How to avoid successive deallocations/allocations in C++?

Question

Consider the following code:

class A
{
    B* b; // an A object owns a B object

    A() : b(NULL) { } // we don\'t know what b will be when constructing A

             


        

Answer 1

How about allocating the memory for B once (or for it's biggest possible variant) and using placement new?

A would store char memB[sizeof(BiggestB)]; and a B*. Sure, you'd need to manually call the destructors, but no memory would be allocated/deallocated.

   void* p = memB;
   B* b = new(p) SomeB();
   ...
   b->~B();   // explicit destructor call when needed.