CODEWITHSUNDEEP
c++smart-pointersunique-ptrdeep-copy
XYZCODE123
XYZCODE123

Reputation: 372

Why std::make_unique calls copy constructor

I'm wondering why make_unique calls copy constructor but does not call default constructor.

Node()
{
    std::cout << "default";
}

~Node(){
    std::cout << " Delete";
}

Node(const Node& other ){
    std::cout << "copy";
}

int main(){
    Node<int,int,int,int> test1; //Calling default Cons
    std::unique_ptr<Node<int,int,int,int>> test2  = 
                               std::make_unique<Node<int,int,int,int>>(test1);//Nothing called

    Node<int,int,int,int> *test3 = test2.get();
    Node<int,int,int,int> test4 = Node<int,int,int,int>(*test3);// Calling copy Cons

    std::unique_ptr<Node<int,int,int,int>> test5 = 
                            std::make_unique<Node<int,int,int,int>(test4);//Calling copy Cons
}

For example in code shown above: Frist, we create Node object -> calling default constructor. Then we wrap this object into smart-pointer object -> no calls.

But if we make deep copy of Node object -> calling copy constructor And then wrap copy into smart-pointer object-> calling copy constructor.

It's somehow connected to the creation of new control block?

Upvotes: 4

Views: 7707

Answers (1)

Michael Chourdakis
Michael Chourdakis

Reputation: 11158

std::unique_ptr<Node<int,int,int,int>> test5 = 
   std::make_unique<Node<int,int,int,int>(test4);// You copy from test4

std::make_unique creates a new pointer. It doesn't reuse an existing pointer. So since test4 is already there, it has to copy it. After test5 is constructed, test4 is still there (so can't do a move) and test5 is holding a new object, copied from test4.

Upvotes: 6

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