How to properly duplicate an object given its shared_ptr

Question

I'm trying to make a duplicate of an object of a custom class Event. I have a shared pointer to the object that I've obtained from its allocation:

std::shared_ptr<Event> e = std::make_shared<Event>();

In order to get a true duplicate of e (not just a copy of the pointer) I've tried:

std::shared_ptr<Event> o = std::make_shared<Event>(*e);

But I'm not sure if this is the correct way as it seems that if I delete e it also deletes o...

Btw, I haven't defined a copy constructor Event::Event(const Event &orig) but in my understanding this is not necessary as the compiler provides a default copy constructor. The event class only contains variables and no further pointers.

Answer 1

std::make_shared is just a simple template function that creates the objects, passing all arguments to the constructor :

template<class T, class... Args>
shared_ptr<T> make_shared(Args&&... args)
{
  return shared_ptr<T>( new T( std::forward<Args>( args )... ) );
}

In your particular case :

std::shared_ptr<Event> o = std::make_shared<Event>(*e);

the object is copied.

If your code is such :

void foo() {
    // create new object using default constructor
    std::shared_ptr<Event> e = std::make_shared<Event>();
    // create new object using copy constructor constructor
    std::shared_ptr<Event> o = std::make_shared<Event>(*e);
}

then of course both objects are destroyed, when they go out of scope.

Answer 2

What you tried should work correctly, if the dynamic type of *e is Event, and not some class derived from Event. (If *e is actually an object derived from Event then you will create a new Event (not the derived type) as a copy of the base class part of *e i.e. you will "slice" *e).

Since you create e using make_shared<Event>() you know that in this case it really is an Event, so std::make_shared<Event>(*e) should make a new shared_ptr that owns a copy of *e.