C++ Handbook

`concept` — C++ Keyword

`concept` — C++ Keyword

The concept keyword in C++20: defines a named set of constraints on template parameters.

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.

concept

Defines a named constraint (a predicate on template parameters) that can be used to constrain templates and improve error messages. Introduced in C++20.

Syntax

template <parameter-list>
concept name = constraint-expression;

Example

#include <concepts>
#include <print>

// Define a concept
template <typename T>
concept Numeric = std::is_arithmetic_v<T>;

// Constrain a function template
template <Numeric T>
T add(T a, T b) {
    return a + b;
}

// Concept in a requires clause
template <typename T>
    requires Numeric<T>
T multiply(T a, T b) {
    return a * b;
}

// Abbreviated function template (C++20)
Numeric auto square(Numeric auto x) {
    return x * x;
}

int main() {
    std::println("{}", add(3, 4));          // 7
    std::println("{:.1f}", add(1.5, 2.5)); // 4.0
    std::println("{}", square(5));          // 25
}

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;
}