constexpr and Compile-Time Computation

C++ supports Compile-Time Computation, allowing developers to execute functions and construct structures during compilation. This shifts computation overhead from runtime to compile time, saving CPU cycles.

constexpr vs. const

While both keywords declare read-only values, their semantics differ:

• const: Means "read-only" at runtime. The initialization value can be determined at runtime (e.g. reading user input or system time).
• constexpr: Means "constant expression". The value must be known at compile time. All constexpr variables are implicitly const.

Code Walkthrough

This example demonstrates compile-time calculations using constexpr and consteval functions.

#include <iostream>
#include <array>
// constexpr functions can run at compile-time OR runtime
constexpr int factorial(int n) {
    return (n <= 1) ? 1 : n * factorial(n - 1);
}
// consteval functions MUST run at compile-time
consteval int square(int n) {
    return n * n;
}
int main() {
    // Computed at compile-time because the target size requires constant expression
    std::array<int, factorial(5)> my_arr; 
    std::cout << "Array size: " << my_arr.size() << std::endl; // 120
// Evaluated at runtime since input is dynamic
    int val = 4;
    int fact_val = factorial(val); 
    std::cout << "Fact: " << fact_val << std::endl;
// Compile-time evaluation enforced by consteval
    constexpr int sq = square(10); 
    // int bad_sq = square(val); // COMPILE ERROR: val is not compile-time constant
return 0;
}

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