class memory layout in vc 6.0

"Sigurd Lerstad" <>
Tue, 18 Sep 2007 00:07:41 +0200

I'm trying to figure out how vc 6.0 does class memory layout.

In order to explain what I'm doing right now, I'm going to use an example:

class base0
virtual void f();

class base1

class derived :
    public base0, // derived shares the vfptr with base0 when it is the
first base class
    public base1 // empty (when used as a base class we can use 0 size)
base1 m_member1; // sizeof(1), offset(4) (when used as a member, 0-sized
classes become 1-size classes)
// 3 bytes of padding
int m_member2; // sizeof(4), offset(8)
char m_member3;
// 3 bytes of padding

    virtual void f2();

sizeof(derived) = 16

also, alignment of a class is the maximum alignment of its members.

This example shows what I've gathered so far, yet there are some cases I've
encountered that I just don't understand.. I've even encountered situations
where offset of a base class is dependendant on classes that come AFTER it
in the class inheritance list, also, it seems there are situations where a
0-size base class does affect things, but how I don't know.

If someone can correct or add to what I've got so far, I'd appreciate it.


Sigurd Lerstad

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