Re: Passing shared_ptr to function that creates obj that shared_ptr points to

21 Jun 2003

      ...
Hi Drew,
You should probably add:
void Fn(DerivedPtr& pObj)
{
  pObj = DerivedPtr(new Derived());
  pObj->m_value = 10;
}
since having a DerivedPtr (or a Derived* for that matter) point to 
an instance of Base is generally a bad idea.  Consider what would 
happen if you called pDerived->method_only_in_derived().
--Dean
  ----- Original Message ----- 
  From: Drew 
  To: Boost-Users@yahoogroups.com 
  Sent: Thursday, June 19, 2003 8:38 AM
  Subject: [Boost-Users] Passing shared_ptr to function that 
creates obj that shared_ptr points to
[Moderator's note: The only apparent connection this message has 
to Boost is in the subject line. I am approving it on the assumption
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...

--- In Boost-Users@yahoogroups.com, "Dean Brettle" <dean@b...> wrote:
that the missing declarations for ParentPtr and DerivedPtr are 
typedefs using boost::shared_ptr.]
...
Hello everyone...
I am attempting to do the following:
class Parent
  {
  public:
    int m_value;
  };
class Derived : public Parent
  {
  };
void Fn(ParentPtr& pObj)
  {
    pObj = ParentPtr(new Parent());
    pObj->m_value = 10;
  }
int main()
  {
    ParentPtr pParent;
    DerivedPtr pDerived;
Fn(pDerived);
return 0;
  }
If I pass the parent pointer, it compiles and works fine. If I
pass
...
the derived pointer I get:
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
  Does anyone know how to solve my problem?
TIA,
  Drew
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