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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.
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To check for null variables, you can use a strict equality operator ( === ) to compare the variable with null . This is demonstrated below, where the boolean expression evaluates to true for only for null and evaluates to false for other falsy values.
Use the is operator to check if a variable is null in Python, e.g. if my_var is None: . The is operator returns True if the values on the left-hand and right-hand sides point to the same object and should be used when checking for singletons like None . Copied! Note that there isn't a null value in Python.
Conclusion. To check null in JavaScript, use triple equals operator(===) or Object.is() method. To find the difference between null and undefined, use the triple equality operator or Object.is() method. To loosely check if the variable is null, use a double equality operator(==).
It is a good idea to check for null explicitly because: You can catch the error earlier. You can provide a more descriptive error message.
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.
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Your note about threads is worrisome. I'm pretty sure you have a race condition that can lead to a crash. If a thread deletes an object and zeros the pointer, another thread could make a call through that pointer between those two operations, leading to this being non-null and also not valid, resulting in a crash. Similarly, if a thread calls a method while another thread is in the middle of creating the object, you may also get a crash.
Short answer, you really need to use a mutex or something to synchonize access to this variable. You need to ensure that this is never null or you're going to have problems.
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