Factory and placement new

Hi!

I'm planning something described below:

struct Base
{
     Base(int i, int j) : i_(i), j_(j) {};
     virtual ~Base() {};

     int i_;
     int j_;
};

struct Derived : public Base
{
     Derived(int i, int j) : Base(i,j) {};
     virtual ~Derived() {};
}

class Factory
{
public:

     virtual Base* create(int arg1, int arg2)
     {
         if( !free_ )
             return new Derived(arg1, arg2);

        Base* ptr = ...; // extract from free list
        ptr->~Base();
        new (ptr) Derived(arg1, arg2);
        return ptr;
     };

     virtual void release(Base* ptr)
     {
         // add ptr to free list
     };

};

Does this work? I mean that does the Base-pointer point always to the
start of the allocated memory block so that the subsequent
Derived-objects created with the placement new shall fit to the space?

My own humble guess is that this works and is quite safe if you only
have a single factory object in use at a time (means that we only have
one Base-derived type in use) but things get dangerous if there is any
possibility that Base-derived objects get released to a factory that
didn't create them (because there is no guarantee that sizeof(Derived)
== sizeof(Derived2))?.

Do you find any other pitfalls or reasons why I shouldn't do this?

--
     jpa

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