Why can't assign const initialize while compile by pointer

Question

I'm curious about why my research result is strange

#include <iostream>

int test()
{
    return 0;
}

int main()
{
    /*include either the next line or the one after*/
    const int a = test(); //the result is 1:1
    const int a = 0; //the result is 0:1

    int* ra = (int*)((void*)&a);
    *ra = 1;
    std::cout << a << ":" << *ra << std::endl;
    return 0;
}

why the constant var initialize while runtime can completely change, but initialize while compile will only changes pointer's var?

Answer 1

The function isn't really that relevant here. In principle you could get same output (0:1) for this code:

int main() {
    const int a = 0;
    int* ra = (int*)((void*)&a);
    *ra = 1;
    std::cout << a << ":" << *ra;
}

a is a const int not an int. You can do all sorts of senseless c-casts, but modifiying a const object invokes undefined behavior.

By the way in the above example even for std::cout << a << ":" << a; the compiler would be allowed to emit code that prints 1:0 (or 42:3.1415927). When your code has undefinded behavior anything can happen.

PS: the function and the different outcomes is relevant if you want to study internals of your compiler and what it does to code that is not valid c++ code. For that you best look at the output of the compiler and how it differs in the two cases (https://godbolt.org/).