constexpr & Compile-Time Computation

constexpr marks a function (or variable) that can be evaluated at compile time, when given compile-time-known inputs — doing real work before the program even runs, with zero runtime cost.

constexpr int factorial(int n) {
    return n <= 1 ? 1 : n * factorial(n - 1);
}

constexpr int result = factorial(5); // computed at COMPILE time
int n = getUserInput();
int runtimeResult = factorial(n);    // same function, now at RUNTIME

The same constexpr function works both ways — the compiler evaluates it at compile time whenever it can prove the inputs are known that early, and falls back to ordinary runtime execution otherwise. This is the modern, far more readable replacement for most of what template metaprogramming used to be needed for.