How is possible that accessing nullptr works? [duplicate]

Question

This question already has an answer here:

• Why am I able to make a function call using an invalid class pointer 6 answers
• Calling class method through NULL class pointer [duplicate] 10 answers

I have a simple class:

class B
{
public:
    int getData() { return 3; }
};

then, I initialize a pointer to it with nullptr:

B *foo{ nullptr };

And then, trying to use it comes the surprise:

int t = foo->getData();

and t is now 3. How is that possible without constructing the class? Is it because getData() does not use "this"? That broke all my knowledge about pointers.

Is that expected behavior? I am working in Visual Studio 2013.

Answer 1

Is that expected behavior?

No, it's UB, anything is possible.

Is it because getData() does not use "this"?

Yes, it might work because this won't be used in the special case, but nothing is guaranteed.

Answer 2

It is undefined behavior, so you really should be very scared.

It might happen to apparently do something (on your implementation of C++), because the getData function is not virtual and don't use any member of B. So the generated code does not dereference the null pointer.

Answer 3

It is undefined behaviour, you are accessing a member method from a nullptr. No particular behaviour or outcome would be defined.

In this case though; given (cppreference):

The keyword this is a prvalue expression whose value is the address of the object, on which the member function is being called.

Since the NULL value of this is not being dereference - the member method is not virtual and doesn't access any member data of this - it "seems" to work.