C++ Inline Functions, constexpr and consteval: Zero-Overhead Computing

In high-performance C++ systems (game engines, low-latency financial systems), the microsecond overhead of invoking a small function repeatedly can degrade throughput. C++ provides three powerful tools to optimize or completely pre-compute results at compile time: inline, constexpr, and consteval.

1. The inline Keyword

inline int maximum(int a, int b) {
    return (a > b) ? a : b;
}

2. Compile-Time Evaluation with constexpr (C++11)

#include <iostream>

// Evaluated at compile time if argument is a constant expression!
constexpr unsigned long long factorial(int n) {
    return (n <= 1) ? 1 : (n * factorial(n - 1));
}

int main() {
    // Computed entirely by the compiler during compilation!
    constexpr auto fact5 = factorial(5); // 120 (hardcoded binary constant)
    std::cout << "5! = " << fact5 << std::endl;
    return 0;
}

3. Strict Compile-Time Enforcement with consteval (C++20)

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

int main() {
    constexpr int val = square(10); // OK: Evaluated at compile time (100)

    // int runtimeInput = 10;
    // int err = square(runtimeInput); // COMPILE ERROR! consteval forbids runtime calls!
    return 0;
}