Why class size depend only on data members and not on member functions?

Question

I want to know the detail description on the size of a class. I want to know if there is only data members & member function without any virtual keyword then why the class size depends only on data members. For an eg:

class A {
    int a;
public:
    int display() { 
    cout << "A=" << a << endl;
   }
};

When I check the sizeof(A) i found that it is 4 byte. Why it is so? Why member function has no effect on the size of class A?

Thanks

Answer 1

Because the class's functions are not saved inside the object itself. Think of it in terms of C programming, every function of class A takes one secret parameter, the this pointer, so in actuality they are just functions with one extra parameter.

For example imagine it like this:

int display(A* thisptr)
{
   //do something
   printf("%d",thisptr->a); 
   return;
}

So the display function is saved as just a simple function with one extra parameter. The name though is mangled depending on the compiler.

I believe that different rules apply for virtual functions which would involve function pointers but since I am not sure maybe someone else can enlighten us on this matter.