Checking for NULL pointer in C C

Question

In a recent code review  a contributor is trying to enforce that all NULL checks on pointers be performed in the following manner   int   some ptr         if  some ptr    NULL           Handle null-pointer error   else          Proceed     instead of  int   some ptr         if  some ptr           Proceed   else          Handle null-pointer error     I agree that his way is a little more clear in the sense that it s explicitly saying  Make sure this pointer is not NULL   but I would counter that by saying that anyone who s working on this code would understand that using a pointer variable in an if statement is implicitly checking for NULL   Also I feel the second method has a smaller chance of introducing a bug of the ilk   if  some ptr   NULL    which is just an absolute pain to find and debug   Which way do you prefer and why

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I m a huge fan of the fact that C C   doesn t check types in the boolean conditions in if  for and while statements  I always use the following   if  ptr   if   ptr    even on integers or other type that converts to bool   while i--           Something to do i times    while cin  gt  gt  a  gt  gt  b           Do something while you ve input     Coding in this style is more readable and clearer to me  Just my personal opinion   Recently  while working on OKI 431 microcontroller  I ve noticed that the following   unsigned char chx   if  chx           is more efficient than  if  chx    1           because in later case the compiler has to compare the value of chx to 1  Where chx is just a true false flag

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Just one more point in favor of the foo    NULL practice  If foo is  say  an int   or a bool    then the if  foo  check can accidentally be interpreted by a reader as testing the value of the pointee  i e  as if   foo   The NULL comparison here is a reminder that we re talking about a pointer   But I suppose a good naming convention makes this argument moot

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Actually  I use both variants    There are situations  where you first check for the validity of a pointer  and if it is NULL  you just return exit out of a function   I know this can lead to the discussion  should a function have only one exit point    Most of the time  you check the pointer  then do what you want and then resolve the error case  The result can be the ugly x-times indented code with multiple if s

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if  foo  is clear enough  Use it

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Personally I ve always used if  ptr    NULL  because it makes my intent explicit  but at this point it s just a habit   Using   in place of    will be caught by any competent compiler with the correct warning settings   The important point is to pick a consistent style for your group and stick to it  No matter which way you go  you ll eventually get used to it  and the loss of friction when working in other people s code will be welcome

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If style and format are going to be part of your reviews  there should be an agreed upon style guide to measure against   If there is one  do what the style guide says   If there s not one  details like this should be left as they are written   It s a waste of time and energy  and distracts from what code reviews really ought to be uncovering   Seriously  without a style guide I would push to NOT change code like this as a matter of principle  even when it doesn t use the convention I prefer   And not that it matters  but my personal preference is if  ptr    The meaning is more immediately obvious to me than even if  ptr    NULL    Maybe he s trying to say that it s better to handle error conditions before the happy path   In that case I still don t agree with the reviewer   I don t know that there s an accepted convention for this  but in my opinion the most  normal  condition ought to come first in any if statement   That way I ve got less digging to do to figure out what the function is all about and how it works   The exception to this is if the error causes me to bail from the function  or I can recover from it before moving on   In those cases  I do handle the error first   if  error condition    bail or fix      return if not fixed      If I m still here  I m on the happy path   By dealing with the unusual condition up front  I can take care of it and then forget about it   But if I can t get back on the happy path by handling it up front  then it should be handled after the main case because it makes the code more understandable   In my opinion   But if it s not in a style guide then it s just my opinion  and your opinion is just as valid   Either standardize or don t   Don t let a reviewer pseudo-standardize just because he s got an opinion

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This is one of the fundamentals of both languages that pointers evaluate to a type and value that can be used as a control expression  bool in C   and int in C  Just use it

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Frankly  I don t see why it matters   Either one is quite clear and anyone moderately experienced with C or C   should understand both   One comment  though   If you plan to recognize the error and not continue executing the function  i e   you are going to throw an exception or return an error code immediately   you should make it a guard clause   int f void  p        if   p    return -1            p is not null     return 0      This way  you avoid  arrow code

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Pointers are not booleans Modern C C   compilers emit a warning when you write if  foo   bar  by accident    Therefore I prefer   if  foo    NULL           null case   else          non null case     or   if  foo    NULL           non null case   else          null case     However  if I were writing a set of style guidelines I would not be putting things like this in it  I would be putting things like      Make sure you do a null check on the pointer

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I ll start off with this  consistency is king  the decision is less important than the consistency in your code base  In C   NULL is defined as 0 or 0L in C    If you ve read The C   Programming Language Bjarne Stroustrup suggests using 0 explicitly to avoid the NULL macro when doing assignment  I m not sure if he did the same with comparisons  it s been a while since I read the book  I think he just did if some ptr  without an explicit comparison but I am fuzzy on that  The reason for this is that the NULL macro is deceptive  as nearly all macros are  it is actually 0 literal  not a unique type as the name suggests it might be   Avoiding macros is one of the general guidelines in C     On the other hand  0 looks like an integer and it is not when compared to or assigned to pointers   Personally I could go either way  but typically I skip the explicit comparison  though some people dislike this which is probably why you have a contributor suggesting a change anyway   Regardless of personal feelings this is largely a choice of least evil as there isn t one right method  This is clear and a common idiom and I prefer it  there is no chance of accidentally assigning a value during the comparison and it reads clearly  if  some ptr      This is clear if you know that some ptr is a pointer type  but it may also look like an integer comparison  if  some ptr    0      This is clear-ish  in common cases it makes sense    But it s a leaky abstraction  NULL is actually 0 literal and could end up being misused easily  if  some ptr    NULL      C  11 has nullptr which is now the preferred method as it is explicit and accurate  just be careful about accidental assignment  if  some ptr    nullptr      Until you are able to migrate to C  0x I would argue it s a waste of time worrying about which of these methods you use  they are all insufficient which is why nullptr was invented  along with generic programming issues which came up with perfect forwarding    The most important thing is to maintain consistency  In C C is a different beast  In C NULL can be defined as 0 or as   void   0   C99 allows for implementation defined null pointer constants   So it actually comes down to the implementation s definition of NULL and you will have to inspect it in your standard library  Macros are very common and in general they are used a lot to make up for deficiencies in generic programming support in the language and other things as well   The language is much simpler and reliance on the preprocessor more common  From this perspective I d probably recommend using the NULL macro definition in C

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In my experience  tests of the form if  ptr  or if   ptr  are preferred  They do not depend on the definition of the symbol NULL  They do not expose the opportunity for the accidental assignment  And they are clear and succinct    Edit  As SoapBox points out in a comment  they are compatible with C   classes such as auto ptr that are objects that act as pointers and which provide a conversion to bool to enable exactly this idiom  For these objects  an explicit comparison to NULL would have to invoke a conversion to pointer which may have other semantic side effects or be more expensive than the simple existence check that the bool conversion implies   I have a preference for code that says what it means without unneeded text  if  ptr    NULL  has the same meaning as if  ptr  but at the cost of redundant specificity  The next logical thing is to write if   ptr    NULL     TRUE  and that way lies madness  The C language is clear that a boolean tested by if  while or the like has a specific meaning of non-zero value is true and zero is false  Redundancy does not make it clearer

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The C Programming Language  K amp R  would have you check for null    ptr to avoid an accidental assignment

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I use if  ptr   but this is completely not worth arguing about   I like my way because it s concise  though others say    NULL makes it easier to read and more explicit  I see where they re coming from  I just disagree the extra stuff makes it any easier   I hate the macro  so I m biased   Up to you   I disagree with your argument  If you re not getting warnings for assignments in a conditional  you need to turn your warning levels up  Simple as that   And for the love of all that is good  don t switch them around    Note in C  0x  we can do if  ptr    nullptr   which to me does read nicer   Again  I hate the macro  But nullptr is nice   I still do if  ptr   though  just because it s what I m used to

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Most compilers I ve used will at least warn on the if assignment without further syntax sugar  so I don t buy that argument  That said  I ve used both professionally and have no preference for either  The    NULL is definitely clearer though in my opinion

[c++] Checking for NULL pointer in C/C++

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