Why pass a string by reference and make it a constant?

Question

Just a general question say the signature of a function is:

void f( const string & s )...

Why is it necessary to pass this by reference if you are not actually changing the string (since it is constant)?

Answer 1

There are two alternatives to passing by reference - passing by pointer, and passing by value.

Passing by pointer is similar to passing by reference: the same argument of "why pass a pointer if you do not want to modify it" could be made for it.

Passing by value requires making a copy. Copying a string is usually more expensive, because dynamic memory needs to be allocated and de-allocated under the cover. That is why it is often a better idea to pass a const reference / pointer than passing a copy that you are not planning to change.