Why do we need the concept keyword in C++20?

c++ 20 concepts

In C++20, the “concept” keyword was introduced as a new language feature to enable the definition of constraints on template parameters. In C++, templates are a powerful feature that allows the creation of generic code that can work with any data type or object, but sometimes it is necessary to restrict the set of types that a template can be instantiated with.

For example, consider a function that takes an iterator and performs some operation on a sequence of elements. If the iterator type does not support certain operations, the function may fail at runtime or produce unexpected results. With concepts, the programmer can specify a set of requirements that the iterator type must meet, such as the ability to increment, dereference, and compare.

Concepts provide a way to make templates more robust and self-documenting by stating the assumptions and requirements that the template relies on explicitly. They also improve error messages by allowing the compiler to check the constraints and report errors before attempting to instantiate the template.

In summary, the “concept” keyword in C++20 provides a way to define constraints on template parameters, which can improve the safety, reliability, and usability of generic code.