virtual and sizeof

jackandking · 05-04-2009, 04:36 AM

class A{
virtual void foo(){};
};
sizeof(A)=1

class B : public virtural A{
virtual void foo(){};
};
sizeof(B)=? and why

dwhitney67 · 05-04-2009, 05:13 AM

I do not no how you drew the conclusion that the size of class A is 1 byte.

Anytime a class has a virtual method declared within it, the class size will at a minimum, be 4 bytes. The is the number of bytes necessary to hold the address of the the location of the virtual table associated with this class.

When you subclass A, to form class B, the address of the virtual table is still 4 bytes; thus using the code as you have presented, the size of class B will also be 4 bytes.

jackandking · 05-04-2009, 08:16 PM

Yes, you are right about sizeof(A)=4, sorry for my thoughtless.

About sizeof(B)=4, I am confused, how about sizeof(C) list below:
class C : public A{
virtual void foo(){};
};

dwhitney67 · 05-05-2009, 04:56 AM

It would not kill you to write a simple application with the classes that you have defined, include <iostream>, and print out the sizeof() value returned for each of the classes.

Class C will also have a sizeof() 4.

jackandking · 05-05-2009, 09:08 PM

sorry for my laziness.

why sizeof(C) is 4?
what's the difference between class B and C?
I thought virtually inheritant should import some difference to class B.

amysaraantony · 05-06-2009, 11:37 AM

Quote:

Originally Posted by jackandking

sorry for my laziness.

Bad Boy jackandking .... Really Bad Boy !!!

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