Concepts

Learn template constraints with C++20 concepts.

Concepts

Concepts (C++20) are type constraints placed on template parameters. You write template <typename T> requires std::integral<T> auto twice(T x). When the constraint fails the compiler gives a clear error message.

Concepts replace verbose SFINAE error messages with a readable tool. The standard library ships ready concepts like std::integral, std::floating_point and std::convertible_to. A constraint is given with a requires clause or the short form template <std::integral T>.

When the constraint fails

A concept is checked at compile time. If the twice template carries std::integral<T>, calling twice(std::string{...}) fails to compile. The error says 'string does not satisfy integral'; no deep SFINAE stack appears.

Concepts also support expression requirements written as requires (T a, T b) { a + b; }. This tests at compile time that type T supports the + operator.

Exercise Constrained twice template with concepts

Write template <typename T> requires std::integral<T> auto twice(T x) with body return x * 2;. In main call twice(6) and print the result via std::cout.

Show example solution

#include <iostream>
#include <concepts>

template <typename T>
requires std::integral<T>
auto twice(T x) {
    return x * 2;
}

int main() {
    std::cout << twice(6);
    return 0;
}

Test yourself

Match each concept to its role

std::integral<T>

Integer constraint
requires std::integral<T>

Requires clause
template <std::integral T>

Short form
std::floating_point<T>

Floating constraint

Complete the requires clause

template <typename T> ____ std::integral<T> auto twice(T x) { return x * 2; }

Show answer

template <typename T> requires std::integral<T> auto twice(T x) { return x * 2; }

Which line is NOT a concept-constrained template?
- template <std::integral T>
- template <typename T> requires std::integral<T>
- requires std::integral<T>
- template <typename T>
Show answer

template <typename T> — This line has no concept constraint.

Next lesson: Ranges

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