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The All-in-One Guide to C++20

Use-cases for Concepts

Understand how to apply C++20 concepts as compile-time predicates to enforce template requirements. Learn to improve type safety using concepts in class templates, generic member functions, and variadic templates with fold expressions.

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First and foremost, concepts are compile-time predicates. A compile-time predicate is a function that is executed at compile-time and returns a boolean.

Before I dive into the various use-cases of concepts, I want to demystify concepts and present them simply as functions returning a boolean at compile time.

Compile-time predicates

A concept can be used in a control structure, which is executed at run time or compile-time. Run the code below.

C++
#include <compare>
#include <iostream>
#include <string>
#include <vector>
struct Test{};
int main() {
    std::cout << "\n";
    std::cout << std::boolalpha;
    std:: cout << "std::three_way_comparable<int>: "
               << std::three_way_comparable<int> << "\n";
    std::cout << "std::three_way_comparable<double>: ";
    if (std::three_way_comparable<double>) std::cout << "True";
    else std::cout << "False";
    std::cout << "\n\n";
    static_assert(std::three_way_comparable<std::string>);
 
    std::cout << "std::three_way_comparable<Test>: ";
    if constexpr(std::three_way_comparable<Test>) std::cout << "True";
    else std::cout << "False";
    std::cout << '\n';
    std::cout << "std::three_way_comparable<std::vector<int>>: ";
    if constexpr(std::three_way_comparable<std::vector<int>>) std::cout << "True";
    else std::cout << "False";
    std::cout << '\n';
}

In the program above, I use the concept ...