Using concepts to constrain auto parameters

Yes, in C++20, you can use concepts to constrain auto parameters in functions. This allows you to specify requirements on the deduced type of an auto parameter, ensuring that only arguments satisfying the concept are accepted.

Here's an example that demonstrates using concepts to constrain auto parameters:

1#include <concepts>
2#include <iostream>
3
4template <typename T>
5concept Addable = requires(T a, T b) {
6  a + b;
7};
8
9void add(Addable auto a, Addable auto b) {
10  std::cout << "Result: " << a + b << '\n';
11}
12
13int main() {
14  add(1, 2);
15  add(3.14, 2.71);
16  add(std::string{"Hello"},
17    std::string{" world"});
18}

In this example, the Addable concept is defined to check if a type T supports the + operator. The requires expression inside the concept ensures that the expression a + b is valid for instances a and b of type T.

The add function uses auto parameters constrained by the Addable concept. The Addable auto syntax specifies that the deduced type of the auto parameter must satisfy the Addable concept.

In the main function, we call the add function with different types of arguments. The compiler will deduce the type of each auto parameter and check if it satisfies the Addable concept before instantiating the function.

The output of this program will be:

1Result: 3
2Result: 5.85
3Result: Hello world

By using concepts to constrain auto parameters, you can write more generic and reusable code while still enforcing specific requirements on the deduced types. The concept constraint ensures that only arguments satisfying the concept are accepted, providing type safety and clarity to the function interface.

Note that you can also use concepts to constrain auto return types and auto variables in a similar way. This allows you to express requirements on the deduced types and catch potential type mismatches at compile-time.

Constraining auto parameters with concepts is a powerful feature in C++20 that combines the flexibility of auto type deduction with the type safety and expressiveness of concepts.