How to write a class with smart pointer?

Question

Suppose I have a class:

class State {
  std::shared_ptr<Graph> _graph;
public:
  State():_graph(new Graph){}
};

With regards to rule of three , apparently no need to free _graph in destructor as it is a smart pointer. The question is, do I need to write copy constructor and assignment operator for it?

Considering following:

State s1;
State s2 = s1;

What will happen with the second line?

Looks like it will be s2._graph = s1._graph; , pointer shared, so we are safe?

Answer 1

Default generated copy ctors and assignment operators use the ones provided in the class members.

The shared_ptr copy constructor "shares ownership of the object".

The shared_ptr assignment operator replaces and shares.

If this is the behavior you want, their is no need to explicitly declare the copy ctor and the assignment operator.