All Callable Concepts and Invocability Traits

Callable concepts

• std::invocable
• std::regular_invocable
• std::predicate
• std::relation
• std::equivalence_relation
• std::strict_weak_order

Invocability traits

• std::is_invocable
• std::is_invocable_r
• std::is_nothrow_invocable
• std::is_nothrow_invocable_r
• std::invoke_result

Related helpers

• std::invoke
• std::invoke_r
• std::function
• std::reference_wrapper

Practical rules

• Use concepts in public templates when readability matters.
• Use traits inside low-level metaprogramming and compatibility layers.
• Prefer expressing callable requirements directly instead of relying on template substitution failures alone.

Small worked example

#include <concepts>
#include <functional>

template <std::predicate<int> Predicate>
bool accepts(Predicate predicate, int value) {
	return std::invoke(predicate, value);
}

This is the modern public-template shape: put the requirement in the signature so users see it immediately instead of discovering it through template errors later.

Concepts vs traits

• use std::invocable or std::predicate in public templates and overload sets
• use std::is_invocable_v or related traits inside compatibility layers, static assertions, or conditional metaprogramming
• use std::invoke_result_t when another type depends on the result of calling a callable

Related pages

• All callable wrappers and adapters
• Header reference: <concepts>
• Header reference: <type_traits>