Simply: why is the nullptr keyword in the C++0X/C++11 standard lowercase, whereas its close companion NULL is uppercase? I do realise that the two adhere to different standards; nullptr is a keyword, whereas NULL is a macro. But since they will be used in similar situations, wouldn't it be clearer for new users if they followed the same convention?
(Not that it's that much of a bother to remember - I'm just curious about the design decision.)
NULL is a macro, and a common convention is that macros are uppercase, while nullptr is not a macro but a keyword and is lower case as all other keywords.
But since they will be used in similar situations, wouln't it be clearer for new users if they followed the same convention?
The potential source of confusion would change and someone else would be asking why a keyword is upper case (assuming NULLPTR, as null would break backwards compatibility in the language)
Lower case definitely fits better with general naming conventions of C++. Using all caps wouldn't do anything to help the new users at all.
And I don't believe it would actually be all that helpful to the old users who grew up with using NULL in C (or were in the cap of C++ users who used NULL instead of 0).
Certainly, I don't see anything that would amount to a justification of breaking the naming convention in this case.
Okay I am editing my original answer because I misread the question. I am going to claim that it is lowercase for the same reason the true and false keywords are lowercase.
Related
I have read the document about naming rule of C++, they seems to be usable names.
However, in practice, when I tried to create a variable/enum with a name like iter, yes, no, out, i, Error, etc. , Visual Studio will strangely use italic font for them.
I can only guess that they are reserved for special thing, and IDE (e.g. refactoring/rename process) might act strangely if I use such names.
Is it safe to use those names in practice? Am I just too panic?
Sorry if it is too newbie or an inappropriate question.
I doubt about it for a few weeks but too afraid to ask.
These names are valid and will not cause any "harm", the standard only says:
Each name that contains a double underscore (_ _) or begins with an underscore followed by an uppercase letter (2.11) is reserved to the
implementation for any use.
Each name that begins with an underscore is reserved to the implementation for use as a name in the global namespace.
Which means that all your names are fine to use in user-code. Visual Studio might just have a thing for these names as i and iter are usually used in looping.
These names are not reserved in standard C++, as explained by Rick Astley. An implementation may choose to accept additional reserved words to provide language extensions, such as ref class in C++/CLI. In some cases, such as with ref class, where ref is a contextual keyword, these extensions only make otherwise ill-formed programs well-formed in the scope of the extended language. In other cases, an otherwise well-formed program may change its meaning or become ill-formed. In the former case, the implementation is still conforming to the C++ standard, as long as it issues all mandatory diagnostics; in the latter case, it is certainly not conforming.
It is considered good practice to make the latter kind of extensions optional e.g. using a command line option, so that the implementation still has a mode in which it is fully standards compliant. My immediate guess is that VC++ in fact does allow you to write well-formed programs containing yes, no, i, iter which will behave as required by the standard (implementation bugs notwithstanding).
The IDE is a different beast, though. It is considered to be outside of the scope of the C++ standard, and might discourage or even stop you from writing perfectly well-formed code. That would still be a quality of implementation issue, or an issue of customer satisfaction, if you will.
I already checked this post Can I use if (pointer) instead of if (pointer != NULL)? and some other posts on net.
But it is not stating any difference between two statements.
Problem: As I run cpplint.py on my cpp code, I found issues where I check pointers for NULL.
I preferred to check using simple
if(pointer) //statement1
but cpplint says you should check like
if(pointer != NULL) //statement2
So I just want to know , Are there any benefits of statement2 over statement1 ? Are there some scenarios in which statement1 may create problem ?
Working: As per my knowledge there is no difference in working of both statements. Its just a change of coding style.
I prefer to use like statement1, because
Its Simple, Readable
No Tension of missing (=) by mistake over equality(==) in a comparison
But cpplint is raising this as issue, then there might be some benefit that I missed.
Note: Java also doesn't support statement1.
No, if pointer is really a pointer type there is no difference, so everything here is a question of coding style. Coding style in turn depends on habits in different communities so there can't be a general recommendation.
I personally prefer the first because it is shorter and more to the point and avoids the use of the bogus macro NULL.
In C NULL can be very different things (integer or pointer) and in C++ its use is even deprecated nowadays. You should at least use nullptr, there.
You are using Hungarian notation, where it's possible to tell if a variable is a pointer. As long as it is - either native or smart - there's no difference. However, when someone changes it to another indirect type (e.g., std::optional<>), then the second will fail. So my suggestion is to keep on using the first: it's not Java, it's C++.
In C++, assuming ptr is a pointer, the comparisons if (ptr) and if (ptr != NULL) are functionally equivalent.
In C++11 and later, it is often considered preferable to use the alternative if (ptr != nullptr).
For a simple check of a pointer, the differences in these options are really stylistic. The mechanisms might differ slightly, but the end result is the same.
cpplint, like most automated checkers, tends to - by default - complain more about breaches of some style guidelines more than others. Whether any particular set of guidelines is right or wrong depends on what is needed for your project.
For class types that can sensibly be compared with a pointer (e.g. smart pointer types) the preferred test depends on what set of operations (comparison operators, implicit conversions, etc) that type supports.
In C, onsider :
int *ptr=malloc(10*sizeof *ptr);
free(ptr); // though the memory is freed, the ptr is not auto-set to NULL
if (ptr)
{
printf ("ptr is not null\n");
}
So you are expected to put
ptr=NULL; // ptr is explicitly made to point at nothing
// The above step is mandatory.
after the free.
So as a response in the the if-statement, one might recommend to do
if ( ptr == NULL ) // This is mostly a coding style & improves readability?
or better
if ( NULL == ptr ) // less chances of error
Well, the [ site ] says about cpplintthat it is :
An automated checker to make sure a C++ file follows Google's C++ style guide
So again, it is somebody's style that matters. Say , if you contribute to somebody's code in google, they expect you to follow this style where it facilitates easy collaboration.
There is one scenario that may create a problem using statement1.
Consider the following code which could have two different meanings.
bool* is_done = ...;
// Is this checking if `is_done` is not null, or actually
// intended to check if `*is_done` is true?
if (is_done) {
...
}
If you intended to do a null check, you're fine. But if your original intent is to check if *is_done is true but missed an asterisk by accident, this code may result in a totally unwanted behavior and require you to spend X hours to figure out the culprit.
This could've avoided by explicitly checking the statement like
// Now this results in a compile error and forces you to write
// `*is_done` instead.
if (is_done == true) {
...
}
This is applicable to any types that could be implicitly converted to bool like std::unique_ptr.
Someone may argue that the above case is too rare and still prefer the statement1 in favor of simplicity. I think it is fair and both styles are acceptable. But some organizations, like Google, may encourage you to follow their coding style to keep the lesson they previously learned.
There is no difference between both if(pointer) and if(pointer != NULL). if(pointer) is used for the code optimization.
Consider this in the (possibly nested) scope of a method (function, block, etc.):
int _ = 42;
Are there any technical reasons to avoid a local variable named _?
Some guidance, for the purpose of this question:
I know _ generally prefixes Objective-C instance variables.
Leave that aside. Commentary on other clashes with convention welcomed.
I like pretty code too, but statements of taste or pure opinion (e.g. "It's {confusing, unreadable, unmaintainable}") are strongly discouraged here†.
I'm primarily interested in answering this for Objective-C, but answers
related to C or C++ are also encouraged.
† Buy me a pint, and you can tell me all about it. :)
C99 §7.1.3 says that all identifiers beginning with at least one underscore are reserved for use by the implementation, as file-scope identifiers only.1 _ is an identifier that begins with at least one underscore, so you're not supposed to define it in any way at file scope.2
However, as a local variable name, _ is fair game for the application programmer. Only identifiers beginning with either two underscores, or an underscore and then an uppercase letter, are reserved unconditionally.
These rules are honored more in the breach than the observance, as the footnotes demonstrate.
1 Yes, that means the very common practice of starting "internal use only" function names with _ followed by a lowercase letter is technically a conformance violation.
2 GNU gettext is a prominent third-party library that breaks this rule; the programmer is encouraged to #define _(x) gettext(x) as shorthand.
To answer your question simply - no, there aren't any technical reasons to avoid it. Lots of other reasons though.
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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?
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 unique_ptr, shared_ptr, 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.
if (foo) is clear enough. Use it.
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.
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.
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."
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.
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.
The C Programming Language (K&R) would have you check for null == ptr to avoid an accidental assignment.
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.
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.
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.
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 >> a >> 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.
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.
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.
I want to redefine NULL in my program such as
#define MYNULL ((void*)0)
But this definition is not working in the following statement:
char *ch = MYNULL;
Error : can not convert from void* to char *
What would be the best way to define NULL?
#define MYNULL NULL
is the safest, I see no reason in doing so but if you really want to, go ahead.
Here's how C and C++ do it respectively:
#define NULL 0 //C++
#define NULL ((void*)0) //C
Generally speaking, defining 0 for NULL is a bad habit, you actually want it to be part of the language. C++0x adresses this.
This is what Bjarne Stroustrup has to say on this:
Should I use NULL or 0?
In C++, the definition of NULL is 0, so there is only an aesthetic difference. I prefer to avoid macros, so I use 0. Another problem with NULL is that people sometimes mistakenly believe that it is different from 0 and/or not an integer. In pre-standard code, NULL was/is sometimes defined to something unsuitable and therefore had/has to be avoided. That's less common these days.
If you have to name the null pointer, call it nullptr; that's what it's called in C++11. Then, "nullptr" will be a keyword.
#ifdef __cplusplus
#define MYNULL 0
#else
#define MYNULL ((void*)0)
#endif
will work in both of them.
What exactly is the problem with getting your NULL from where you're supposed to?, i.e.,
#include <stddef.h>
or
#include <cstddef>
as alluded to in #Johannes Rudolph's answer, any trickery you do is not likely be very future proof in the face of things like nullptr etc.
EDIT: while stdlib (and many others) are mandated to include a NULL, stddef is the most canonical header [and has been for decades].
PS In general, it's just a bad idea to get involved in this sort of trickery unless you have a really good reason. You didnt expand on the thinking that led you to feeling the need to do this. If you could add some detail on that, it's likely to lead to better answers. Other people answering the question should have pointed this out in their answers too, but I guess does FGITW as FGITW does best :D
EDIT 2: As pointed out by #Yossarian: The single justification for doing this is if there isnt a NULL defined in an appropriately language-agnostic form elsewhere in your system. Naked compilers with no headers and/or if you're writing your own custom standard library from scratch are examples of such a circumstance. (In such a bare-bones scenario, I'd go with #lilburne's answer (be sure to use 0 as much as possible))
#define MYNULL 0
will work in C++
Don't do this. There is nothing that says that NULL has to be the value zero, it's implementation specific.
It could be a value that represents the end of memory, some special place in memory, or even an object that represents no value exists.
Doing this is very dangerous, may break portability, and will most certainly screw with code-aware editors. It isn't buying you anything, trust your library's definition.
EDIT: Evan is correct! The code itself will say zero, under the hood the compiler can do what it wants with implementation specific details. Thanks Evan!
I think that anyone that doesn't know that setting a pointer in C/C++ to 0 is the same as setting it to NULL, nullptr, or any other equivalent shouldn't be messing with code. The difference in readability between
char* ch = NULL
and
char* ch = 0;
is minimal. When it comes to expressions the forms
if (NULL == ch) {
}
if (0 == ch) {
}
if (nullptr == ch) {
}
are no more readable than
if (!ch) {
}
In contrast to what some people state here, 0 is a perfectly valid definition for NULL in C. Thus you have to be careful when you give NULL as an argument to a variadic function, because it may be mistaken as the integer value 0, ending in non-portability.
http://c-faq.com/null/null2.html
BTW, the comp.lang.c FAQ is a highly recommended read for every C programmer. See for example here:
http://c-faq.com/null/null1.html
containing such gems of nearly-forgotten wisdom like "As mentioned above, there is a null pointer for each pointer type, and the internal values of null pointers for different types may be different." Which means that calloc or memset are NOT a portable initialization for pointers.
#define NULL 0 //for C
is the perfect definition in C
e.g.
char *ch = NULL ;
*ch++ ;// will cause error
it causes error since ch pointing to nothing while executing increment statement
is known by compiler by seeing the value of pointer in LOOK-UP table to be 0
if u try to update this pointer then u are actually changing the contents of
CODE region which start at 0 physical address.
FOR that reason the first entry of page table prior to code region starts
is kept empty
What exactly is the problem with getting your NULL from where you're supposed to?, i.e.,
#include <stddef.h>
or
#include <cstddef>
as alluded to in #Johannes Rudolph's answer, any trickery you do is not likely be very future proof in the face of things like nullptr etc.
EDIT: while stdlib (and many others) are mandated to include a NULL, stddef is the most canonical header [and has been for decades].
PS In general, it's just a bad idea to get involved in this sort of trickery unless you have a really good reason. You didnt expand on the thinking that led you to feeling the need to do this. If you could add some detail on that, it's likely to lead to better answers. Other people answering the question should have pointed this out in their answers too, but I guess does FGITW as FGITW does best :D
EDIT 2: As pointed out by #Yossarian: The single justification for doing this is if there isnt a NULL defined in an appropriately language-agnostic form elsewhere in your system. Naked compilers with no headers and/or if you're writing your own custom standard library from scratch are examples of such a circumstance. (In such a bare-bones scenario, I'd go with #lilburne's answer (be sure to use 0 as much as possible))