Question

I recently had a serious bug, where I forgot to return a value in a function. The problem was that even though nothing was returned it worked fine under Linux/Windows and only crashed under Mac. I discovered the bug when I turned on all compiler warnings.

So here is a simple example:

#include <iostream>

class A{
public:
    A(int p1, int p2, int p3): v1(p1), v2(p2), v3(p3)
    {
    }

    int v1;
    int v2;
    int v3;
};

A* getA(){
    A* p = new A(1,2,3);
//  return p;
}

int main(){

    A* a = getA();

    std::cerr << "A: v1=" << a->v1 << " v2=" << a->v2 << " v3=" << a->v3 << std::endl;  

    return 0;
}

My question is how can this work under Linux/Windows without crashing? How is the returning of values done on lower level?

Answer 1

On Intel architecture, simple values (integers and pointers) are usually returned in eax register. This register (among others) is also used as temporary storage when moving values in memory and as operand during calculations. So whatever value left in that register is treated as the return value, and in your case it turned out to be exactly what you wanted to be returned.