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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 remove_const<TO>::type;
  8. };
  9.  
  10. template<typename FROM, typename TO>
  11. struct CopyConstImpl<const FROM, TO> {
  12. 	using type = typename add_const<TO>::type;
  13. };
  14.  
  15. template<typename FROM, typename TO>
  16. struct CopyConst {
  17. 	using type = typename CopyConstImpl<typename remove_reference<FROM>::type, TO>::type;
  18. };
  19.  
  20. struct Foo {
  21.   auto bar() -> typename CopyConst<decltype(*this), int>::type& {
  22.   	return x;
  23.   }
  24.   int x = 5;
  25. };
  26.  
  27. int main() {
  28. 	static_assert(is_same<int&, decltype(declval<Foo>().bar())>::value,
  29. 		"Non-const foo.bar() result must yield a non-const reference");
  30. 	//static_assert(is_same<const int&, decltype(declval<const Foo>().bar())>::value,
  31. 	//	"Const foo.bar() result must yield a const reference");
  32. 	const Foo foo;
  33. 	foo.bar();
  34.  
  35. 	return 0;
  36. }
Compilation error #stdin compilation error #stdout 0s 3336KB
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stdin
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Standard input is empty
compilation info 
prog.cpp: In function ‘int main()’:
prog.cpp:33:10: error: passing ‘const Foo’ as ‘this’ argument of ‘CopyConst<Foo&, int>::type& Foo::bar()’ discards qualifiers [-fpermissive]
  foo.bar();
          ^
stdout
copy 
Standard output is empty
https://ideone.com/W4fTa4
language:
C++14 (gcc 8.3)
created:
9 years ago
visibility:
public

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