The constexpr keyword

The constexpr keyword tells the compiler that a certain function is intended to be evaluated at compile time if all the conditions allowing for compile-time evaluation are fulfilled. Otherwise, it will execute at runtime, like a regular function.

A constexpr function has a few restrictions; it is not allowed to do the following:

• Allocate memory on the heap
• Throw exceptions
• Handle local static variables
• Handle thread_local variables
• Call any function, which, in itself, is not a constexpr.

Note

Back in C++11, constexpr functions were only allowed to contain a single return statement, requiring the programmer to resort to recursion for more advanced constexpr functions; however, in C++14 this restriction has been removed. The constexpr functions may now contain several statements, declare variables, and even mutate variables.

With the constexpr keyword, writing a compile-time evaluated function is as easy as writing a regular function, as its parameters are regular parameters instead...