memory layouts of C++ classes

> After walking the heap to reverse engineer the memory layout
> of a class in MFC, I figured this forum might provide a better
> answer. How does a C++ compiler build the memory layout of a class
> and a derived class?




First, memory layout is not defined by the language... It is an
implementation issue; however, a common approach is the following...


Given:


Class A{
    field a;
}


Class B{
    field b;
}


Class C{
    field c;
}


No Inheritance:


    For class A, the layout would be:


                  first field: pointer to virtual table A
                  second field: the field a.


Single Inheritance:


    For B derived from A (class B : public A), the layout would be:


                  first field: pointer to virtual table for B... first
                                            entries in this table are inherited/overriden
                                            virtual functions of A.


                  second field: the field a.
                  third field: the field b.


Multiple Inheritance:


      For C derived from both A and B (class C : public A, public B), the
        layout would be:


                  first field: pointer to virtual table for C... first
                                            entries in this table are inherited/overriden
                                            virtual functions of A.


                  second field: the field a.
                  third field: pointer to virtual for C...first
                                            entries in this table are inherited/overriden
                                            virtual functions of B.


                  fourth field: the field a in B's class.
                  fifth field the field b
                  sixth field: the field c.




Virtual Inheritance:


          As you can see in the above example, field A is replicated twice in
          Class C. If you want only only one field A, you would need to
          do the following:




class A
                class B : public virtual A
                class C : public virtual A, public B




            The layout for C would be the following:


                  first field: pointer to virtual table for C... first
                                            entries in this table are inherited/overriden
                                            virtual functions of A.


                  second field: pointer to field a.
                  third field: the field a. (points to second field)
                  fourth field: pointer to virtual for C...first
                                            entries in this table are inherited/overriden
                                            virtual functions of B.


                  fifth field: pointer to field a (should point to second field)
                  sixth field: field a (never used, cheaper to allocate here as
                                                              opposed to doing a malloc).
                  seventh field the field b
                  eighth field: the field c.








> Also, what is the difference in memory layout
> of a regular C++ compiler and the memory layouts used by SOM compilers
> to provide binary consistency?


This would be interesting to hear... does someone out there know???


Thank you.


Steve Simmons


[What are SOM compilers? -John]
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