4 bytes in the a non-virtual class get reserved,can't figure out what?

[michelle1278]

I have the following class hierarchy:

Class Base
{
void baseFoo();
//some more functions
private:
int baseIdx;
};

class Derived : public Base
{
void derFoo();
//some more functions
private:
int spvar;
};

then object of type Derived:
Derived derObj;

When I checked the memory layout of "derObj", the first 4 bytes of the object are reserved. The value of the int baseIdx starts at the 5th byte.

I cannot understand why those first 4 bytes are reserved, there's no virtual functions/ vtable involved in this class hierarchy.

 

[teriviret]

What do the first 4 bytes look like? Do they point to a vtable or is it something else? Did you turn on RTTI, maybe?

 

[ArkM]

In regular compilation mode (no RTTI and other exotics) all member addresses are OK: &object == &base member var (VC++ SP5)...

 

[Zyrenthian]

I believe it is because they are declared as a class. I would assume that this is related to why a sizeof(class) will not always be the size of the member data but a sizeof(struct) will be. You could change them to a struct instead of a class and retain the inheritance if you want to ensure the memory is as expected.

Matt

 

[ArkM]

No any 'sizeof differences' between classes and structures in C++.
sizeof(class|struct) != sum(sizeof(members)) when memory align needed (or a class/structure has virtual functions - vtbl ptr added). RTTI support (as usually) implemented via vtbl (see teriviret's post)...