How to return smart pointers (shared_ptr), by reference or by value?

Question

Let\'s say I have a class with a method that returns a shared_ptr.

What are the possible benefits and drawbacks of returning it by reference or by value

Answer 1

Regarding any smart pointer (not just shared_ptr), I don't think it's ever acceptable to return a reference to one, and I would be very hesitant to pass them around by reference or raw pointer. Why? Because you cannot be certain that it will not be shallow-copied via a reference later. Your first point defines the reason why this should be a concern. This can happen even in a single-threaded environment. You don't need concurrent access to data to put bad copy semantics in your programs. You don't really control what your users do with the pointer once you pass it off, so don't encourage misuse giving your API users enough rope to hang themselves.

Secondly, look at your smart pointer's implementation, if possible. Construction and destruction should be darn close to negligible. If this overhead isn't acceptable, then don't use a smart pointer! But beyond this, you will also need to examine the concurrency architecture that you've got, because mutually exclusive access to the mechanism that tracks the uses of the pointer is going to slow you down more than mere construction of the shared_ptr object.

Edit, 3 years later: with the advent of the more modern features in C++, I would tweak my answer to be more accepting of cases when you've simply written a lambda that never lives outside of the calling function's scope, and isn't copied somewhere else. Here, if you wanted to save the very minimal overhead of copying a shared pointer, it would be fair and safe. Why? Because you can guarantee that the reference will never be mis-used.