Using multiple type parameters in C++20 concepts

Yes, you can define concepts that take multiple type parameters in C++20. This allows you to express relationships and constraints between different types. When using such a concept to constrain a template, you need to provide the additional type arguments explicitly.

Here's an example that demonstrates a concept with multiple type parameters:

1#include <concepts>
2#include <iostream>
3#include <string>
4
5template <typename T, typename U>
6concept Convertible = std::is_convertible_v<T, U>;
7
8template <typename T, typename U>
9  requires Convertible<T, U>
10U convert(T value) {
11  return static_cast<U>(value);
12}
13
14int main() {
15  double result1 = convert<int, double>(42);
16  std::cout << "Converted value: "
17    << result1 << '\n';
18
19  std::string result2 = convert<
20    const char*, std::string>("Hello");
21  std::cout << "Converted value: "
22    << result2 << '\n';
23}

In this example, the Convertible concept takes two type parameters, T and U. It uses the std::is_convertible_v type trait to check if T is implicitly convertible to U.

The convert function template is constrained by the Convertible concept using a requires clause. The requires Convertible<T, U> clause specifies that T and U must satisfy the Convertible concept, meaning that T must be convertible to U. The function takes a parameter of type T and returns a value of type U.

When calling the convert function, you need to provide both the T and U template arguments explicitly, as shown in the main function.

The output of this program will be:

1Converted value: 42
2Converted value: Hello

By using multiple type parameters in concepts, you can express more complex relationships and constraints between types. This allows you to write more expressive and reusable templates that enforce specific type requirements.

Remember that when using a concept with multiple type parameters to constrain a template, you need to provide the additional type arguments explicitly when instantiating the template. The requires clause is used to specify the concept constraint on the template parameters.