how to pass a string through a function c++

I'm just learning programming in c++. In my homework I need to compare three strings given by the user by a prompt. I know that for a function to modify certain values directly instead of returning a value I need to use pointers. I also know that strings already behave like pointers. This is my code so far but I don't know where I'm wrong:

#include"std_lib_facilities.h"

void switching (string x, string y){
        string flag;
        if (x>y){flag=y;
                y=x;
                x=flag;
        }

}

int main(){
        string  s1, s2, s3;
        cout<<"introduce three words: ";
        cin>>s1>>s2>>s3;
                switching(s1,s2);
                cout<<s1<<" "<<s2<<" "<<s3;
                cout<<endl;
                switching(s2,s3);
                cout<<s1<<" "<<s2<<" "<<s3;
                cout<<endl;
                switching(s1,s2);
                cout<<s1<<" "<<s2<<" "<<s3;
                cout<<endl;
        return 0;
}