char buffer[MAX_BUFFER_SIZE]; // static array
...
class Base{...};
class D1 :public Base {...};
class D2 :public Base {...};
class D3 :public Base {...};
...

Base *ProduceObject(ObjectType type)
{
   switch(type)
   {
       case Type1: return new (buffer) D1(); // creating an object using palcement new
       case Type2: return new (buffer) D2();
       case Type3: return new (buffer) D3();
   }
}

void *operator new(unsigned int size, void* ptr)
{
    return ptr;
}

char buffer[1000]; // static array

class Base {
public:
    unsigned int A;
    unsigned int N;
    void SetA(unsigned int a)
    {
        A=a;
    }
};
class D1 :public Base {
public:
    void *operator new(unsigned int size, void* ptr)
    {
    }
    void SetN()
    {
        N=0;
    }
};
class D2 :public Base {
public:
    void SetN()
    {
        N=1;
    }
};
class D3 :public Base {
public:
    void SetN()
    {
        N=2;
    }
};

Base *ProduceObject(unsigned char type)
{
    switch(type)
    {
    case 0: return new (buffer) D1(); // creating an object using palcement new
    case 1: return new (buffer) D2();
    case 2: return new (buffer) D3();
    }
}
int _tmain(int argc, _TCHAR* argv[])
{


    Base *p;
    p=ProduceObject(0);
    p->SetA(10);
}