https://nimrod.blog/posts/what-does-cpp-object-layout-look-like/

No inheritance

• non-static class member variables are laid out in declaration order, with padding and alignment
• alignof(type): returns the alignment of a type
• alignas(int constexpr or type): specifies alignment requirement for a var
  • can use with class/class member var/var
• attribute(packed): disregard padding and alignment and put member vars consecutive…

Single inheritance

• base class is stacked on top of derived class
• if there is vptr, it is put before base class to make sure vptr is still accessible with object slicing…

inside vtable

• vcall offset
• vbase offset
• delta: offset to top
  • always = 0 for single inherence
  • useful with object slicing with multi inheritance… (<=0 delta)
• typeinfo pointer
• function pointers for virtual functions
  • ABI defines “__cxa_pure_virtual” as the placeholder
  • pure virtual functions: func ptr to “__cxa_pure_virtual”
  • vptr in instantiated objects point to the begin of this list, not the top of vtable!
• if dtor is virtual, there will be 2 dtor func ptr included: complete obj dtor and deleting dtor
  • 1 for objects on stacks, 1 for objects on heap
  • 3 types of destructor:
    • base object destructor: runs the destructors for non-static data members and non-virtual direct base classes
      • the compiler generates the function to execute code in the object destruction then call other destructors (of member var and base classes).
    • complete object destructor: runs base object destructor and the destructors for the virtual base classes
    • deleting destructor: runs complete object destructor and calls the appropriate deallocation function (e.g. op delete)

Multi inheritance

• Parent classes will be stacked on top of each other. Each subobject will retains its own vptr if exists.
  • vtable for each subobject will point differently…
  • subobject destructor implemented using thunk: offset back to the true base then call the true destructor