为什么shared_ptr可以通过加入“受保护的访问权限”来访问

Question

I made some testing with shared_ptr,and i can't think out the matter below.I just started to learn the boost library. Is there anybody can tell me the reason?

#include <boost\shared_ptr.hpp>
#include <iostream>

class A 
{
public:  
    virtual void sing()
    {
        std::cout<<"A";
    }
protected:  virtual ~A() {};

};

class B : public A 
{
public:  
    virtual void sing() 
    {   
        std::cout << "B"; 
    }
    virtual ~B() {};
};


int foo()
{   
    boost::shared_ptr<A> pa(new B());
    pa->sing();

    delete static_cast<B*>(pa.get());

    delete pa.get(); //this line has a problem error C2248: “A::~A”: can't access protected memmber(declared in class“A")   
    return 0;
}

int main()
{
    foo();
    return 0;
}

but it can be compiled when that line is commented out. Surely it doesn't mean that the shared_ptr will delete the pointer internally maintained out of the main function, just like what i did. Is there any difference between the pointer returned by pa.get() and the pointer internally maintained?

Answer 1

I believe that delete is called during destruction of the shared_ptr on the type of the pointer passed into the constructor. Have a look at the constructor here:

http://www.boost.org/doc/libs/1_49_0/libs/smart_ptr/shared_ptr.htm#constructors

So when your pa goes out of scope, B::~B( ) is called rather than the destructor of the type contained - A::~A ( which would be impossible because it's declared protected ).

Answer 2

Actually, it's a bit more complicated than that: the machinery behind shared_ptr is quite complicated.

First, let us prove there is no specific access rights granted to shared_ptr :

int main() {
    A* a = new B();
    std::shared_ptr<A> p(a); // expected-error
}

This will result in an error because the destructor of A is not accessible. Interestingly, the error occurs at the point of construction, which is a clue...

So, what is the magic behind shared_ptr ?

Internally, a shared_ptr keeps much more than a simple pointer and reference counts. A shared_ptr is built with a deleter , in charge of destructing the instance of the object. Where the design really shines is that this deleter is instantiated in the constructor and thus may know more type information than the bare shared_ptr type lets on.

A simplified demo:

template <typename T>
struct shared_holder {
    typedef void (*Disposer)(T*);

    explicit shared_holder_base(T* t, Disposer d): _ptr(t), _disposer(d) {}

    void dispose() { _disposer(_ptr); _ptr = 0; }

    T* _ptr;
    Disposer _disposer;
};

template <typename U, typename T>
void dispose(T* t) { delete static_cast<U*>(t); }

template <typename T>
class shared_ptr {
    typedef shared_holder<T> holder;
public:
    shared_ptr(): _holder(0), _ptr(0) {}

    template <typename U>
    explicit shared_ptr(U* u):
        _holder(new holder(u, dispose<U, T>)), _ptr(_holder->_ptr) {}

private:
    holder* _holder;
    T* _ptr;
};

The key insight is that the disposer is instantiated from the static type known by the constructor; this is why:

• shared_ptr<A>(new B) works
• A* a = new B; shared_ptr<A>(a) A* a = new B; shared_ptr<A>(a) does not

You can read the Boost headers, the machinery behind the shared_ptr is quite interesting.

As an exercise for the reader, why does shared_ptr<T> has a _ptr member ?

Answer 3

When you have:

boost::shared_ptr<A> pa(new B());

...you are calling boost::shared_ptr constructor and are dealing with TWO template parameters:

1. shared_ptr template type T ( A in your case);

   get() returns T* so when you tried:

    delete pa.get(); 

   ...you tried to access ~A() which is accessible only to A s children and therefore got an error.

   In the following line:

     delete static_cast<B*>(pa.get()); 

   ...you were downcasting A* to B* and called delete on B* thus invoking ~B() to which you had access. ( ~B() is always calling ~A() for ~A() is declared as virtual )

2. shared_ptr constructor argument template type Y ( B in your case) with the requirement that Y* must be convertible to T* (you can upcast B* to A* as B inherits A in your case).

   ~shared_ptr() calls delete on Y* ( B* in your case; ~B() can access ~A() and calls it) and this is what happens when pa goes out of scope (and this is how shared_ptr accessed base class protected destructor which was your original question).

boost::shared_ptr keeps internally a single pointer. When creating boost::shared_ptr you are passing to its constructor a SINGLE pointer which can be regarded as / converted to pointer to any of those TWO template argument types.

Answer 4

I believe it is because the definition of the boost template declares the sharedpointer a friend of the <Class T> class from your template instantiation.

This is a snipet I copied from the boot shared pointer header file.

// Tasteless as this may seem, making all members public allows member templates
// to work in the absence of member template friends. (Matthew Langston)

#ifndef BOOST_NO_MEMBER_TEMPLATE_FRIENDS

private:

    template<class Y> friend class shared_ptr;
    template<class Y> friend class weak_ptr;


#endif