Copy constructor with smart pointer - c++

Question

I have a class with one std::unique_ptr as class member. I was wondering, how to correctly define the copy constructor, since I'm getting the following compiler error message: error C2248: std::unique_ptr<_Ty>::unique_ptr : cannot access private member declared in class 'std::unique_ptr<_Ty>. My class design looks something like:
template <typename T>
class Foo{
public:
Foo(){};
Foo( Bar<T> *, int );
Foo( const Foo<T> & );
~Foo(){};
void swap( Foo<T> & );
Foo<T> operator = ( Foo<T> );
private:
std::unique_ptr<Bar> m_ptrBar;
int m_Param1;
};
template < typename T >
Foo<T>::Foo( const Foo<T> & refFoo )
:m_ptrBar(refFoo.m_ptrBar),
m_Param1(refFoo.m_Param1)
{
// error here!
}
template < typename T >
void Foo<T>::swap( Foo<T> & refFoo ){
using std::swap;
swap(m_ptrBar, refFoo.m_ptrBar);
swap(m_Param1, refFoo.m_Param1);
}
template < typename T >
Foo<T> Foo<T>::operator = ( Foo<T> Elem ){
Elem.swap(*this);
return (*this);
}

Answer 1

Assuming the goal is to copy-construct the uniquely-owned Bar,
template < typename T >
Foo<T>::Foo( const Foo<T> & refFoo )
: m_ptrBar(refFoo.m_ptrBar ? new Bar(*refFoo.m_ptrBar) : nullptr),
m_Param1(refFoo.m_Param1)
{
}

Answer 2

Unique_ptr documentation:
Stores a pointer to an owned object. The object is owned by no other unique_ptr.
The object is destroyed when the unique_ptr is destroyed.
You cant copy it because two objects can't own it.
Try switching to a std::shared_ptr.
EDIT I should point out that this would make both objects have a pointer to that same object. If you want to copy the uniquely owned object Cubbi's solution is the correct one.

Answer 3

A possibility is to create a new clone_ptr type for this.
Below is a rudimentary example of a clone_ptr that invokes the correct copy constructor (and destructor) of a derived object. This is done here by creating a "type erasure" helper when the clone_ptr is created.
Other implementations may be found on the Internet.
#include <memory>
namespace clone_ptr_detail
{
template <class T>
class clone_ptr_helper_base
{
public:
virtual ~clone_ptr_helper_base() {}
virtual T* clone(const T* source) const = 0;
virtual void destroy(const T* p) const = 0;
};
template <class T, class U>
class clone_ptr_helper: public clone_ptr_helper_base<T>
{
public:
virtual T* clone(const T* source) const
{
return new U(static_cast<const U&>(*source));
}
virtual void destroy(const T* p) const
{
delete static_cast<const U*>(p);
}
};
}
template <class T>
class clone_ptr
{
T* ptr;
std::shared_ptr<clone_ptr_detail::clone_ptr_helper_base<T>> ptr_helper;
public:
template <class U>
explicit clone_ptr(U* p): ptr(p), ptr_helper(new clone_ptr_detail::clone_ptr_helper<T, U>()) {}
clone_ptr(const clone_ptr& other): ptr(other.ptr_helper->clone(other.ptr)), ptr_helper(other.ptr_helper) {}
clone_ptr& operator=(clone_ptr rhv)
{
swap(rhv);
return *this;
}
~clone_ptr()
{
ptr_helper->destroy(ptr);
}
T* get() const { /*error checking here*/ return ptr; }
T& operator* () const { return *get(); }
T* operator-> () const { return get(); }
void swap(clone_ptr& other)
{
std::swap(ptr, other.ptr);
ptr_helper.swap(other.ptr_helper);
}
};
See usage example: http://ideone.com/LnWa3
(But perhaps you don't really need to copy your objects, and might rather explore the possibilities of move semantics. For example, you can have a vector<unique_ptr<T>>, as long as you don't use functions that copy the contents.)