Using requires clause in function templates

In C++20, you can use a requires clause in a function template to constrain its arguments based on a concept. The requires clause allows you to specify the requirements that the template arguments must satisfy in order for the function template to be considered a viable overload.

Here's an example that demonstrates using a requires clause in a function template:

1#include <concepts>
2#include <iostream>
3
4template <typename T>
5concept Printable = requires(T a) {
6  std::cout << a;
7};
8
9template <typename T>
10requires Printable<T>
11void print(T value) {
12  std::cout << "Value: " << value << '\n';
13}
14
15int main() {
16  print(42);
17  print("Hello");
18  print(3.14);
19}

In this example, the Printable concept is defined to check if a type T can be printed to std::cout. The requires expression inside the concept ensures that the expression std::cout << a is valid for an instance a of type T.

The print function template uses a requires clause to constrain its argument value based on the Printable concept. The requires Printable<T> clause specifies that the template argument T must satisfy the Printable concept in order for the function template to be considered a viable overload.

In the main function, we call the print function with different types of arguments. The compiler will check if each argument type satisfies the Printable concept before instantiating the function template.

The output of this program will be:

1Value: 42
2Value: Hello
3Value: 3.14

By using a requires clause in a function template, you can enforce specific requirements on the template arguments based on concepts. This allows you to write more expressive and type-safe code, ensuring that the function is only called with arguments that meet the specified requirements.

Note that the requires clause can be used in various places within a template declaration, such as function parameters, return types, and template parameters. It provides a powerful way to constrain and specialize templates based on concepts.