constexpr declares that a function or variable can be evaluated at compile time. In C++11, constexpr functions are limited to a single return statement (plus static_assert and typedef), and constexpr variables must be initialized with constant expressions. The compiler evaluates constexpr functions when all arguments are constant expressions.

Before constexpr, compile-time computation required template metaprogramming with recursive template instantiation. constexpr allows writing computation in ordinary function syntax that the compiler evaluates during compilation, using the same language constructs as runtime code.

#include <print>

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

constexpr auto grid_size = 8;
constexpr auto total_cells = grid_size * grid_size;

int main() {
    constexpr auto f5 = factorial(5);
    static_assert(f5 == 120, "factorial(5) must be 120");

    std::println("factorial(5) = {}", f5);
    std::println("total cells in {}x{} grid = {}", grid_size, grid_size, total_cells);

    // Also callable at runtime
    auto n = 7;
    std::println("factorial({}) = {}", n, factorial(n));
}