What is Happening in this C++ Code? Why s.u.a = 20?

Question

I was expecting the output to look like: 10 20 10 20

But the output came like this: 10 20 20 20

What is happening behind the code?

#include <iostream>

using namespace std;

typedef struct sdata{
    int a;
    int b;
    union udata{
        int a;
        int b;
    }u;
}Data;

int main()
{
    Data s;
    s.a = 10;
    s.b = 20;
    s.u.a = 10;
    s.u.b = 20;
    cout << s.a << " " << s.b << " " << s.u.a << " " << s.u.b;
    return 0;
}

I'd recommend reading up on what unions do. You seem to be using them, but expecting them to behave like structs. — Thomas Jager, Jul 26 '19 at 12:53

score 1 · Accepted Answer · answered Jul 26 '19 at 12:55

Unlike a struct, a union can only hold one member at a time. Each member starts at the same memory address, so writing to one affects the other.

In this case your union has two fields of type int. So if you set one than the other will contain exactly the same value. So when you set s.u.b to 20 it also sets s.u.a to that value as well.

What is Happening in this C++ Code? Why s.u.a = 20?

1 Answers1