Like C, C++ doesn’t define layouts, just semantic constraints that must be met. Therefore different implementations do things differently. One good explanation is in a book that is otherwise outdated and doesn’t describe any current C++ implementation: The Annotated C++ Reference Manual (usually called the ARM). It has diagrams of key layout examples. There is a very brief explanation in Chapter 2 of TC++PL3.
Basically, C++ constructs objects simply by concatenating sub objects. Thus
is represented by two
ints next to each other, and
is represented by an
A followed by an
int; that is, by three
ints next to each other.
Virtual functions are typically implemented by adding a pointer (the “vptr”) to each object of a class with virtual functions. This pointer points to the appropriate table of functions (the “vtbl”). Each class has its own vtbl shared by all objects of that class.