Requires Expressions

Thanks to requires expressions, you can define powerful concepts. A requires expression has the following form:

requires (parameter-list(optional)) {requirement-seq}

• parameter-list: A comma-separated list of parameters, such as in a function declaration
• requirement-seq: A sequence of requirements, consisting of simple, type, compound, or nested requirements

Let’s discuss its type one by one.

Simple requirements

The following concept Addable is a simple requirement:

template <typename T>
concept Addable = requires (T a, T b) {
    a + b; 
};

The concept Addable requires that the addition a + b of two values of the same type T is possible.