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  1. #include <iostream>
  2.  
  3. template <typename T>
  4. class has_size {
  5. private:
  6.   typedef char Yes;
  7.   typedef Yes No[2];
  8.  
  9.   template <typename U, U> struct really_has;
  10.  
  11.   template<typename C> static Yes& Test(really_has <size_t (C::*)() const, &C::size>*);
  12.   template<typename> static No& Test(...);
  13.  
  14. public:
  15.     static bool const value = sizeof(Test<T>(0)) == sizeof(Yes);
  16. };
  17.  
  18. struct NoSize {};
  19.  
  20. struct Sized { size_t size() const { return 0; } };
  21.  
  22. struct BadlySized { int size() { return 0; } };
  23.  
  24. struct MultiSized { size_t size() const { return 0; } int size(int) { return 0; } };
  25.  
  26. int main() {
  27.   std::cout << "NoSize: " << has_size<NoSize>::value << "\n";
  28.   std::cout << "Sized: " << has_size<Sized>::value << "\n";
  29.   std::cout << "BadlySized: " << has_size<BadlySized>::value << "\n";
  30.   std::cout << "MultiSized: " << has_size<MultiSized>::value << "\n";
  31. }
Success #stdin #stdout 0s 3340KB
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stdin
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Standard input is empty
stdout
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NoSize: 0
Sized: 1
BadlySized: 0
MultiSized: 1
https://ideone.com/JKo8p5
language:
C++ (gcc 8.3)
created:
10 years ago
visibility:
public

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