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

From: Drew (drewtenn_at_[hidden])
Date: 2003-06-20 22:42:56

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_at_[hidden], "Dean Brettle" <dean_at_b...> wrote:
> 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_at_[hidden] 
>   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 
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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