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  1. #include <iostream>
  2. #include <type_traits>
  3. using namespace std;
  4.  
  5. template<typename FROM, typename TO>
  6. struct CopyConstImpl {
  7. 	using type = typename std::remove_const<TO>::type;
  8. };
  9.  
  10. template<typename FROM, typename TO>
  11. struct CopyConstImpl<const FROM, TO> {
  12. 	using type = typename std::add_const<TO>::type;
  13. };
  14.  
  15. template<typename FROM, typename TO>
  16. struct CopyConst {
  17. 	using type = typename CopyConstImpl<typename std::remove_reference<FROM>::type, TO>::type;
  18. };
  19.  
  20. struct Foo {
  21.   template<typename T>
  22.   auto bar(T&&) -> typename CopyConst<T, int>::type& {
  23.   	static_assert(is_same<typename decay<T>::type, int>::value,
  24.   		"Because probably the only reason this is templated now is for this behavior"
  25.   		"and what you really wanted to say was 'give me an int please'");
  26.   	return x;
  27.   }
  28.   int x = 5;
  29. };
  30.  
  31. int main() {
  32. 	static_assert(is_same<int&, decltype(declval<Foo>().bar(declval<int>()))>::value,
  33. 		"Non-const arg to foo.bar() result must lead to non-const reference result");
  34. 	static_assert(is_same<const int&, decltype(declval<Foo>().bar(declval<const int>()))>::value,
  35. 		"Const arg to foo.bar() result must lead to const reference result");
  36.  
  37. 	return 0;
  38. }
Success #stdin #stdout 0s 3456KB
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stdout 
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https://ideone.com/wgaDJJ
inout on parameter
language:
C++14 (gcc 8.3)
created:
8 years ago
visibility:
public

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