SFINAE: decltype on operator[]

Question

Based on answers here and here I am trying to use the following

template <typename T>
using operator_square_brackets = decltype(&T::operator[]);

It fails on visual studio with

error C2760: syntax error: expected ')' not ']'

Any ideas on how to fix this?

Answer 1

If you want to detect whether a type has a certain function or overloaded operator you have to call that function or operator. This is important because you might have several overloads of a function or operator and overload resolution always depends on the caller.

Here is a small example, based on CppCon 2014: Walter E. Brown "Modern Template Metaprogramming: A Compendium, Part II" on how to detect operator[] in a type.

I have no idea why VC is giving you such a weird error which looks more like a parsing error. I would have expected something like »reference to overloaded function could not be resolved; did you mean to call it?«.

#include <string>
#include <type_traits>
#include <vector>

// in C++17 std::void_t
template < typename... >
using void_t = void;


template < typename T, typename Index >
using subscript_t = decltype(std::declval<T>()[std::declval<Index>()]);

template < typename, typename Index = size_t, 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 {};


struct A
{
  void operator[](size_t) {}
};

struct B {};

int main ()
{
  static_assert(has_subscript< std::vector<int> >::value    == true , "!");
  static_assert(has_subscript< std::vector<double> >::value == true , "!");
  static_assert(has_subscript< A >::value                   == true , "!");
  static_assert(has_subscript< A, std::string >::value      == false, "!");
  static_assert(has_subscript< B >::value                   == false, "!");
  static_assert(has_subscript< double[5] >::value           == true , "!");
  static_assert(has_subscript< double* >::value             == true , "!");
  static_assert(has_subscript< double >::value              == false, "!");
}