How to check for the existence of a subscript operator?

Question

I want to write a type trait which uses SFINAE to check a type for the existence of a subscript expression. My initial attempt below seems to work when the subscript expression

Answer 1

Once you have C++11, it's a lot easier to write type traits... you don't need to use the ellipsis overload trick. You can just use your decltype expression directly with the help of the magical:

template 
using void_t = void;

We have our base case:

template
struct has_subscript_operator : std::false_type { };

And our expression SFINAE valid case:

template
struct has_subscript_operator()[std::declval()])
>> : std::true_type { };

And then you can write the same alias:

template 
using has_subscript_operator_t = typename has_subscript_operator::type;

You can also use @Yakk's favorite method, which given this boilerplate that he copies in every answer:

namespace details {
  templatestruct voider{using type=void;};
  templateusing void_t=typename voider::type;

  templateclass Z, class, class...Ts>
  struct can_apply:
    std::false_type
  {};
  templateclass Z, class...Ts>
  struct can_apply>, Ts...>:
    std::true_type
  {};
}
templateclass Z, class...Ts>
using can_apply=details::can_apply;

You can then simply write properties:

template 
using subscript_t = decltype(std::declval()[std::declval()]);

template 
using has_subscript = can_apply;