In a member function, can this == NULL?

Question

Lets say I have the following class

struct FooList {
    int data;
    FooList *next;

    // basic constructor
    FooList(int d, FooList *n): data(d), next(n) {}
    // basic destructor
    ~FooList() { if (next) delete next; }

    FooList *functional_append(FooList *) const;
};

And then in my implementation, I define

FooList *FooList::functional_append(FooList *other) const {
    if (this == NULL)
        return other;
    else
        return new FooList(data, next->functional_append(other));
}

Does the condition on the second line make any sense? Will this ever be NULL? I know that if functional_append was declared virtual then this would throw a segmentation fault once this == NULL, but as a simple member function would it still work like I expect it to?

Note: I am looking for a reason why the above will or will not work (like the equivalent C code), not just "No, it can't happen" or "Yes, if force it to like this ...".

Yes, it can. Initialize `FooList *ptr = NULL;`, and then call `ptr->functional_append(...);`. — barak manos, Dec 22 '14 at 15:14
It is UB that very often works. Until some hapless victim uses it as a MI base class. — Hans Passant, Dec 22 '14 at 15:18

πάντα ῥεῖ · Answer 1 · 2014-12-22T15:35:58.330

2

You can do something like

 MyClass* p = NULL;
 p->foo();

and foo() might by actually called, and a test for this == NULL yields true.

Nevertheless the above sample is simply raising undefined behavior, at the point when p is dereferenced. A check for this == NULL to determine any further logical behavior, is beyond all hope.

edited Dec 22 '14 at 15:35

answered Dec 22 '14 at 15:11

πάντα ῥεῖ

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Well I believe you never be able to see this as NULL in this scenario... – ravi Dec 22 '14 at 15:17
@ravi I didn't say that (besides in my comment). I just stated it's going to be UB. – πάντα ῥεῖ Dec 22 '14 at 15:23

ravi · Accepted Answer · 2014-12-22T15:20:12.490

1

Will this ever be NULL?

Yes, this could be NULL. But that would be the case your program is behaving incorrectly ( i.e maybe corrupted memory or something else). Then there would be many more things to check for rather than putting this == NULL.

In normal scenarios this can never be NULL as when you call a function on a valid object then this is implicitly passed by compiler to that function.

edited Dec 22 '14 at 15:20

answered Dec 22 '14 at 15:13

ravi

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In a member function, can this == NULL?

2 Answers2