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  1. #include <iostream>
  2.  
  3. // A factory class that will not call anything except constructor. Even if it could.
  4. template<typename T> class HonestFactory
  5. {
  6. 	// A factory class that would like to mess with private methods. But it can't.
  7. 	friend class HeinousFactory;
  8.  
  9. 	// This will ensure only Heinous one can use it.
  10. 	HonestFactory() { }
  11.  
  12. 	// Real one should be a variadic template.
  13. 	T Create()
  14. 	{
  15. 		return T{};
  16. 	}
  17. };
  18.  
  19. class Foo
  20. {
  21. 	friend class HonestFactory<Foo>; // We trust the honest one.
  22. 	Foo() { }
  23. 	int m_foo {0};
  24.  
  25. public:
  26. 	int m_bar {1};
  27. };
  28.  
  29. class HeinousFactory
  30. {
  31. public:
  32. 	Foo CreateFoo()
  33. 	{
  34. 		HonestFactory<Foo> factory;
  35. 		Foo foo = factory.Create();
  36. 		//foo.m_foo = 1; // Error.
  37. 		foo.m_bar = 1;
  38. 		return foo;
  39. 	}
  40. };
  41.  
  42. int main()
  43. {
  44. 	//Foo foo1; // No.
  45. 	//HonestFactory<Foo> factory; // Nope.
  46. 	HeinousFactory factory;
  47. 	Foo foo2 = factory.CreateFoo();
  48. 	std::cout << foo2.m_bar << std::endl;
  49. }
Success #stdin #stdout 0s 3468KB
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https://ideone.com/4vUgbK
language:
C++14 (gcc 8.3)
created:
9 years ago
visibility:
public

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