Re: [Boost-Users] Re: Passing shared_ptr to function that creates obj that shared_ptr points to

23 Jun 2003

      This is normal behaviour for a pointer too, so shared_ptr is correct to
disallow this :

void AddObject(CBase&* pObj);
...
  CDerived* pDerived = 0;.
  AddObject(p);   // also won't compile

If you think about it this makes sense.  Imagine, if that behaviour were
allowed this would cause some problems:

class CDerived2 : public CBase  {};

void AddObject(CBase&* pObj)
{
  pObj = new CDerived2;
}
...
  CDerived* pDerived  = 0;
  AddObject(pDerived);   // eeek!  pDerived now points to a CDerived2
object!

Normally in this situation I would return a shared_ptr<> rather than take
one by reference. (I prefer that style anyway, but lets not get into that
:-) ). You will then have to do a cast to the derived object ptr:

BasePtr AddObject();
...
  BasePtr p = AddObject();
  if (DerivedPtr pDerived = shared_dynamic_cast<CDerived>(p))
  {
    //  ...
  }

HTH

Sam

Drew wrote:
...
To be a little more detailed, here is what I am trying to accomplish:
typedef boost::shared_ptr<CBase> BasePtr;
typedef boost::shared_ptr<CDerived> DerivedPtr;
AddObject(pBase); // works
AddObject(pDerived); // does not work
void AddObject(BasePtr& pObj)
{
  std::vector<BasePtr>::iterator pos = find(myList.begin(),
myList.end(), 5);
if (pos != myList.end()) {
    pObj= *pos;
  }
  else {
    myList.push_back(pObj);
  }
}
error C2664: 'Fn' : cannot convert parameter 1 from 'DerivedPtr'
to 'ParentPtr &'
A reference that is not to 'const' cannot be bound to a non-lvalue
However,
AddObject(base);  // works
AddObject(derived); // works
void AddObject(CBase& base)
{
}
This is a generic function that adds an element if it does not exist.
Why when I use a shared_ptr does this not work?
I dont want a specific lookup for all of my objects.
Thanks guys...

Re: [Boost-Users] Re: Passing shared_ptr to function that creates obj that shared_ptr points to

Sam Partington