Find out the size of a polymorphic object

Question

I have an pointer Base* base_ptr to an polymorphic object. Is it possible to find out the size of the dynamic type of said object?

AFAIK, sizeof(*base_ptr) yilds the size of the static type of base_ptr. I'm beginning to suspect this isn't possible, but maybe I'm overlooking something.

Note: I'm aware that I could add a virtual function to my type hierarchy which returns the size, but this is not a desirable solution in my case.

EDIT: sizeof(base_ptr) -> sizeof(*base_ptr)

`sizeof(base_ptr)` gives you the size of the pointer, not of any class. — Kerrek SB, Nov 14 '11 at 14:12
It looks like even the g++-specific doesn't provide this information. — aschepler, Nov 14 '11 at 14:34

score 13 · Accepted Answer · answered Nov 14 '11 at 14:11

13

No, you can't do that in C++ - at least in a portable way. The best bet would be to have getSize() member function implemented in each class.

answered Nov 14 '11 at 14:11

sharptooth

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score 8 · Answer 2 · answered Nov 14 '11 at 14:18

Yes. You can implement a virtual function in the base class which returns the size:

class Base
{
   virtual int size() { return sizeof(Base); }
};
class Derived : public Base
{
   virtual int size() { return sizeof(Derived); }
};

//......
Base* b = new Derived;
int size = b->size(); //will call Derived::size() and return correct size

score 2 · Answer 3 · edited May 23 '17 at 12:01

2

You can use CRTP idiom, if possible, as I described here: https://stackoverflow.com/a/14730166/908336

edited May 23 '17 at 12:01

Community

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answered Feb 06 '13 at 13:31

Masood Khaari

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Find out the size of a polymorphic object

3 Answers3

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