[boost] Using shared_ptr in an object hierarchy

From: David B. Held (dheld_at_[hidden])
Date: 2002-03-24 13:41:20

I have something like this:

class Base
{
public:
    virtual void foo();
};

class Derived
{
public:
    virtual void foo();
    void bar();
};

typedef boost::shared_ptr<Base> PBase;
typedef boost::shared_ptr<Derived> PDerived;

std::vector<PBase> BaseObjs;
std::vector<PDerived> DerivedObjs;

I need DerivedObjs so I can call Derived::bar() without having
to do type checking and casting, but all Deriveds will go into
both BaseObjs and DerivedObjs, so I can call foo() 
polymorphically.  Here was my first attempt:

PDerived p(new Derived());

BaseObjs.push_back(p);
DerivedObjs.push_back(p);

Unfortunately, this didn't let me call foo() polymorphically from
BaseObjs.  I guess when it converted from PBase to PDerived,
the underlying pointer lost the information that it was actually a
Derived*.  I don't know.  So I tried this:

Derived* p = new Derived();

BaseObjs.push_back(PBase(p));
DerivedObjs.push_back(PDerived(p));

This allows me to call foo() polymorphically, but, of course, fails
when everything gets cleaned up, since I passed ownership of
p to two different smart pointers. :(  At this point, some advice
would be appreciated. ;)

Dave