Re: Factory and placement new

"Heinz Ozwirk" <>
3 Aug 2006 08:52:27 -0400
<> schrieb im Newsbeitrag


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

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

       Base* ptr = ...; // extract from free list
       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))?.

First of all, I would call as soon as an object is released. Why do you want

to wait until it will be recycled? That way objects not taken from the free
list would never be destroyed. Not really a good idea if their destructor
does something important like flushing buffers or closing files.

Second, you code might stop working when you have to deal with multiple
inheritance. If you have more than one base class, only one of them can
start at the same address as the derived class.

Third. Since that factory code only works with a single type, it should
return and accept the type it really handles, Derived in your example. As
you already mentioned, it will not work for classes of different sizes. And
you probably cannot assume that classes, that are now of the same size, will

remain so in the future. So it really is not a good idea, to use pointers to

the base class. At least not with some extra work.


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