Should accessors return values or constant references?

Question

Suppose I have a class Foo with a std::string member str. What should get_str return?

std::string Foo::ge         


        

Answer 1

Returning by value means you do not have to have an internal std::string stored somewhere in the class for which you return.

In a pure virtual method it is preferable not to assume that the std::string will be there and therefore to return a std::string by value.

In a concrete class where there is clearly a std::string member and you are just going to return a reference to it, you can, for efficiency, return it by const reference. Even if you have to change it later, you do not need to change functionality that uses the class.

In a multi-threaded model where the inner string might change between calls, of course, you probably need to return by value (assuming that users of the class will get a "snapshot" view of the string value at the time of the completion of the call).

Returning by reference is usually more efficient. I do however have a non-mutable reference-counted string class that you can return by value efficiently and I used to use that quite frequently.

By the way, some would recommend returning a std::string by const value. I do not think it is the best way to do it, as it prevents allowing the user to "swap" it into a local variable.