Share to learn: How an object in C/C++ is stored


No Inheritance


class A
{
public:
    int a;
};
 
class B
{
public:
    int b;
};
 
class C: public A, public B
{
public:
    int c;
};
 
void main()
{
    C obj;
    cout << &obj << endl << &obj.a << endl << &obj.b << endl << &obj.c << endl;
}

Output:

0012FF58

0012FF5C

0012FF60

if change the order of inheritance into:

class C : public B, public A

the output is:

0012FF58

0012FF5C

0012FF58

0012FF60

The storage of a derived class is dependent on the declare order.

Single Inheritance

In the case that a virtual function is added to A:

class A
{
public:
int a;
virtual void fA() {}
};
...
class C: public A, public B
...

Output:

0012FF54

0012FF58

0012FF5C

0012FF60

the new address of obj now is 0012FF54 instead of 0012FF58. First extra 4 bytes is used to for __vfptr. And one more thing, if I change the order of inheritance (don't remove the virtual function in A), the output does not change (!!!).

It seems that in the multiple inheritance case, the class which contains virtual methodhas higher priority when being storaged.

Multiple Inheritance

Next, add a new virtual method to B and change to order of inheritance in turn, a C object now will contain two __vfptr corresponding with the A and B (or B and A up to the declaration order).

Output:

0012FF50

0012FF54

0012FF5C

0012FF60

The number of __vfptrs which an object contains are up to the number of classes it inherits.

(To be continued ... )