upcast at runtime. (Morph from Base Class to derive Class)

Question

class B{
  private:
    int a;
}
class D: public B{
  private:
    int b;
}

B* b = new B;

Now for some reason I want turn b into a D* Type of Object. e.g. retain the information of B and become D with extra Informations required.

What I am currently thinking of is. static_cast to do the upcasting. the additional attributes will be set to null or garbage. then assign the additional attributes manually. But this leads to a dangling pointer risk. If the copy constructor is not coded with enough care.

So what else could be the proper Solution ?
and is there any solution of the Puzzle if we think it from a PHP perspective ?

Answer 1

If you do a static_cast<> then the additional attributes will not be null or garbage, they will actually be outside the memory allocated for the object. Use dynamic_cast<>, which will properly fail if the variable does not contain the correct type.

Also, the similarity between C++ and PHP in this case is merely superficial; do not use it to gain further "insight" into the C++ code.

Answer 2

b points to an object of type B. It can't become a D. If you want a D, you have to allocate one and copy the fields from *b.

Never think of things from a PHP perspective unless you're writing something in PHP. Never write anything in PHP.

Answer 3

Reconstructing a base object to a Derived object is generally not possible. First of all, The allocation only allocated enought space for B and not for D. If you allocated enought space for a D to create the B object (which you currently didn't, and is a bit hard to do anyway), then you could use hacks to 'reconstruct' a object like this :

// EXTREME HACK, BE CAREFUL, AND USE WITH CARE
// IF YOU DON'T UNDERSTAND IT, DON'T USE IT.
// you should have allocated (sizeof (D)) space to your b pointer.
D* d_object = new (b) D(B(*b));
// you should consider any previous pointer pointing toward the old B* as dangling.