Using Concepts with Class Templates

You can use concepts to constrain the types allowed for a class template by placing the concept name before the typename in the template parameter list. Here's an example:

1#include <concepts>
2
3template <std::integral T>
4class IntegralContainer {
5public:
6  IntegralContainer(T val) : value(val) {}
7  T value;
8};
9
10int main() {
11  // Valid
12  IntegralContainer<int> intContainer(42);
13  
14  // Invalid, double is not an integral type
15  IntegralContainer<double> doubleContainer(3.14);
16}

1error: 'IntegralContainer': the associated constraints are not satisfied
2the concept 'std::integral<double>' evaluated to false

In this example, the IntegralContainer class template is constrained to only accept integral types using the std::integral concept. Attempting to instantiate the template with a non-integral type like double will result in a compilation error.