Concepts

Concepts are a major feature in C++20 that provides a set of requirements for types. The basic idea behind concepts is the compile-time validation of template arguments. For example, to specify that the template argument must have a default constructor, we use the default_constructible concept in the following way:

template <default_constructible T>
void make_T() { return T(); }

In the preceding code, we missed the typename keyword. Instead, we set a concept that describes the T parameter of the template function.

We can say that concepts are types that describe other types – meta-types, so to speak. They allow the compile-time validation of template parameters along with a function invocation based on type properties. We will discuss concepts in detail in Chapter 3, Details of Object-Oriented Programming, and Chapter 4, Understanding and Designing Templates.