Calling class method through NULL class pointer [duplicate]

Answer 1

Calling member functions using a pointer that does not point to a valid object results in undefined behavior. Anything could happen. It could run; it could crash.

In this case, it appears to work because the this pointer, which does not point to a valid object, is not used in print.

Answer 2

probably it runs because your class pointer is not using any member variable in print function...If in print function you try to access a it will not run... as uninitialized class pointer can't have initialized member variable...

Answer 3

Most answers said that undefined behaviour can include "appearing" to work, and they are right.

Alexander Malakhov's answer gave implementation details which are kind of common and explain why your situation appeared to work, but he made a slight misstatement. He wrote "which is alright while execution since you do not use this arg" but meant "which appeared to be alright while execution since you do not use this arg".

But be warned, your code still is undefined behaviour. It printed what you wanted AND it transfered the balance of your bank account to mine. I thank you.

(SO style says this should be a comment but it's too long. I made it CW though.)

Answer 4

As others said, it is undefined behavior. Regarding the reason why it appears to work is that you are not trying to access the member variable a inside the print(). All the instances of the class share same memory for the code of print() hence this pointer is not required to access the method. However, if you try to access a inside the method you are most likely to get an access violation exception.