In the following code, what is the benefit of using (!!p) instead of (p != NULL)?
AClass *p = getInstanceOfAClass();
if( !!p )
// do something
else
// do something without having valid pointer
It is pretty much the same, although I consider the !!p to be bad style, and usually indicates a coder trying to be clever.
That's a matter of style, in fact they are equivalent. See this very similar question for discussion.
IMO comparing against null pointer is clearer.
I thing GMan’s original comment should be the accepted answer:
I wonder what's wrong with just if (p)
The point is: nothing is wrong with it, and this should be the preferred way. First off, !!p is “too clever”; it’s also completely unnecessary and thus bad (notice: we’re talking about pointers in an if statement here, so Anacrolix’ comment, while generally valid, doesn’t apply here!).
The same goes for p != NULL. While this is possible, it’s just not needed. It’s more code, it’s completely redundant code and hence it makes the code worse. The truest thing Jeff Atwood ever said was that “the best code is no code at all.” Avoid redundant syntax. Stick to the minimum (that still conveys the complete meaning; if (p) is complete).
Finally, if (p) is arguably the most idiomatic way to write this in C++. C++ bends over backwards to get this same behaviour for other types in the language (e.g. data streams), at the cost of some very weird quirks. The next version of the standard even introduces new a syntax to achieve this behaviour in user-defined types.
For pointers, we get the same for free. So use it.
/EDIT: About clarity: sharptooth writes that
IMO comparing against null pointer is clearer.
I claim that this is objectively wrong: if (p) is clearer. There is no possible way that this statement could mean anything else, neither in this context nor in any other, in C++.
As far as I can see, it's just a shorter way to convert it into a boolean value. It applies the ! twice, though, whereas p != NULL does one comparison. So I guess the benefit is just shorter code, albeit more cryptic if you don't know what !!p is supposed to mean.
They are the same, but I recommend to use
NULL != p
It is more readable.
There is no difference in the given example.
However the assumption that this applies to all cases is incorrect. a = not not b is not the same as a = b, as far as integer types are concerned.
In C, 0 is false. Anything but 0 is true. But not 0 is 1, and nothing else. In C++, true casts to 1 as an integer, not only for backward compatibilty with C, but because 1 is not 0, and 1 is the most common value used to denote true in C bool types, including the official C bool type, and BOOL used in Win32.
While for the example code given, !!p is unnecessary because the result is cast to a bool for evaluation of the if condition, that doesn't rule out the use of !! for purposes of casting booleans to expected integer values. Personally in this example, to maximize the probability that type changes and semantics are clear, I would use NULL != p or p != NULL to make it absolutely clear what is meant.
This technique is known as the double-bang idiom, and this guy provides some good justifications.
Do !!NOT use double negation. A simple argument is that since C++ is a limited English subset and english just does not have a double negation then english speakers will have a lot of difficulty to parse what is going on.
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Closed 10 years ago.
Okay, we know that the following two lines are equivalent -
(0 == i)
(i == 0)
Also, the first method was encouraged in the past because that would have allowed the compiler to give an error message if you accidentally used '=' instead of '=='.
My question is - in today's generation of pretty slick IDE's and intelligent compilers, do you still recommend the first method?
In particular, this question popped into my mind when I saw the following code -
if(DialogResult.OK == MessageBox.Show("Message")) ...
In my opinion, I would never recommend the above. Any second opinions?
I prefer the second one, (i == 0), because it feel much more natural when reading it. You ask people, "Are you 21 or older?", not, "Is 21 less than or equal to your age?"
It doesn't matter in C# if you put the variable first or last, because assignments don't evaluate to a bool (or something castable to bool) so the compiler catches any errors like "if (i = 0) EntireCompanyData.Delete()"
So, in the C# world at least, its a matter of style rather than desperation. And putting the variable last is unnatural to english speakers. Therefore, for more readable code, variable first.
If you have a list of ifs that can't be represented well by a switch (because of a language limitation, maybe), then I'd rather see:
if (InterstingValue1 == foo) { } else
if (InterstingValue2 == foo) { } else
if (InterstingValue3 == foo) { }
because it allows you to quickly see which are the important values you need to check.
In particular, in Java I find it useful to do:
if ("SomeValue".equals(someString)) {
}
because someString may be null, and in this way you'll never get a NullPointerException. The same applies if you are comparing constants that you know will never be null against objects that may be null.
(0 == i)
I will always pick this one. It is true that most compilers today do not allow the assigment of a variable in a conditional statement, but the truth is that some do. In programming for the web today, I have to use myriad of langauges on a system. By using 0 == i, I always know that the conditional statement will be correct, and I am not relying on the compiler/interpreter to catch my mistake for me. Now if I have to jump from C# to C++, or JavaScript I know that I am not going to have to track down assignment errors in conditional statements in my code. For something this small and to have it save that amount of time, it's a no brainer.
I used to be convinced that the more readable option (i == 0) was the better way to go with.
Then we had a production bug slip through (not mine thankfully), where the problem was a ($var = SOME_CONSTANT) type bug. Clients started getting email that was meant for other clients. Sensitive type data as well.
You can argue that Q/A should have caught it, but they didn't, that's a different story.
Since that day I've always pushed for the (0 == i) version. It basically removes the problem. It feels unnatural, so you pay attention, so you don't make the mistake. There's simply no way to get it wrong here.
It's also a lot easier to catch that someone didn't reverse the if statement in a code review than it is that someone accidentally assigned a value in an if. If the format is part of the coding standards, people look for it. People don't typically debug code during code reviews, and the eye seems to scan over a (i = 0) vs an (i == 0).
I'm also a much bigger fan of the java "Constant String".equals(dynamicString), no null pointer exceptions is a good thing.
You know, I always use the if (i == 0) format of the conditional and my reason for doing this is that I write most of my code in C# (which would flag the other one anyway) and I do a test-first approach to my development and my tests would generally catch this mistake anyhow.
I've worked in shops where they tried to enforce the 0==i format but I found it awkward to write, awkward to remember and it simply ended up being fodder for the code reviewers who were looking for low-hanging fruit.
Actually, the DialogResult example is a place where I WOULD recommend that style. It places the important part of the if() toward the left were it can be seen. If it's is on the right and the MessageBox have more parameters (which is likely), you might have to scroll right to see it.
OTOH, I never saw much use in the "(0 == i) " style. If you could remember to put the constant first, you can remember to use two equals signs,
I'm trying always use 1st case (0==i), and this saved my life a few times!
I think it's just a matter of style. And it does help with accidentally using assignment operator.
I absolutely wouldn't ask the programmer to grow up though.
I prefer (i == 0), but I still sort of make a "rule" for myself to do (0 == i), and then break it every time.
"Eh?", you think.
Well, if I'm making a concious decision to put an lvalue on the left, then I'm paying enough attention to what I'm typing to notice if I type "=" for "==". I hope. In C/C++ I generally use -Wall for my own code, which generates a warning on gcc for most "=" for "==" errors anyway. I don't recall seeing that warning recently, perhaps because the longer I program the more reflexively paranoid I am about errors I've made before...
if(DialogResult.OK == MessageBox.Show("Message"))
seems misguided to me. The point of the trick is to avoid accidentally assigning to something.
But who is to say whether DialogResult.OK is more, or less likely to evaluate to an assignable type than MessageBox.Show("Message")? In Java a method call can't possibly be assignable, whereas a field might not be final. So if you're worried about typing = for ==, it should actually be the other way around in Java for this example. In C++ either, neither or both could be assignable.
(0==i) is only useful because you know for absolute certain that a numeric literal is never assignable, whereas i just might be.
When both sides of your comparison are assignable you can't protect yourself from accidental assignment in this way, and that goes for when you don't know which is assignable without looking it up. There's no magic trick that says "if you put them the counter-intuitive way around, you'll be safe". Although I suppose it draws attention to the issue, in the same way as my "always break the rule" rule.
I use (i == 0) for the simple reason that it reads better. It makes a very smooth flow in my head. When you read through the code back to yourself for debugging or other purposes, it simply flows like reading a book and just makes more sense.
My company has just dropped the requirement to do if (0 == i) from its coding standards. I can see how it makes a lot of sense but in practice it just seems backwards. It is a bit of a shame that by default a C compiler probably won't give you a warning about if (i = 0).
Third option - disallow assignment inside conditionals entirely:
In high reliability situations, you are not allowed (without good explanation in the comments preceeding) to assign a variable in a conditional statement - it eliminates this question entirely because you either turn it off at the compiler or with LINT and only under very controlled situations are you allowed to use it.
Keep in mind that generally the same code is generated whether the assignment occurs inside the conditional or outside - it's simply a shortcut to reduce the number of lines of code. There are always exceptions to the rule, but it never has to be in the conditional - you can always write your way out of that if you need to.
So another option is merely to disallow such statements, and where needed use the comments to turn off the LINT checking for this common error.
-Adam
I'd say that (i == 0) would sound more natural if you attempted to phrase a line in plain (and ambiguous) english. It really depends on the coding style of the programmer or the standards they are required to adhere to though.
Personally I don't like (1) and always do (2), however that reverses for readability when dealing with dialog boxes and other methods that can be extra long. It doesn't look bad how it is not, but if you expand out the MessageBox to it's full length. You have to scroll all the way right to figure out what kind of result you are returning.
So while I agree with your assertions of the simplistic comparison of value types, I don't necessarily think it should be the rule for things like message boxes.
both are equal, though i would prefer the 0==i variant slightly.
when comparing strings, it is more error-prone to compare "MyString".equals(getDynamicString())
since, getDynamicString() might return null.
to be more conststent, write 0==i
Well, it depends on the language and the compiler in question. Context is everything.
In Java and C#, the "assignment instead of comparison" typo ends up with invalid code apart from the very rare situation where you're comparing two Boolean values.
I can understand why one might want to use the "safe" form in C/C++ - but frankly, most C/C++ compilers will warn you if you make the typo anyway. If you're using a compiler which doesn't, you should ask yourself why :)
The second form (variable then constant) is more readable in my view - so anywhere that it's definitely not going to cause a problem, I use it.
Rule 0 for all coding standards should be "write code that can be read easily by another human." For that reason I go with (most-rapidly-changing value) test-against (less-rapidly-changing-value, or constant), i.e "i == 0" in this case.
Even where this technique is useful, the rule should be "avoid putting an lvalue on the left of the comparison", rather than the "always put any constant on the left", which is how it's usually interpreted - for example, there is nothing to be gained from writing
if (DateClass.SATURDAY == dateObject.getDayOfWeek())
if getDayOfWeek() is returning a constant (and therefore not an lvalue) anyway!
I'm lucky (in this respect, at least) in that these days in that I'm mostly coding in Java and, as has been mentioned, if (someInt = 0) won't compile.
The caveat about comparing two booleans is a bit of a red-herring, as most of the time you're either comparing two boolean variables (in which case swapping them round doesn't help) or testing whether a flag is set, and woe-betide-you if I catch you comparing anything explicitly with true or false in your conditionals! Grrrr!
In C, yes, but you should already have turned on all warnings and be compiling warning-free, and many C compilers will help you avoid the problem.
I rarely see much benefit from a readability POV.
Code readability is one of the most important things for code larger than a few hundred lines, and definitely i == 0 reads much easier than the reverse
Maybe not an answer to your question.
I try to use === (checking for identical) instead of equality. This way no type conversion is done and it forces the programmer do make sure the right type is passed,
You are right that placing the important component first helps readability, as readers tend to browse the left column primarily, and putting important information there helps ensure it will be noticed.
However, never talk down to a co-worker, and implying that would be your action even in jest will not get you high marks here.
I always go with the second method. In C#, writing
if (i = 0) {
}
results in a compiler error (cannot convert int to bool) anyway, so that you could make a mistake is not actually an issue. If you test a bool, the compiler is still issuing a warning and you shouldn't compare a bool to true or false. Now you know why.
I personally prefer the use of variable-operand-value format in part because I have been using it so long that it feels "natural" and in part because it seems to the predominate convention. There are some languages that make use of assignment statements such as the following:
:1 -> x
So in the context of those languages it can become quite confusing to see the following even if it is valid:
:if(1=x)
So that is something to consider as well. I do agree with the message box response being one scenario where using a value-operand-variable format works better from a readability stand point, but if you are looking for constancy then you should forgo its use.
This is one of my biggest pet peeves. There is no reason to decrease code readability (if (0 == i), what? how can the value of 0 change?) to catch something that any C compiler written in the last twenty years can catch automatically.
Yes, I know, most C and C++ compilers don't turn this on by default. Look up the proper switch to turn it on. There is no excuse for not knowing your tools.
It really gets on my nerves when I see it creeping into other languages (C#,Python) which would normally flag it anyway!
I believe the only factor to ever force one over the other is if the tool chain does not provide warnings to catch assignments in expressions. My preference as a developer is irrelevant. An expression is better served by presenting business logic clearly. If (0 == i) is more suitable than (i == 0) I will choose it. If not I will choose the other.
Many constants in expressions are represented by symbolic names. Some style guides also limit the parts of speech that can be used for identifiers. I use these as a guide to help shape how the expression reads. If the resulting expression reads loosely like pseudo code then I'm usually satisfied. I just let the expression express itself and If I'm wrong it'll usually get caught in a peer review.
We might go on and on about how good our IDEs have gotten, but I'm still shocked by the number of people who turn the warning levels on their IDE down.
Hence, for me, it's always better to ask people to use (0 == i), as you never know, which programmer is doing what.
It's better to be "safe than sorry"
if(DialogResult.OK == MessageBox.Show("Message")) ...
I would always recommend writing the comparison this way. If the result of MessageBox.Show("Message") can possibly be null, then you risk a NPE/NRE if the comparison is the other way around.
Mathematical and logical operations aren't reflexive in a world that includes NULLs.
This question already has answers here:
What is the difference between these (bCondition == NULL) and (NULL==bCondition)? [duplicate]
(6 answers)
Closed 9 years ago.
I have received code from someone working earlier on it, and it contains a lot of lines like
while(false==find && false == err && k<kmax)
if(true==refract(ep1,ep2,n1,RI_blood, RI_collagen))
and my favorite line is
if(false == (ret_s<0))
The other code is done really well, documented just fine, but these lines with these odd conditions are throwing me off, and I wonder why they are done that way.
Especially that false==(ret_s<0) is completely confusing, and you kind of need to read that line like three times to understand what they want there.
Is this a common programming style, don't I understand the reasoning for that, or is that just bad style?
Edit: I don't feel this is similar to if(object==NULL) vs if(NULL==object), since this isn't about accidental assigning but about obfuscated if clauses...
Is this a common programming style?
No.
don't I understand the reasoning for that?
Some people like to explicitly compare booleans with true or false, even though the result is exactly the same boolean value. The logic is presumably that, by making the code harder to read and more surprising, people will think harder about it and make fewer assumptions about its behaviour. Or perhaps just that code should be hard to maintain, since it was hard to write.
Others like to write comparisons with constants backwards, which prevents mistakes like if (x = 5) when you meant if (x == 5). Any modern compiler will warn you about this mistake, so again its only real purpose is to make the code harder to read.
Combining these two behaviours gives the bizarre code you posted.
Or is that just bad style?
It's a style. I'm no judge of style, but if you like to keep maintainence programmers on their toes, it certainly does that. Personally, I like my code to be readable, but that's just me.
my favorite line is
I once encountered return a && !b implemented in about ten lines of code. The first line was switch(a).
Yoda Conditions
Using if(constant == variable) instead of if(variable == constant), like if(4 == foo). Because it's like saying "if blue is the sky" or "if tall is the man".
Its a safe guard against assignment in C++.
In C++ it is perfectly legal to do this
if (foo = true) ....
In this case the single = is an assignment and would replace the value of foo.
This is not legal and will generate a compiler error
if (true = foo) ....
Constants and literals are often put on the left because it prevents accidental assignments. Consider typing:
if(foo == bar)
as:
if(foo = bar)
The second might appear to work... but silently clobber foo. If foo is a constant, this error is not longer possible.
It's a self-protection technique that prevents you from accidentally typing an assignment operator (=) instead of equality operator (==), which can introduce strange bugs. Putting the constant value on the left hand side will introduce a compiler error, while putting a variable on the LHS will just silently compile.
Perhaps the original programmer thought that explicit comparison to true or false was clearler than if(condition) or if(!condition) and coded things in that way. I haven't seen this particular style before however.
It's quite subjective but I find while(!find && !err && k<kmax) easier to read.
The code could have been written for a shop where there is a site standard that every conditional statement must include a comparison operator, in order to avoid accidentally leaving out part of the comparison. (Maybe that's a stretch, but you did say that the rest of the code was very good.) That, coupled with a standard or habit of putting the constant on the left to avoid accidentally using = instead of == would give pretty much the code you showed. It doesn't explain the use of 'false' instead of the more natural 'true', though. Maybe it's a (misguided on multiple levels) attempt to gain microefficiency by comparing to zero instead of 1 at the machine level.
for my is only bad style,
if(false == (ret_s<0))
is equals to in C#
if(!(ret_s<0))
In one C++ coding style guide,
I found one particular recommendation (page 41, recommendation number 53):
Always have non-lvalues on the left side (0 == i instead of i == 0).
And I don't uderstand what is this good for? Are to sticking to this practice?
I'm not and I don't know why is his a good practice. The only advantage I can think of is that is will avoid mistaking an unintentional assignment with a comparison (if (foo = 0){} versus if (foo == 0){})
Have you got any other ideas why should I use it?
Yes, you guessed it right. It's the good, old Yoda condition!!!
As you say, the reason some people use it is to occasionally avoid typing = when they mean ==.
Since it only catches some cases, where you're comparing a lvalue with a constant or rvalue, and every compiler I know of will warn you if you make that mistake, there's very little point in doing it.
At least to native English speakers, it makes the code read as if it's written backwards; so a "Yoda condition" some call it do. Like many rules in corporate style guides, it dates back to a time when dealing with unforgiving compilers was a higher priority than writing readable code.
Or are we all sticking to our taught "&&, ||, !" way?
Any thoughts in why we should use one or the other?
I'm just wondering because several answers state thate code should be as natural as possible, but I haven't seen a lot of code with "and, or, not" while this is more natural.
I like the idea of the not operator because it is more visible than the ! operator. For example:
if (!foo.bar()) { ... }
if (not foo.bar()) { ... }
I suggest that the second one is more visible and readable. I don't think the same argument necessarily applies to the and and or forms, though.
"What's in a name? That which we call &&, || or !
By any other name would smell as sweet."
In other words, natural depends on what you are used to.
Those were not supported in the old days. And even now you need to give a special switch to some compilers to enable these keywords. That's probably because old code base may have had some functions || variables named "and" "or" "not".
One problem with using them (for me anyway) is that in MSVC you have to include iso646.h or use the (mostly unusable) /Za switch.
The main problem I have with them is the Catch-22 that they're not commonly used, so they require my brain to actively process the meaning, where the old-fashioned operators are more or less ingrained (kind of like the difference between reading a learned language vs. your native language).
Though I'm sure I'd overcome that issue if their use became more universal. If that happened, then I'd have the problem that some boolean operators have keywords while others don't, so if alternate keywords were used, you might see expressions like:
if ((x not_eq y) and (y == z) or (z <= something)) {...}
when it seems to me they should have alternate tokens for all the (at least comparison) operators:
if ((x not_eq y) and (y eq z) or (z lt_eq something)) {...}
This is because the reason the alternate keywords (and digraphs and trigraphs) were provided was not to make the expressions more readable - it was because historically there have been (and maybe still are) keyboards and/or codepages in some localities that do not have certain punctuation characters. For example, the invariant part of the ISO 646 codepage (surprise) is missing the '|', '^' and '~' characters among others.
Although I've been programming C++ from quite some time, I did not know that the keywords "and" "or" and "not" were allowed, and I've never seen it used.
I searched through my C++ book, and I found a small section mentioning alternative representation for the normal operators "&&", "||" and "!", where it explains those are available for people with non-standard keyboards that do not have the "&!|" symbols.
A bit like trigraphs in C.
Basically, I would be confused by their use, and I think I would not be the only one.
Using a representation which is non-standard, should really have a good reason to be used.
And if used, it should be used consistently in the code, and described in the coding standard.
The digraph and trigraph operators were actually designed more for systems that didn't carry the standard ASCII character set - such as IBM mainframes (which use EBCDIC). In the olden days of mechanical printers, there was this thing called a "48-character print chain" which, as its name implied, only carried 48 characters. A-Z (uppercase), 0-9 and a handful of symbols. Since one of the missing symbols was an underscore (which rendered as a space), this could make working with languages like C and PL/1 a real fun activity (is this 2 words or one word with an underscore???).
Conventional C/C++ is coded with the symbols and not the digraphs. Although I have been known to #define "NOT", since it makes the meaning of a boolean expression more obvious, and it's visually harder to miss than a skinny little "!".
I wish I could use || and && in normal speech. People try very hard to misunderstand when I say "and" or "or"...
I personally like operators to look like operators. It's all maths, and unless you start using "add" and "subtract" operators too it starts to look a little inconsistent.
I think some languages suit the word-style and some suit the symbols if only because it's what people are used to and it works. If it ain't broke, don't fix it.
There is also the question of precedence, which seems to be one of the reasons for introducing the new operators, but who can be bothered to learn more rules than they need to?
In cases where I program with names directly mapped to the real world, I tend to use 'and' and 'or', for example:
if(isMale or isBoy and age < 40){}
It's nice to use 'em in Eclipse+gcc, as they are highlighted. But then, the code doesn't compile with some compilers :-(
Using these operators is harmful. Notice that and and or are logical operators whereas the similar-looking xor is a bitwise operator. Thus, arguments to and and or are normalized to 0 and 1, whereas those to xor aren't.
Imagine something like
char *p, *q; // Set somehow
if(p and q) { ... } // Both non-NULL
if(p or q) { ... } // At least one non-NULL
if(p xor q) { ... } // Exactly one non-NULL
Bzzzt, you have a bug. In the last case you're testing whether at least one of the bits in the pointers is different, which probably isn't what you thought you were doing because then you would have written p != q.
This example is not hypothetical. I was working together with a student one time and he was fond of these literate operators. His code failed every now and then for reasons that he couldn't explain. When he asked me, I could zero in on the problem because I knew that C++ doesn't have a logical xor operator, and that line struck me as very odd.
BTW the way to write a logical xor in C++ is
!a != !b
I like the idea, but don't use them. I'm so used to the old way that it provides no advantage to me doing it either way. Same holds true for the rest of our group, however, I do have concerns that we might want to switch to help avoid future programmers from stumbling over the old symbols.
So to summarize: it's not used a lot because of following combination
old code where it was not used
habit (more standard)
taste (more math-like)
Thanks for your thoughts
From time to time, I run into communications issue with other programmers, when we talk about NULL. Now NULL could be
a NULL pointer
the NUL character
an empty data element in some sort of database.
NUL seems to be the most confusing. It is the ASCII character 0x00.
I tend to use '\0' in my code to represent it. Some developers in my group
tend to prefer to simply use 0, and let the compiler implicitly cast it to a char.
What do you prefer to use for NUL? and why?
I use '\0' for the nul-character and NULL for pointers because it is clearest in both cases.
BTW, both 0 and '\0' are ints in C and either one will be converted to char when stored in a char variable.
I like the pre-defined NULL macro, as it preserves the semantic meaning, rather than some other use of the number 0.
There are many English words which are spelled or spoken alike, yet which have different meanings. Like in English, use the context in which the discussion is taking place to guide you toward the intended meaning.
For dealing with strings, I alwayse represent the null character as '\0'.
For pointers, I try to use implicit-conversion-to-boolean (if (!myPtr) or if (myPtr)) for pointer nullity.
If I need a default value for a pointer, it's NULL, e.g. struct list_head = { 0.0, NULL };).
END_OF_STRING is silly, since it's extra indirection that simply confuses new readers (anyone who doesn't immediately recognize '\0' should step away from the keyboard).
One other thing—I think the difference between a null value and an empty value is extremely important when talking about data modeling. This is especially true when discussing C-style strings or nullable database fields. There's a huge difference between someone telling you "I have no name" and "My name is ."
#BKB:
I see the point in his advice, but "NULL" makes it clearer that the context is pointers. It's like using "0.0" for floating-point values, as '\0' when dealing with characters. (Likewise, I prefer seeing 0 if a char is being used in an arithmetic context.)
Bjarne further states in this FAQ that NULL is #defined as 0 anyway, so standard code shouldn't have a problem with it. I agree that the all-caps notation is ugly, but we'll have to wait until 0x (where nullptr will be available, as a keyword.)
If I remember correctly most C compilers define NULL like this:
#define NULL ((void*)0)
This is to ensure that NULL is interpreted as being a pointer type (in C). However this can cause issues in the much more type strict world of C++. Eg:
// Example taken from wikibooks.org
std::string * str = NULL; // Can't automatically cast void * to std::string *
void (C::*pmf) () = &C::func;
if (pmf == NULL) {} // Can't automatically cast from void * to pointer to member function.
Therefore in the current C++ standard null pointers should be initialized with the literal 0. Obviously because people are so used to using the NULL define I think a lot of C++ compilers either silently ignore the issue or redefine NULL to be 0 in C++ code. Eg:
#ifdef __cplusplus
#define NULL (0)
#else
#define NULL ((void*)0)
#endif
The C++x0 standard now defines a nullptr keyword to represent null pointers. Visual C++ 2005's CLI/C++ compiler also uses this keyword when setting managed pointers to null. In current compilers you can create a template to emulate this new keyword.
There is a much more detailed article on wikibooks.org discussing this issue.
A one-L NUL, it ends a string.
A two-L NULL points to no thing.
And I will bet a golden bull
That there is no three-L NULLL.
(The name of the original author is, alas, lost to the sands of time.)
NULL for databases, NIL for code.
While, on the whole, I would advice using named constants, this is one exception. To me, defining:
#define NULL 0
#define END_OF_STRING '\0'
makes as much sense as defining:
#define SEVEN 7
which is none. And yes, I am aware that NULL is already defined by the compiler, but I never use it. For pointers, 0; for chars, '\0'. Longer does not always mean more expressive.
I quite like
#define ASCII_NUL ('\0')
I only very occasionally mistype '\0' as '0'. But when I have done it, I've found the error very hard to spot by code inspection, with hilarious consequences. So I don't like '\0' much, and prefer ASCII_NUL or 0 (of course the latter has the wrong type in C++). Obviously I use '\0' where demanded by consistency with existing code, or style guides.
The Google C++ style guide, which contains a few things I like and a few I don't, but seems mostly sound, prefers NULL to 0 for pointers. It points out that NULL might not be defined simply as 0 (or 0L), especially in implementations where sizeof(void*) might not be sizeof(int) (or sizeof(long int)).
0 and NULL are both specified to be of integral type, and when converted to a pointer type they both must yield a null pointer value. But they aren't necessarily of the same integral type. So you might conceivably get some useful warnings or errors in some situations by using NULL.
For communication I use NULL. If I'm working with a developer who cannot grasp the concept of NULL for different data-types then I'd be concerned.
For implementation it's case-specific. Numbers are 0 (post-fixed f for floating-point), pointers are NULL and character strings are 0.
Systems that don't use binary 0 for NULL are getting harder to find. They also tend to have various portability issues. Why? Because on these systems neither memset nor calloc can clear out a struct that contains pointers correctly.
const char END_OF_STRING = '\0';
So when you say:
str[i] = END_OF_STRING;
or
if (*ptr == END_OF_STRING)
there is absolutely no question what you mean.
We use NULL for pointers and NULLCHAR for characters, using
#define NULLCHAR '\0'
Sort of related: Slashdot recently had a story on the comp.lang.c FAQ section on null pointers, which I found quite interesting.