why pass by reference better?

Question

Possible Duplicate:
Is it better in C++ to pass by value or pass by constant reference?

see these 2 program.

bool isShorter(const string s1, const string s2);

int main()
{
    string s1 = "abc";
    string s2 = "abcd";
    cout << isShorter(s1,s2); 
}

bool isShorter(const string s1, const string s2)
{
    return s1.size() < s2.size();
}

and

bool isShorter(const string &s1, const string &s2);

int main()
{
    string s1 = "abc";
    string s2 = "abcd";
    cout << isShorter(s1,s2); 
}

bool isShorter(const string &s1, const string &s2)
{
    return s1.size() < s2.size();
}

Why second one is better?

Answer 1

Because it doesn't have to copy the strings.

Answer 2

I suggest you read this SO Question - Is it better in C++ to pass by value or pass by constant reference?

Answer 3

If you're really interested in some cases when passing by value may be better, you might want to see this.