CppCon 2014: Walter E. Brown "Modern Template Metaprogramming: A Compendium, Part II"

https://www.youtube.com/watch?v=a0FliKwcwXE&t=29m7s

We can use the void_t trick shown in the above video to detect whether a type has a member function, overloaded operator, etc. In this example we want to detect whether a type has a subscript operator with subscript int.

The void_t uses expression SFINAE to detect the existence of something. The code is ultra short.

template < typename T, typename Index > using subscript_t = decltype(std::declval<T>()[std::declval<Index>()]); template < typename, typename Index = int, typename = void_t<> > struct has_subscript : std::false_type {}; template < typename T, typename Index > struct has_subscript< T, Index, void_t< subscript_t<T,Index> > > : std::true_type {};

We could make it even shorter by omitting subscript_t. For explanation watch the video.