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The All-in-One Guide to C++20

Function Execution and Variable Initialization

Learn the differences between const, constexpr, consteval, and constinit in C++20. Understand how these keywords affect function execution at run time or compile time, and how they influence variable initialization for optimized program behavior.

We'll cover the following...

Now it’s time to write about the differences between const, constexpr, consteval, and constinit. First, I discuss function execution and then variable initialization.

Function execution

The following program has three versions of a square function.

C++
#include <iostream>
int sqrRunTime(int n) {
    return n * n;
}
consteval int sqrCompileTime(int n) {
    return n * n;
}
constexpr int sqrRunOrCompileTime(int n) {
    return n * n;
}
int main()  {
    // constexpr int prod1 = sqrRunTime(100); ERROR
    constexpr int prod2 = sqrCompileTime(100);
    constexpr int prod3 = sqrRunOrCompileTime(100);
    int x = 100;
    int prod4 = sqrRunTime(x);
    // int prod5 = sqrCompileTime(x); ERROR
    int prod6 = sqrRunOrCompileTime(x);
}

As the name suggests: the ordinary function sqrRunTime (line 3) runs at run time, the consteval function ...