c++ - Checking if this is null

Question

Does it ever make sense to check if this is null?

Say I have a class with a method; inside that method, I check this == NULL, and if it is, return an error code.

If this is null, then that means the object is deleted. Is the method even able to return anything?

Update: I forgot to mention that the method can be called from multiple threads and it may cause the object to be deleted while another thread is inside the method.

Answer 1

Does it ever make sense to check for this==null? I found this while doing a code review.

In standard C++, it does not, because any call on a null pointer is already undefined behavior, so any code relying on such checks is non-standard (there's no guarantee that the check will even be executed).

Note that this holds true for non-virtual functions as well.

Some implementations permit this==0, however, and consequently libraries written specifically for those implementations will sometimes use it as a hack. A good example of such a pair is VC++ and MFC - I don't recall the exact code, but I distinctly remember seeing if (this == NULL) checks in MFC source code somewhere.

It may also be there as a debugging aid, because at some point in the past this code was hit with this==0 because of a mistake in the caller, so a check was inserted to catch future instances of that. An assert would make more sense for such things, though.

If this == null then that means the object is deleted.

No, it doesn't mean that. It means that a method was called on a null pointer, or on a reference obtained from a null pointer (though obtaining such a reference is already U.B.). This has nothing to do with delete, and does not require any objects of this type to have ever existed.