constexpr specifies that an object's value is a constant expression and must be computable at compile time. Unlike const, which permits runtime initialization, constexpr objects must be initialized with values known during compilation. The initializer must consist of constant expressions only.

const objects in C can be initialized at runtime, so they cannot be used in contexts requiring compile-time constants (array sizes in some contexts, case labels). constexpr guarantees compile-time evaluation, making objects usable wherever constant expressions are required.

#include <stdio.h>

constexpr int array_size = 10;
constexpr double pi = 3.14159265358979;

int main(void) {
    int arr[array_size];  // OK: array_size is a constant expression

    constexpr int doubled = array_size * 2;
    printf("Array size: %d, doubled: %d\n", array_size, doubled);
    printf("Pi: %f\n", pi);
}