C++ Handbook

`requires` — C++ Keyword

`requires` — C++ Keyword

The requires keyword in C++20: introduces constraints on templates or checks expressions at compile time.

How to use this reference page

Use reference pages to confirm names, categories, nearby facilities, and the constraints that matter before writing or reviewing code.

  • Scan the top of the page first to identify the primary types, functions, or algorithm families involved.
  • Use the nearby-page links when your question is really about a companion header, related algorithm family, or broader subsystem.
  • Validate tricky behavior with a small compileable example before relying on memory for details like invalidation, ordering, allocation, or lifetime rules.

requires

Has two related uses in C++20: (1) a requires-clause that constrains a template or function, and (2) a requires-expression that tests whether expressions are well-formed.

Syntax

// Requires-clause: constrains the template
template <typename T> requires Constraint<T>
void func(T);

// Requires-expression: evaluates to bool
requires (T a) {
    { a + a } -> std::same_as<T>;
    a.size();
}

Example

#include <concepts>
#include <print>

template <typename T>
concept Addable = requires(T a, T b) {
    { a + b } -> std::convertible_to<T>;
};

template <Addable T>
T sum(T a, T b) {
    return a + b;
}

// Inline requires-clause
template <typename T>
    requires std::integral<T>
T next(T n) { return n + 1; }

int main() {
    std::println("{}", sum(3, 4));          // 7
    std::println("{:.1f}", sum(1.1, 2.2)); // 3.3
    std::println("{}", next(41));           // 42
}

Notes

Example in practice

int main() {
    // Pick one facility from this reference page.
    // Write the smallest program that exercises its main precondition,
    // complexity rule, or lifetime constraint before scaling up.
    return 0;
}