Notes

Tells the compiler “compute this at compile time, if at all possible.”

constexpr int square(int x) { return x * x; }

int arr[square(4)];             // 16 - computed at compile time, valid array size
constexpr int val = square(5);  // 25, baked into the binary, zero runtime cost

vs #define

#define MAX_HEALTH 100          // dumb text substitution, no type, no scope
constexpr int MAX_HEALTH = 100; // an actual typed constant, respects scope,
                                  // shows up properly in a debugger

constexpr gets everything a macro constant gives, plus type safety, without any of the macro pitfalls.

Doesn’t force compile-time — it allows it

int x = getUserInput();
int y = square(x);   // still works fine, just computed at runtime instead

A constexpr function can still be called with a value only known at runtime — it just falls back to running normally, like any other function.

Related

Preprocessor directives