Question

I have following code snippet:

class ABC{
public:
        int a;
        void print(){cout<<"hello"<<endl;}
};

int main(){
        ABC *ptr = NULL:
        ptr->print();
        return 0;
}

It runs successfully. Can someone explain it?

Answer 1

Under the hood most compilers will transform your class to something like this:

struct _ABC_data{  
    int a ;  
};  
// table of member functions 
void _ABC_print( _ABC_data* this );  

where _ABC_data is a C-style struct
and your call ptr->print(); will be transformed to:

_ABC_print(nullptr)

which is alright while execution since you do not use this arg.

---

UPDATE: (Thanks to Windows programmer for right comment)
Such code is alright only for CPU which executes it.
There is absolutely positively no sane reason to exploit this implementation feature. Because:

1. Standard states it yields undefined behavior
2. If you actually need ability to call member function without instance, using static keyword gives you that with all the portability and compile-time checks