Programming with constant expressions

An expression prefixed with the constexpr keyword tells the compiler that the expression should be evaluated at compile time:

constexpr auto v = 43 + 12; // Constant expression

The constexpr keyword can also be used with functions. In that case, it 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:

• Handle local static variables
• Handle thread_local variables
• Call any function, which, in itself, is not a constexpr function

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

Consider the following constexpr function...