使用智能指针复制构造函数

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.)