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The All-in-One Guide to C++20

Requires Expressions

Explore how requires expressions enable you to define reusable and self-explanatory concepts in C++20. Understand the syntax and types of requirements, including simple, type, compound, and nested, to enhance your template programming skills and enforce type safety effectively.

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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.

🔑 Avoid anonymous concepts: ...