which tool to use to view memory layout of multiple inheritance

Question

Is there a tool that I can use to see the multiple inheritance memory layout of compiled C++ code?

Answer 1

Yes, but you're not going to like it much. What you need to do is use the -S flag (on gcc, check your documentation for other compilers) and generate the assembler code, then read that.

Answer 2

I don't know what exactly you want to know. For this simple example

class classA { };

class classB { };

class classC : public classA, public classB {

};

$ g++ -fdump-class-hierarchy test.cpp

Outputs the following into a file test.cpp.002t.class

Class classA
   size=1 align=1
   base size=0 base align=1
classA (0xb7b06780) 0 empty

Class classB
   size=1 align=1
   base size=0 base align=1
classB (0xb7b067bc) 0 empty

Class classC
   size=1 align=1
   base size=1 base align=1
classC (0xb7a736e0) 0 empty
  classA (0xb7b067f8) 0 empty
  classB (0xb7b06834) 0 empty

See the gcc manpage for details. Changing classA to this:

class classA { int aObj; virtual void aFun() { } };

Suddenly pops up a virtual table:

Vtable for classA
classA::_ZTV6classA: 3u entries
0     (int (*)(...))0
4     (int (*)(...))(& _ZTI6classA)
8     classA::aFun

Class classA
   size=8 align=4
   base size=8 base align=4
classA (0xb7b4d7f8) 0
    vptr=((& classA::_ZTV6classA) + 8u)

Class classB
   size=1 align=1
   base size=0 base align=1
classB (0xb7b4d9d8) 0 empty

Vtable for classC
classC::_ZTV6classC: 3u entries
0     (int (*)(...))0
4     (int (*)(...))(& _ZTI6classC)
8     classA::aFun

Class classC
   size=8 align=4
   base size=8 base align=4
classC (0xb7aba820) 0
    vptr=((& classC::_ZTV6classC) + 8u)
  classA (0xb7b4da14) 0
      primary-for classC (0xb7aba820)
  classB (0xb7b4da50) 0 empty

What looks so strange (_ZTI6classA and _ZTI6classC and _ZTV6classC) is actually a pointer to the RTTI information gcc created for the classes. Using -fno-rtti shows that it will assign a null pointer to the second entries of the vtables then. Nice, have fun digging in those information.