How to implement deep copy feature in some smart pointer?

问题

unique_ptr is quite useful. However, it is not copyable. If virutal clone (deep copy) methods are provided for its pointed class, I think it will become more useful. Is it necessary or any better way to implement it? Any similar smart pointer exist in some library? Here is a version

template<class T>
class deep_ptr: private unique_ptr<T>
{
public:
    using unique_ptr<T>::operator *;
    using unique_ptr<T>::operator ->;
    using unique_ptr<T>::operator bool;
    using unique_ptr<T>::release;
    using unique_ptr<T>::reset;
    using unique_ptr<T>::get;

    // add (DEFAULT_CONSTRUCTOR)(MOVE_CONSTRUCTOR)(MOVE_ASSIGNMENT_METHOD) ...

    explicit deep_ptr(T* p) : unique_ptr(p) {}

    deep_ptr(deep_ptr const& r) : unique_ptr(r->clone()) {}

    deep_ptr& operator=(deep_ptrconst& r)
    { if (this != &r) reset(r->clone()); return *this; }
};

Juse feel it is very useful but never see similar things. ???

回答1:

Unless I am misunderstanding what you are looking for, if a class has a clone method, that should be sufficient to get what you are looking for.

Sample code:

#include <iostream>
#include <memory>

struct A
{
   virtual ~A() {}
   virtual A* clone() = 0;
};

struct B : A
{
   B(int in = 0) : x(in) {}
   B(B const& copy) : x(copy.x) {}
   virtual ~B() {std::cout << "In B::~B()\n";}

   virtual A* clone() { return new B(*this); }
   int x;
};

int main()
{
   std::unique_ptr<A> p1(new B(10));
   std::unique_ptr<A> p2(p1->clone());
   return 0;
}

Output from running the above program:

In B::~B()
In B::~B()

回答2:

Without a clone method (just a copy-constructor) the following should work:

template <typename T>
class deep_ptr
{
public:
  deep_ptr() : i_() {}

  deep_ptr(std::nullptr_t) : i_(nullptr) {}

  template <typename U>
    deep_ptr(U* u) : i_(u ? new inner_impl<U>(*u) : nullptr) {}

  ~deep_ptr() { delete i_; }

  deep_ptr(const deep_ptr& p) : i_(p.i_ ? p.i_->copy() : nullptr) {}

  deep_ptr& operator=(const deep_ptr& p)
  {
    if (!p.i_) { i_ = nullptr; }
    else { i_ = p.i_->copy(); }
  }

  deep_ptr(deep_ptr&& p) : i_(p.i_) { p.i_ = nullptr; }

  deep_ptr& operator=(deep_ptr&& p)
  {
    i_ = p.i_;
    p.i_ = nullptr;
  }

  const T* operator->() const { return get(); }

  const T* get() const
  {
    if (i_) { return *i_; }
    return nullptr;
  }

  const T& operator*() const { return *static_cast<T*>(*i_); }

  T* operator->() { return get(); }

  T* get()
  {
    if (i_) { return *i_; }
    return nullptr;
  }

  T& operator*(){ return *static_cast<T*>(*i_); }

private:
  struct inner
  { 
    virtual inner* copy() const = 0;
    virtual operator const T*() const  = 0;
    virtual operator T*() = 0;
    virtual ~inner() {}
  };

  inner* i_;

  template <typename U>
  struct inner_impl : inner
  {
    inner_impl(const U& u) : u_(u) {}
    inner_impl* copy() const override { return new inner_impl(u_); }
    operator const T*() const override { return &u_; }
    operator T*() override { return &u_; }

    U u_;
  };
};

来源：https://stackoverflow.com/questions/24334888/how-to-implement-deep-copy-feature-in-some-smart-pointer

标签

c++

smart-pointers