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  1. #include <iostream>
  2.  
  3. class B;
  4.  
  5. class A {
  6.   double d;
  7. public:
  8.   A(double a) : d(a) {}
  9.   friend class B;
  10.   double operator*(B& b); // auto operator*(B& b) -> decltype(d * B::i) と書けない
  11. };
  12.  
  13. class B {
  14.   int i;
  15. public:
  16.   B(int j) : i(j) {}
  17.   friend class A;
  18.   auto operator*(A& a) -> decltype(i * A::d) {
  19.     return i * a.d;
  20.   }
  21. };
  22.  
  23. auto A::operator*(B& b) -> decltype(d * B::i)
  24. {
  25.   return d * b.i;
  26. }
  27.  
  28. template<class T, class U>
  29. auto mul(T x, U y) -> decltype(x * y)
  30. {
  31.   return x * y;
  32. }
  33.  
  34. int main()
  35. {
  36.   A a(3.0);
  37.   B b(5);
  38.  
  39.   std::cout << mul(a, b) << std::endl;
  40.   std::cout << a * b << std::endl;
  41. }
Success #stdin #stdout 0s 3340KB
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https://ideone.com/LDs6Av
language:
C++14 (gcc 8.3)
created:
11 years ago
visibility:
public

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