when reading "Beyond the C++ Standard Library: An Introduction to Boost " ,I got a very interesting example:
class A
{
public:
virtual void sing()=0;
protected:
virtual ~A() {};
};
class B : public A
{
public:
virtual void sing( )
{
std::cout << "Do re mi fa so la"<<std::endl;;
}
};
and I do some testing:
int main()
{
//1
std::auto_ptr<A> a(new B); //will not compile ,error: ‘virtual A::~A()’ is protected
//2
A *pa = new B;
delete pa; //will not compile ,error: ‘virtual A::~A()’ is protected
delete (dynamic_cast<B*>(pa)); //ok
//3
boost::shared_ptr<A> a(new B);//ok
}
what I am very curious here is how ~shared_ptr works? how it deduce the derived class B ?
Thanks advance for your help!
thanks all, I write a simple sample about how ~shared_ptr works
class sp_counted_base
{
public:
virtual ~sp_counted_base(){}
};
template<typename T>
class sp_counted_base_impl : public sp_counted_base
{
public:
sp_counted_base_impl(T *t):t_(t){}
~sp_counted_base_impl(){delete t_;}
private:
T *t_;
};
class shared_count
{
public:
static int count_;
template<typename T>
shared_count(T *t):
t_(new sp_counted_base_impl<T>(t))
{
count_ ++;
}
void release()
{
--count_;
if(0 == count_) delete t_;
}
~shared_count()
{
release();
}
private:
sp_counted_base *t_;
};
int shared_count::count_(0);
template<typename T>
class myautoptr
{
public:
template<typename Y>
myautoptr(Y* y):sc_(y),t_(y){}
~myautoptr(){ sc_.release();}
private:
shared_count sc_;
T *t_;
};
int main()
{
myautoptr<A> a(new B);
}
the key is:
Starting with Boost release 1.53, shared_ptr can be used to hold a pointer to a dynamically allocated array. This is accomplished by using an array type ( T[] or T[N] ) as the template parameter.
A shared_ptr can share ownership of an object while storing a pointer to another object. This feature can be used to point to member objects while owning the object they belong to. The stored pointer is the one accessed by get(), the dereference and the comparison operators.
By moving the shared_ptr instead of copying it, we "steal" the atomic reference count and we nullify the other shared_ptr . "stealing" the reference count is not atomic, and it is hundred times faster than copying the shared_ptr (and causing atomic reference increment or decrement).
All the instances point to the same object, and share access to one "control block" that increments and decrements the reference count whenever a new shared_ptr is added, goes out of scope, or is reset. When the reference count reaches zero, the control block deletes the memory resource and itself.
Surprisingly, the key here is not boost::shared_ptr
destructor but its constructor(s).
If you look into boost/shared_ptr.hpp
, you will see that shared_ptr<T>
does not 'simply' have a constructor expecting a T *
but :
template<class Y>
explicit shared_ptr( Y * p );
In //3
when you construct a boost::shared_ptr
from a B *
, no conversion to A *
takes place, and the shared_ptr
internals are built with the actual B
type. Upon destruction of the object, deletion occurs on a B
pointer (not through a base class pointer).
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