Base class unique_ptr to derived class shared_ptr

Question

I have a base class which I am passing with unique_ptr as a reference to a function and I want to copy/move it to a derived class shared_ptr (or unique_ptr what I want it is too guarantee no memory leaked throughout the execution).

I am doing this:

std::shared_ptr<Derived> f(unique_ptr<Base>& b_ptr){

    std::shared_ptr<Derived> d_ptr= std::make_shared<Base>(std::move(b_ptr));
    return d_ptr;
}

This is what I would like to have, but I know that this is not the right way, I would like to know how could I do something like this.

Give up ownership, downcast, pass ownership to shared pointer. — Cheers and hth. - Alf

Russell Greene Russell Greene · Accepted Answer · 2015-07-18T06:21:38

You are almost correct.

std::make_shared always creates a new object, using perfect forwarding, so unless there is a constructor taking a std::unique_ptr&, thats not what you want.

In order to do this, all you need is the std::shared_ptr's constructor from a std::unique_ptr then use the std::static_pointer_cast function:

#include <memory>
#include <iostream>

class Parent{ virtual ~Parent(); };

class Derived : public Parent { };

std::shared_ptr<Derived> f(std::unique_ptr<Parent>&& b_ptr)
{
    auto shared_base = std::shared_ptr < Parent > {std::move(b_ptr)};

    return std::static_pointer_cast<Derived>(shared_base);
}

Also, i found it a good idea to pass b_ptr by rvalue-ref because that way the user has the explcitly use std::move(...) on their object just to illustrate that they won't have ownership over the unique_ptr anymore.

Base class unique_ptr to derived class shared_ptr

1 Answers