Creating compile-time constant expressions

The possibility to evaluate expressions at compile time improves runtime execution because there is less code to run and the compiler can perform additional optimizations. Compile-time constants can be not only literals (such as a number or string), but also the result of a function execution. If all the input values of a function (regardless of whether they are arguments, locals, or globals) are known at compile time, the compiler can execute the function and have the result available at compile time. This is what generalized the constant expressions introduced in C++11, which were relaxed in C++14. The keyword constexpr (short for constant expression) can be used to declare compile-time constant objects and functions. We have seen this in several examples in the previous chapters. Now it's time to learn how it actually works.

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