The memory layout of a struct/class in cpp¶

1. without virtual function¶

Static in global variable state
Other order located in the struct

2. with virtual function¶

1. Self owning virtual function¶

This will place a vtable_ptr at the start of this struct. which type is void **. And it is pointing to the real and unique virtual function table: A array of void* (function ptr)

2. Inherited from a struct with virtual function¶

vtable_ptr holding by the parent. And the child's virtual functions will push_back to the table that parent owing. But parent will only know the size of function of itself knows.

The child call its virtual function, can get the parent's vtable_ptr, then do a offset binary operation to update it.

3. Inherited from 2 struct with virtual functions¶

1. Both A and B have virtual functions¶

C++
struct A{int a; virtual void f_a(){}};
struct B{int b; virtual void f_b(){}};
struct C: public A, public B
{
    int c;
    virtual void f_c();
}

the f_c function will be push_back to A's vtable

2. A have virtual functions , but B dose not have¶

same as the condition above...

3. A does not virtual functions , but B have¶

C++
struct A{int a; };
struct B{int b; virtual void f_b(){}};
struct C: public A, public B
{
    int c;
    virtual void f_c();
}

the f_c function will be push_back to B's vtable.. BUT the B's vtable will be put at the start of this struct.