Constrain a function with a concept, in the parameter list

An abbreviated function template puts the constraint where the parameter is declared. A wrong argument type is a compile error naming the concept, not a page of template errors.

#include <concepts>
#include <iostream>

// Abbreviated function template: the parameter's type must satisfy integral.
auto doubled(std::integral auto v) { return v * 2; }

int main() {
    std::cout << doubled(21) << '\n';
    // doubled(2.5); // compile error: double does not satisfy integral
}

Observed output:

42

Facts verified 2026-11-04. The code on this page was compiled and run on the box with GCC 16.2.0 at -std=c++23 -Wall -Wextra; the outputs quoted are the observed outputs. MSVC was not tested.

Back to the daily tips index