Strange behavior with global variables

[subith86]

while i was learning about global variables I did some trials and found one problem. Here goes my code.

int cows = 10; //global variable

void farm1()
{
    int cows;
    cout<<"in farm1, cows="<<cows<<endl;
    cows++;
}

int main()
{
    //cout<<"in main, cows="<<cows<<endl;
    farm1();
    //cout<<"back in main, cows="<<cows<<endl;
}

Here line 6 prints some garbage value as expected. But when I uncomment lines 12 and 14, line 6 prints 10. Can somebody please explain why?

[hanst99]

An implementation detail. Your compiler may have optimized your code to use less memory, and thus lets the local and global cows be at the same location.

E.g.

void farm1()
{
    int cows; //hm you want an integer here too
    cout<<"in farm1, cows="<<cows<<endl; // you print it
    cows++; // you increment it and never do anything with the value
}

int main()
{
    cout<<"in main, cows="<<cows<<endl; //have used an integer before
    farm1();
    cout<<"back in main, cows="<<cows<<endl;
}

Your compiler may optimize the cows++ out as it doesn't do anything, and puts the local int cows at the same place as the global int cows cause you never actually give a value to cows. Don't rely on stuff like that, how compilers react to undefined situations is completely up to them.

PS: I say may. Of course I have no idea what your compiler really does.