Selecting branches at compile time with constexpr if

In the previous recipes, we saw how we can impose restrictions on types and functions using static_assert and std::enable_if and how these two are different. Template metaprogramming can become complicated and cluttered when we use SFINAE and std::enable_if to define function overloads or when we write variadic function templates. A new feature of C++17 is intended to simplify such code; it is called constexpr if, and it defines an if statement with a condition that is evaluated at compile time, resulting in the compiler selecting the body of a branch or another in the translation unit. Typical usage of constexpr if is for simplification of variadic templates and std::enable_if-based code.

Getting ready

In this recipe, we will refer to and simplify the code written in previous recipes. Before continuing with this recipe, you should take a moment to go back and review the code we have written in the previous recipes...