(int i) : identity(i) {}
  47. 	bool operator==(const C& other) const
  48. 	{
  49. 		return identity == other.identity;
  50. 	}
  51. private:
  52. 	int identity;
  53. };
  54.  
  55. const char * TF(bool b)
  56. {
  57. 	return b ? "True" : "False";
  58. }
  59.  
  60. int main() {
  61. 	B b1(1);
  62. 	B b2(2);
  63. 	C c1(1);
  64. 	C c2(1);
  65. 	A& a1 = b1;
  66. 	A& a2 = b2;
  67. 	A& a3 = c1;
  68. 	A& a4 = c2;
  69. 	cout << "a1 == b1: " << TF(a1 == b1) << endl;
  70. 	cout << "a1 == a2: " << TF(a1 == a2) << endl;
  71. 	cout << "a2 == a3: " << TF(a2 == a3) << endl;
  72. 	cout << "a3 == a4: " << TF(a3 == a4) << endl;
  73.  
  74. 	// your code goes here
  75. 	return 0;
  76. }
Success #stdin #stdout 0s 3296KB
comments (?)
stdin
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Standard input is empty
stdout
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a1 == b1: True
a1 == a2: False
a2 == a3: False
a3 == a4: True
https://ideone.com/SymduV
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
public

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