consteval functions

consteval functions

Use consteval for functions that are always constant. A consteval function is strictly declared so that every invocation of the function must return a compile-time constant.

The consteval keyword is a subset of constexpr functions (and also implies inline on a function). Although a constexpr function is constant if its arguments are constant, it can also return a dynamic return value for non-constant arguments.

When would you use consteval versus constexpr functions? I mean, when you ask your boss to make you a cup of coffee, do you like to ask politely or do you issue commands? Supposedly constexpr is optional for the C++ compiler, whereas consteval is mandating compile-time evaluation.

Personally, I can't see much difference in general usage, since the compiler will probably optimize a constexpr function at compile-time if it's capable enough. Hence, for regular functions I don't see much benefit to consteval over constexpr. There are some complicated places in C++ where it helps to guarantee a compile-time constant, such as reflexive types and other tricks in compile-time template usage.