Passing smart pointers is an important topic that is seldom addressed. This chapter ends with the C++ core guidelines since they have six rules for passing std::shared_ptr and std::unique_ptr.

The Six Rules #

The following six rules violate the important DRY (don’t repeat yourself) principle for software development. In the end, we only have six rules, which makes life as a software developer a lot easier. Here are the rules:

1. R.32: Take a unique_ptr<widget> parameter to express that a function assumes ownership of a widget.
2. R.33: Take a unique_ptr<widget>& parameter to express that a function reseats the widget.
3. R.34: Take a shared_ptr<widget> parameter to express that a function is part owner.
4. R.35: Take a shared_ptr<widget>& parameter to express that a function might reseat the shared pointer.
5. R.36: Take a const shared_ptr<widget>& parameter to express that it might retain a reference count to the object.
6. R.37: Do not pass a pointer or reference obtained from an aliased smart pointer.