Object memory layout in C++ and how its structure depends on most platforms

Question

I have two classes in C++:

class Base
{
  int baseField;
 public:
  Base();
  ~Base();
  T BaseMethod();
  virtual SomeMethod()=0;
};

class Derived : public Base
{
  int derivedField;
 public:
  Derived()
  ~Derived();
  T DerivedMethod();
  virtual SomeMethod() {...}; // Some implementation
};

My question is how it's represented in memory, i.e. where is this pointer, vptr (vtable) fields and methods pointers of this class. Ok, it's compiler dependent, but there is some unwritten standards. I'm most interested in Visual Studio and gcc compilers.

[Edit: I put one virtual method to improve example, there weren't any virtuals before]

Is there some books writing on this theme in details

Answer 1

Your classes have no virtual table pointer since they have no virtual functions.

And like you said, even if they had, how virtual functions are implemented is not specified by C++ – every compiler can do it differently, there is no requirement for the existence of a vtable.

In practice, the sane implementation (for single inheritance) is to have a virtual table pointer at the beginning of the class, and after it the normal class members. There is no such thing as “method pointers” – normal member functions are resolved at compile time, and virtual functions have their pointers in the vtable in the order of their declaration.

Here’s an example layout:

struct Base {               + Base ------------+     + Base vtable ----+
    virtual void foo();     | * vptr        ---|---> | * void (*foo)() |
    T bases_field;          | * base_field     |     +-----------------+
};                          +------------------+

struct Derived {            + Derived ---------+     + Derived vtable -+
    T derived_field;        | * CBase instance |---> | * void (*foo)() |
};                          | * derived_field  |     +-----------------+
                            +------------------+

(Not to scale for bytes occupied in memory.)

Each instance thus contains a pointer to a virtual table. Per class, there is only one virtual table. I.e. one shared between all instances of CBase and one shared between all instances of CDerived.