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  1.     #include <iostream>
  2.     #include <utility>
  3.  
  4.     struct Foo {};
  5.     struct Bar { Foo _foo; };
  6.     struct Bad {};
  7.  
  8.     template <typename T, typename U> struct same_type: std::false_type {};
  9.  
  10.     template <typename T> struct same_type<T, T>: std::true_type {};
  11.  
  12.     Foo const& to_foo(Bar const& b) { return b._foo; }
  13.     Foo& to_foo(Bar& b) { return b._foo; }
  14.  
  15.     template <typename T>
  16.     struct ToFoo {
  17.       T const& _crt;
  18.       T& _rt;
  19.  
  20.       template <typename U>
  21.       static auto to_foo_exists(U const& crt, U& rt) ->
  22.           decltype(to_foo(crt), to_foo(rt), std::true_type());
  23.  
  24.       static std::false_type to_foo_exists(...);
  25.  
  26.       static bool const value = same_type<decltype(to_foo_exists(_crt, _rt)), std::true_type>::value;
  27.     };
  28.  
  29.     int main() {
  30.       std::cout << ToFoo<Bar>::value << "\n";
  31.       std::cout << ToFoo<Bad>::value << "\n";
  32.     }
Success #stdin #stdout 0s 2828KB
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https://ideone.com/CHJM0
language:
C++14 (gcc 8.3)
created:
12 years ago
visibility:
public

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