Return a const reference or a copy in a getter function?

Question

What\'s better as default, to return a copy (1) or a reference (2) from a getter function?

class foo {
public:
    std::string str () { // (1)
              


        

Answer 1

My rule of thumb is to return a copy for simple basic datatypes such as int, string etc. For a bit more complicated structures where copying may be costlier (like vector you mentioned) I prefer to return a const-reference.

Answer 2

Returning a reference to an object's internals as part of its public interface can be a code smell if not outright bad design.

Before returning a reference to an internal object in a public interface, the designer should pause. Doing so couples users of your class to part of your design. Often it is outright unnecessary, sometimes it indicates further design work is needed. At times it is necessary, as commenters have noted.