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  1. #include <iostream>
  2. #include <vector>
  3.  
  4. template <class T>
  5. struct holder {
  6.     std::vector<T>	data;
  7. };
  8.  
  9. struct gameboy;
  10.  
  11. struct ball { ball(gameboy *) {} };
  12. struct door { door(gameboy *) {} };
  13. struct piss { piss(gameboy *) {} };
  14. struct foo {};
  15.  
  16. struct gameboy
  17. 	: private holder<ball>
  18. 	, private holder<door>
  19. 	, private holder<piss>
  20. {
  21. 	template <class T>
  22. 	T & push_back()
  23. 	{
  24. 		holder<T> & ancestor = static_cast<holder<T> &>(*this);	// compile error here if type mismatch
  25. 		ancestor.data.push_back(T(this));
  26. 		return *ancestor.data.rbegin();
  27. 	}
  28.  
  29. 	template <class T>
  30. 	typename std::vector<T>::size_type size() const 
  31. 	{
  32. 		return holder<T>::data.size(); // or you can just ask your ancestor directly
  33. 	}
  34. };
  35.  
  36. int main()
  37. {
  38. 	gameboy boy;
  39. 	boy.push_back<ball>();
  40. 	boy.push_back<door>();
  41. 	boy.push_back<piss>();
  42. 	boy.push_back<door>();
  43.  
  44. 	std::cout << boy.size<door>() << std::endl;
  45.     // std::cout << boy.size<foo>() << std::endl;   // cannot use foo!
  46.  
  47. 	return 0;
  48. }
Success #stdin #stdout 0.01s 2816KB
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https://ideone.com/3nkale
language:
C++ (gcc 8.3)
created:
13 years ago
visibility:
public

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