Type Traits and std::integral_constant
The original metaprogramming interface: traits answer questions about types, integral_constant carries compile-time values as types.
Traits: questions about types
std::is_integral_v<T>, std::is_same_v<A, B>, std::is_nothrow_move_constructible_v<T>, std::underlying_type_t<E> — each is a compile-time query. Traits come in _t (type result) and _v (value result) flavors; the _v forms are the modern spelling of ::value.
std::integral_constant: a value wearing a type
std::integral_constant<int, 5> is a type whose value is part of it. That lets you pass compile-time numbers through normal overload machinery — dispatch on value:
constexpr int route(std::integral_constant<int, 0>) { return 0; } // empty pack
constexpr int route(std::integral_constant<int, 1>) { return 1; } // one element
You rarely write the type by hand — you usually receive it: a function template parameter std::size_t N produces std::integral_constant<std::size_t, N> when deduced through helpers, and std::bool_constant<B> is integral_constant<bool, B>.
When traits beat if constexpr
Use traits in concepts and constraints (Module 4) and in type-level arithmetic (std::conditional_t<flag, A, B>). Use if constexpr when values in the function body differ. The graded exercises use both together.