C++: When is a class implicitly copied?

When is a class implicitly copied in C++?

I have a class that contains a unique_ptr, and therefore cannot be safely copied, and therefore, I disabled the copy constructor on the class by creating private versions of X(X&) and X& operator = X&.

I immediately ran into the problem that 开发者_开发技巧instances of this class cannot be returned, because returning actually makes a copy of the instance.

Are there any other situations I need to watch out for?

Returning does not copy the instance, it moves the instance. You just forgot to provide a move constructor. In addition, classes are now moved when used in Standard containers in most situations in which they used to be copied.

In short, provide a move constructor and move assignment operator (and swap, preferably) and you should find that almost all situations where copies are implicit, they're now moves.

The situations that come to mind are: functions that receives the class by value, functions that returns then class by value, and any class or container that contains that class. Classes like std::vector will use move semantics whenever possible (you did overload that right)? but will be unable to use functions that require a copy constructor, such as copying the vector. As GMan said though, you can make a copy constructor for your class, and do a deep copy of the std::unique_ptr manually, if you want to make things easier.