C++ Union, garbage value

Question

For the following code

#include <iostream>

using namespace std;

union type
{
    int a;
    char b ;
};

int main()
{
    type first;
    first.b = 'a';
    cout << first.a << " " << first.b << endl;

}

the output is -858993567 a (MSVC) or 4201057 a(g++ MINGW).

but for

#include <iostream>

using namespace std;

union type
{
    int a;
    char b ;
};

int main()
{
    type first;
    first.a = 0;
    first.b = 'a';
    cout << first.a << " " << first.b << endl;

}

the output is 97 a

And these values are fixed under every circumstances (tried rebooting and creating new workspace/file, hence not garbage values).

So, why did the initialization (in the second case) made a difference?

I have tried it both on visual studio (using MSVC) and visual studio code (using g++).

Update 1 I checked on online IDE which probably use Linux g++, and they give the exact expected answer i.e., 97 a, in both the cases.

Your programs exhibit undefined behavior, by way of accessing an object (an inactive member of the union) before its lifetime has started. — Igor Tandetnik, May 30 '20 at 20:42
Undefined behavior is undefined. "Seems to work" is one possible manifestation of undefined behavior. — Igor Tandetnik, May 30 '20 at 20:51
"the exact expected answer" does not exist, because your program has Undefined Behavior. — alter_igel, May 30 '20 at 20:57

score -3 · Answer 1 · answered May 30 '20 at 20:42

-3

If you specify first.a in first code sample - you will get fixed, stable value.

You have union with 4 bytes size and initialize only one byte of them.

answered May 30 '20 at 20:42

Evgeny

1,072
6
6

Check update one – Abhay Patil May 30 '20 at 20:47
Ok, Abhay. You can sleep well, If you plan to compile and run your code on online IDE only. But in other cases you can get undefined behavior with undefined values. – Evgeny May 30 '20 at 21:25

score -3 · Answer 2 · answered May 30 '20 at 20:53

Union is like type who can holds any type but this types musts be writed in definition of union, like this:

union Example {

    char a;
    std::int32_t b;

};

Example ex; /// Create object of union
ex.a = 'c'; /// At this moment a member is valid and b is invalid
std::cout << ex.b; /// This cause undefined behavior

Be aware, size of union object is size of type who need most bytes. In this case size is same as size of b property.

Any one can tell what i have write wrong? – Kaninchen May 31 '20 at 11:03 — Kaninchen, May 31 '20 at 11:03

C++ Union, garbage value

2 Answers2