Checking Member Type with Type Traits

Yes, you can combine type traits with decltype to inspect the type of a class member within a concept. Here's an example:

1#include <concepts>
2#include <type_traits>
3
4template <typename T>
5concept IntegralSized =
6  std::is_integral_v<
7    std::remove_cvref_t<decltype(T::Size)>>;
8
9struct Asteroid {
10  long Size;
11};
12
13// Passes
14static_assert(IntegralSized<Asteroid>);

In this example, the IntegralSized concept checks if the Size member of a class is an integral type.

We use decltype(T::Size) to get the type of the Size member. Since decltype preserves cv-qualifiers and references, we use std::remove_cvref_t to remove any const, volatile, and reference qualifiers from the type.

Finally, we use the std::is_integral_v type trait to check if the resulting type is an integral type. The _v suffix is a C++17 feature that provides a convenient way to get the value member of a type trait as a bool.

The static_assert at the end confirms that the Asteroid class, which has a long Size member, satisfies the IntegralSized concept.

You can use various other type traits like std::is_floating_point, std::is_base_of, std::is_polymorphic, etc., to create concepts that constrain class members based on their types and properties.