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  1. #include <type_traits>
  2. #include <iostream>
  3.  
  4. struct C {};
  5.  
  6. int size (const C & c) {
  7.     std::cout << "global size!\n";
  8.     return 0;
  9. }
  10.  
  11. template <typename B>
  12. class has_size
  13. {
  14.     // This type won't compile if the second template parameter isn't of type T,
  15.     // so I can put a function pointer type in the first parameter and the function
  16.     // itself in the second thus checking that the function has a specific signature.
  17.     template <typename T, T> struct TypeCheck;
  18.  
  19.     typedef char Yes;
  20.     typedef struct { char dummy[2]; } No;
  21.  
  22.     // A helper struct to hold the declaration of the function pointer.
  23.     // Change it if the function signature changes.
  24.     template <typename T> struct Size
  25.     {
  26.         typedef int (T::*fptr)(const C&);
  27.     };
  28.  
  29.     template <typename T> static Yes HasSize(TypeCheck< typename Size<T>::fptr, &T::size >*);
  30.     template <typename T> static No  HasSize(...);
  31.  
  32. public:
  33.     static bool const value = (sizeof(HasSize<B>(0)) == sizeof(Yes));
  34. };
  35. template <class B>
  36. int callSize(B* obj, const C& c, typename std::enable_if<has_size<B>::value>::type* = 0)
  37. {
  38.     obj->size(c);
  39. }
  40. template <class B>
  41. int callSize(B*, const C& c, typename std::enable_if<!has_size<B>::value>::type* = 0)
  42. {
  43.     return size(c);
  44. }
  45.  
  46.  
  47. struct B
  48. {
  49.     int size () { /* ... */ };
  50.  
  51.     template <class U> friend int ::callSize(U*, const C&, typename std::enable_if<has_size<U>::value>::type*);
  52.     template <class U> friend class has_size;
  53.     void doSomething (const C & c)
  54.     {
  55.        int x = ::callSize(this, c); // <----------- problem!
  56.        // ...
  57.     }
  58. private:
  59. #ifdef LOCAL_SIZE
  60.     int size (const C & c) {
  61.        std::cout << "local size!\n";
  62.        return 0;
  63.      }
  64. #endif
  65. };
  66.  
  67. int main() {
  68.     B b;
  69.     b.doSomething(C());
  70. }
Success #stdin #stdout 0s 2884KB
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global size!
https://ideone.com/6TwjRz
language:
C++14 (gcc 8.3)
created:
12 years ago
visibility:
secret

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