invoke_result with member (operator[]) function

Question

How do I call invoke_result correctly for a member function? Or specifically for an operator member function. I tried std::invoke_result<T::operator[], size_type> to no success. What would be the correct syntax in this case?

Answer 1

Don't. Use decltype(std::declval<T&>()[size_type{}]) or something similar (adjust the value category and the cv-qualification as needed).

invoke_result is for when you have an invocable/callable object. You don't have one, so don't try to hammer square pegs into round holes.

Answer 2

What about as follows ?

std::invoke_result<decltype(&T::operator[]), T, size_type>

But this syntax should works with a single, and not template, operator[].

In case of template or overload, you should avoid std::invoke_result and follows the decltype() way suggested by T.C.

Or, maybe, you can wrap the call in a lambda function and apply std::invoke_result to the lambda (if you really, really want to use std::invoke_result).

Regarding the std::invoke_result syntax, take in count that a pointer to a member function is a completely different things, compared to a pointer to a regular function. Anyway, you can roughly see it as a pointer to a regular function receiving an additional argument (in first position) corresponding to the object that call its method.

So, in your example, the first T argument represent the object of type T that call its operator.