Constraining Member Variable Type

You can constrain the type of a class member variable within a concept using std::same_as along with decltype. Here's an example:

1#include <concepts>
2
3template <typename T>
4concept IntegerSized = std::same_as<
5  int, std::remove_cvref_t<decltype(T::Size)>>;
6
7struct Rock {
8  int Size;
9};
10
11// Passes
12static_assert(IntegerSized<Rock>);

In this concept, std::same_as checks if the member T::Size has the same type as int. We use std::remove_cvref_t to remove any const, volatile, or reference qualifiers from the type before the comparison.

The static assertion static_assert(IntegerSized<Rock>) will trigger a compile error if the Size member of Rock is not an int.