When should you use constexpr capability in C++11?

Question

It seems to me that having a "function that always returns 5" is breaking or diluting the meaning of "calling a function". There must be a reason, or a need for this capability or it wouldn't be in C++11. Why is it there? // preprocessor. #define MEANING_OF_LIFE 42 // constants: const int MeaningOfLife = 42; // constexpr-function: constexpr int MeaningOfLife () { return 42; } It seems to me that if I wrote a function that return a literal value, and I came up to a code-review, someone would tell me, I should then, declare a constant value instead of writing return 5.

Answer 1

You can have constexpr functions that compute things, but they must be composed completely of other constexprs.

constexpr int factorial( int n ) {

return n == 0 ? 1 : n * factorial(n-1);

}

The usefulness of constexprs over typical const variables is mostly useful in combination with templates, where template parameters must be known at compile time: any constexpr can be computed by the compiler, and so can be used as a template argument.