[FIXED] Why is this working? I cannot understand the logic of this swapping

Issue

int main() {
    // Complete the program
    string a,b;
    getline(cin,a);
    getline(cin,b);
    cout<<a.size()<<" ";
    cout<<b.size();
    string c=a+b;
    cout<<endl<<c;

    swap(a[0],b[0]);
    cout<<endl<<a<<" "<<b;
    return 0;
}

void swap(string s1,string s2){
    string temp=s1;
    s1=s2;
    s2=temp;
}

Well the target is to swap the first element of both strings, but I created a general function for that and even got it right. But, unexpectedly, I didn't use pass by reference or pointer! Even then, the changes are permanent when I try to output a and b in the end!

Logically it shouldn't work but it is working. Is it something to do with the strings?

Solution

This is almost certainly due to the fact that, somewhere in code that you have not shown us, you have this line (or something very similar):

using namespace std;

With this line included, then that very namespace std defines a function as follows:

void swap(_Ty& _Left, _Ty& _Right);

Where the _Ty template is replaced with char in your swap(a[0],b[0]); call.

Add a simple cout << "My Swap" << endl; line to your swap function, and you'll see it's not being called.

Highly recommended reading: Why is "using namespace std;" considered bad practice?.

Answered By - Adrian Mole

Answer Checked By - Katrina (PHPFixing Volunteer)