Re: Defect Report: Unintended consequences of N2351

From: (Alberto Ganesh Barbati)
Sun, 19 Aug 2007 00:04:01 GMT
Greg Herlihy ha scritto:

On 8/13/07 8:51 PM, in article 46c121f2$0$30614$,
"Joe Gottman" <> wrote:

    shared_ptr<int> p1; //Empty shared_ptr.
    static int x = 7;
    shared_ptr p2(p1, &x); //"Shares ownership" with p1, which is empty.

While this code is guaranteed not to seg-fault and might be a useful way
to create a shared_ptr to a global or static object, p2 now has the
unusual property that p2.use_count() == 0 but p2.get() != 0.

I hope that there is a more compelling example of this constructor's
usefulness - than simply to create a shared_ptr instance that does not
delete its stored pointer.

Of course that's not the motivating example of this feature! At least
two more interesting examples come to my mind. The first one is the

class Base1 { /* ... */ };
class Base2 { /* ... */ };
class Derived : public Base1, public Base2 { /* ... */ };

void foo(shared_ptr<Base2> p);

void bar(shared_ptr<Derived> p)
   foo(shared_ptr<Base2>(p, static_cast<Base2*>(p.get()));
   // same as:
   // foo(static_pointer_cast<Base2>(p));

The use of multiple inheritance is only to show that the "stored
pointers" can actually have completely different values.

Another case that is similar but cannot be achieved through
*_pointer_cast occurs quite often in COM (delegation to sub-object):

class AbstractInterface { /* ... */ };

class ConcreteInterface : public AbstractInterface { /* ... */ };

struct ObjectClass {
   ConcreteInterface m_iface;
   /* ... */

void foo(shared_ptr<AbstractInterface> p);

void bar(shared_ptr<ObjectClass> p)
   foo(shared_ptr<AbstractInterface>(p, &(p->m_iface)));

These two example alone are sufficient, IMHO, to motivate the "new"
constructor. However, I still can't find an example that can motivate
having an empty shared_ptr with a non-null stored pointer...


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