Related
My code is something like:
BYTE *byteVar;
_CERT_TEMPLATE_EXT *structVar;
// assign byteVar
structVar = (_CERT_TEMPLATE_EXT*)byteVar;
// Here I would like to check if in the byteVar there is a _CERT_TEMPLATE_EXT * or not;
// (if the cast was successfull or not.)
I don't know what kind of cast I should use.
Assuming that you are trying to do crypto on Windows, and _CERT_TEMPLATE_EXT is as defined here, then there is no way you can reliably check.
You can look at structVar->pszObjId, and check if it looks like a valid OID. If not, things have gone badly wrong with your application, and you need to debug it. (If the pointer wasn't a CERT_TEMPLATE_EXT, the check will involve undefined behaviour - but so will using the converted value, so you are no worse off).
In general, you need reinterpret_cast to cast between unrelated pointer types, but at least GCC appears to allow just static_cast from unsigned char * (I don't have MSVC here to check readily.). Personally, I think I would stick to a big scary reinterpret_cast.
I have seen below macro in many topmost header files:
#define NULL 0 // C++03
In all over the code, NULL and 0 are used interchangeably. If I change it to.
#define NULL nullptr // C++11
Will it cause any bad side effect ? I can think of the only (good) side effect as following usage will become ill-formed;
int i = NULL;
I have seen below macro in topmost header file:
You shouldn't have seen that, the standard library defines it in <cstddef> (and <stddef.h>). And, IIRC, according to the standard, redefining names defined by standard header files results in undefined behaviour. So from a purely standardese viewpoint, you shouldn't do that.
I've seen people do the following, for whatever reason their broken mind thought of:
struct X{
virtual void f() = NULL;
}
(As in [incorrectly]: "set the virtual table pointer to NULL")
This is only valid if NULL is defined as 0, because = 0 is the valid token for pure-virtual functions (ยง9.2 [class.mem]).
That said, if NULL was correctly used as a null pointer constant, then nothing should break.
However, beware that, even if seemingly used correctly, this will change:
void f(int){}
void f(char*){}
f(0); // calls f(int)
f(nullptr); // calls f(char*)
However, if that was ever the case, it was almost certainly broken anyways.
Far better is to search and replace NULL with nullptr throughout the code.
It may be syntactically safe, but where would you put the #define? It creates code organisation problems.
No. You're not allowed to (re)define standard macros. And if you see
#define NULL 0
at the top of any file other than a standard header (and even there, it
should be in include guards, and typically in additional guards as
well), then that file is broken. Remove it.
Note that good compilers will typically define NULL with something
like:
#define NULL __builtin_null
, to access a compiler builtin which will trigger a warning if it is
used in a non-pointer context.
You shouldn't be defining it at all, unless you're writing your own version of <cstddef>; it certainly shouldn't be in "many topmost header files".
If you are implementing your own standard library, then the only requirement is
18.2/3 The macro NULL is an implementation-defined C++ null pointer constant
so either 0 or nullptr is acceptable, and nullptr is better (if your compiler supports it) for the reason you give.
Maybe Not
If you have a particular format of overloading behaviour:
void foo(int);
void foo(char*);
Then the behaviour of the code:
foo(NULL);
will change depending on whether NULL is changed to nullptr or not.
Of course, there's another question as to whether it's safe to write such code as is present in this answer...
While it might break backwards-compatibility with older stuff that was badly written (either that, or overly clever...), for your newer code, this is a non-issue. You should use nullptr, and not NULL, where you mean nullptr. Also, you should use 0 where you mean zero.
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))
This is a C++ disaster, check out this code sample:
#include <iostream>
void func(const int* shouldnotChange)
{
int* canChange = (int*) shouldnotChange;
*canChange += 2;
return;
}
int main() {
int i = 5;
func(&i);
std::cout << i;
return 0;
}
The output was 7!
So, how can we make sure of the behavior of C++ functions, if it was able to change a supposed-to-be-constant parameter!?
EDIT: I am not asking how can I make sure that my code is working as expected, rather I am wondering how to believe that someone else's function (for instance some function in some dll library) isn't going to change a parameter or posses some behavior...
Based on your edit, your question is "how can I trust 3rd party code not to be stupid?"
The short answer is "you can't." If you don't have access to the source, or don't have time to inspect it, you can only trust the author to have written sane code. In your example, the author of the function declaration specifically claims that the code will not change the contents of the pointer by using the const keyword. You can either trust that claim, or not. There are ways of testing this, as suggested by others, but if you need to test large amounts of code, it will be very labour intensive. Perhaps moreso than reading the code.
If you are working on a team and you have a team member writing stuff like this, then you can talk to them about it and explain why it is bad.
By writing sane code.
If you write code you can't trust, then obviously your code won't be trustworthy.
Similar stupid tricks are possible in pretty much any language. In C#, you can modify the code at runtime through reflection. You can inspect and change private class members. How do you protect against that? You don't, you just have to write code that behaves as you expect.
Apart from that, write a unittest testing that the function does not change its parameter.
The general rule in C++ is that the language is designed to protect you from Murphy, not Machiavelli. In other words, its meant to keep a maintainance programmer from accidentally changing a variable marked as const, not to keep someone from deliberatly changing it, which can be done in many ways.
A C-style cast means all bets are off. It's sort of like telling the compiler "Trust me, I know this looks bad, but I need to do this, so don't tell me I'm wrong." Also, what you've done is actually undefined. Casting off const-ness and then modifying the value means the compiler/runtime can do anything, including e.g. crash your program.
The only thing I can suggest is to allocate the variable shouldNotChange from a memory page that is marked as read-only. This will force the OS/CPU to raise an error if the application attempts to write to that memory. I don't really recommend this as a general method of validating functions just as an idea you may find useful.
The simplest way to enforce this would be to just not pass a pointer:
void func(int shouldnotChange);
Now a copy will be made of the argument. The function can change the value all it likes, but the original value will not be modified.
If you can't change the function's interface then you could make a copy of the value before calling the function:
int i = 5;
int copy = i
func(©);
Don't use C style casts in C++.
We have 4 cast operators in C++ (listed here in order of danger)
static_cast<> Safe (When used to 'convert numeric data types').
dynamic_cast<> Safe (but throws exceptions/returns NULL)
const_cast<> Dangerous (when removing const).
static_cast<> Very Dangerous (When used to cast pointer types. Not a very good idea!!!!!)
reinterpret_cast<> Very Dangerous. Use this only if you understand the consequences.
You can always tell the compiler that you know better than it does and the compiler will accept you at face value (the reason being that you don't want the compiler getting in the way when you actually do know better).
Power over the compiler is a two edged sword. If you know what you are doing it is a powerful tool the will help, but if you get things wrong it will blow up in your face.
Unfortunately, the compiler has reasons for most things so if you over-ride its default behavior then you better know what you are doing. Cast is one the things. A lot of the time it is fine. But if you start casting away const(ness) then you better know what you are doing.
(int*) is the casting syntax from C. C++ supports it fully, but it is not recommended.
In C++ the equivalent cast should've been written like this:
int* canChange = static_cast<int*>(shouldnotChange);
And indeed, if you wrote that, the compiler would NOT have allowed such a cast.
What you're doing is writing C code and expecting the C++ compiler to catch your mistake, which is sort of unfair if you think about it.
In the early days of C++ when it was bolted on top of C, you could not use NULL as it was defined as (void*)0. You could not assign NULL to any pointer other than void*, which made it kind of useless. Back in those days, it was accepted that you used 0 (zero) for null pointers.
To this day, I have continued to use zero as a null pointer but those around me insist on using NULL. I personally do not see any benefit to giving a name (NULL) to an existing value - and since I also like to test pointers as truth values:
if (p && !q)
do_something();
then using zero makes more sense (as in if you use NULL, you cannot logically use p && !q - you need to explicitly compare against NULL, unless you assume NULL is zero, in which case why use NULL).
Is there any objective reason to prefer zero over NULL (or vice versa), or is all just personal preference?
Edit: I should add (and meant to originally say) that with RAII and exceptions, I rarely use zero/NULL pointers, but sometimes you do need them still.
Here's Stroustrup's take on this: C++ Style and Technique FAQ
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.
That said, don't sweat the small stuff.
There are a few arguments (one of which is relatively recent) which I believe contradict Bjarne's position on this.
Documentation of intent
Using NULL allows for searches on its use and it also highlights that the developer wanted to use a NULL pointer, irrespective of whether it is being interpreted by the compiler as NULL or not.
Overload of pointer and 'int' is relatively rare
The example that everybody quotes is:
void foo(int*);
void foo (int);
void bar() {
foo (NULL); // Calls 'foo(int)'
}
However, at least in my opinion, the problem with the above is not that we're using NULL for the null pointer constant: it's that we have overloads of foo() which take very different kinds of arguments. The parameter must be an int too, as any other type will result in an ambiguous call and so generate a helpful compiler warning.
Analysis tools can help TODAY!
Even in the absence of C++0x, there are tools available today that verify that NULL is being used for pointers, and that 0 is being used for integral types.
C++ 11 will have a new std::nullptr_t type.
This is the newest argument to the table. The problem of 0 and NULL is being actively addressed for C++0x, and you can guarantee that for every implementation that provides NULL, the very first thing that they will do is:
#define NULL nullptr
For those who use NULL rather than 0, the change will be an improvement in type-safety with little or no effort - if anything it may also catch a few bugs where they've used NULL for 0. For anybody using 0 today... well, hopefully they have a good knowledge of regular expressions...
Use NULL. NULL shows your intent. That it is 0 is an implementation detail that should not matter.
I always use:
NULL for pointers
'\0' for chars
0.0 for floats and doubles
where 0 would do fine. It is a matter of signaling intent. That said, I am not anal about it.
I stopped using NULL in favor of 0 long ago (as well as as most other macros). I did this not only because I wanted to avoid macros as much as possible, but also because NULL seems to have become over-used in C and C++ code. It seems to be used whenever a 0 value is needed, not just for pointers.
On new projects, I put this in a project header:
static const int nullptr = 0;
Now, when C++0x compliant compilers arrive, all I have to do is remove that line.
A nice benefit of this is that Visual Studio already recognizes nullptr as a keyword and highlights it appropriately.
cerr << sizeof(0) << endl;
cerr << sizeof(NULL) << endl;
cerr << sizeof(void*) << endl;
============
On a 64-bit gcc RHEL platform you get:
4
8
8
================
The moral of the story. You should use NULL when you're dealing with pointers.
1) It declares your intent (don't make me search through all your code trying to figure out if a variable is a pointer or some numeric type).
2) In certain API calls that expect variable arguments, they'll use a NULL-pointer to indicate the end of the argument list. In this case, using a '0' instead of NULL can cause problems. On a 64-bit platform, the va_arg call wants a 64-bit pointer, yet you'll be passing only a 32-bit integer. Seems to me like you're relying on the other 32-bits to be zeroed out for you? I've seen certain compilers (e.g. Intel's icpc) that aren't so gracious -- and this has resulted in runtime errors.
If I recall correctly NULL is defined differently in the headers that I have used. For C it is defined as (void*)0, and for C++ it's defines as just 0. The code looked something like:
#ifndef __cplusplus
#define NULL (void*)0
#else
#define NULL 0
#endif
Personally I still use the NULL value to represent null pointers, it makes it explicit that you're using a pointer rather than some integral type. Yes internally the NULL value is still 0 but it isn't represented as such.
Additionally I don't rely on the automatic conversion of integers to boolean values but explicitly compare them.
For example prefer to use:
if (pointer_value != NULL || integer_value == 0)
rather than:
if (pointer_value || !integer_value)
Suffice to say that this is all remedied in C++11 where one can simply use nullptr instead of NULL, and also nullptr_t that is the type of a nullptr.
I would say history has spoken and those who argued in favour of using 0 (zero) were wrong (including Bjarne Stroustrup). The arguments in favour of 0 were mostly aesthetics and "personal preference".
After the creation of C++11, with its new nullptr type, some compilers have started complaining (with default parameters) about passing 0 to functions with pointer arguments, because 0 is not a pointer.
If the code had been written using NULL, a simple search and replace could have been performed through the codebase to make it nullptr instead. If you are stuck with code written using the choice of 0 as a pointer it is far more tedious to update it.
And if you have to write new code right now to the C++03 standard (and can't use nullptr), you really should just use NULL. It'll make it much easier for you to update in the future.
I usually use 0. I don't like macros, and there's no guarantee that some third party header you're using doesn't redefine NULL to be something odd.
You could use a nullptr object as proposed by Scott Meyers and others until C++ gets a nullptr keyword:
const // It is a const object...
class nullptr_t
{
public:
template<class T>
operator T*() const // convertible to any type of null non-member pointer...
{ return 0; }
template<class C, class T>
operator T C::*() const // or any type of null member pointer...
{ return 0; }
private:
void operator&() const; // Can't take address of nullptr
} nullptr = {};
Google "nullptr" for more info.
I once worked on a machine where 0 was a valid address and NULL was defined as a special octal value. On that machine (0 != NULL), so code such as
char *p;
...
if (p) { ... }
would not work as you expect. You HAD to write
if (p != NULL) { ... }
Although I believe most compilers define NULL as 0 these days I still remember the lesson from those years ago: NULL is not necessarily 0.
I think the standard guarantees that NULL == 0, so you can do either. I prefer NULL because it documents your intent.
Using either 0 or NULL will have the same effect.
However, that doesn't mean that they are both good programming practices. Given that there is no difference in performance, choosing a low-level-aware option over an agnostic/abstract alternative is a bad programming practice. Help readers of your code understand your thought process.
NULL, 0, 0.0, '\0', 0x00 and whatelse all translate to the same thing, but are different logical entities in your program. They should be used as such. NULL is a pointer, 0 is quantity, 0x0 is a value whose bits are interesting etc. You wouldn't assign '\0' to a pointer whether it compiles or not.
I know some communities encourage demonstrating in-depth knowledge of an environment by breaking the environment's contracts. Responsible programmers, however, make maintainable code and keep such practices out of their code.
Strange, nobody, including Stroustroup mentioned that. While talking a lot about standards and aesthetics nobody noticed that it is dangerous to use 0 in NULL's stead, for instance, in variable argument list on the architecture where sizeof(int) != sizeof(void*). Like Stroustroup, I prefer 0 for aesthetic reasons, but one has to be careful not to use it where its type might be ambiguous.
I try to avoid the whole question by using C++ references where possible. Rather than
void foo(const Bar* pBar) { ... }
you might often be able to write
void foo(const Bar& bar) { ... }
Of course, this doesn't always work; but null pointers can be overused.
I'm with Stroustrup on this one :-)
Since NULL is not part of the language, I prefer to use 0.
Mostly personal preference, though one could make the argument that NULL makes it quite obvious that the object is a pointer which currently doesn't point to anything, e.g.
void *ptr = &something;
/* lots o' code */
ptr = NULL; // more obvious that it's a pointer and not being used
IIRC, the standard does not require NULL to be 0, so using whatever is defined in <stddef.h> is probably best for your compiler.
Another facet to the argument is whether you should use logical comparisons (implicit cast to bool) or explicity check against NULL, but that comes down to readability as well.
I prefer to use NULL as it makes clear that your intent is the value represents a pointer not an arithmetic value. The fact that it's a macro is unfortunate, but since it's so widely ingrained there's little danger (unless someone does something really boneheaded). I do wish it were a keyword from the beginning, but what can you do?
That said, I have no problem with using pointers as truth values in themselves. Just as with NULL, it's an ingrained idiom.
C++09 will add the the nullptr construct which I think is long overdue.
I always use 0. Not for any real thought out reason, just because when I was first learning C++ I read something that recommended using 0 and I've just always done it that way. In theory there could be a confusion issue in readability but in practice I have never once come across such an issue in thousands of man-hours and millions of lines of code. As Stroustrup says, it's really just a personal aesthetic issue until the standard becomes nullptr.
Someone told me once... I am going to redefine NULL to 69. Since then I don't use it :P
It makes your code quite vulnerable.
Edit:
Not everything in the standard is perfect. The macro NULL is an implementation-defined C++ null pointer constant not fully compatible with C NULL macro, what besides the type hiding implicit convert it in a useless and prone to errors tool.
NULL does not behaves as a null pointer but as a O/OL literal.
Tell me next example is not confusing:
void foo(char *);
void foo(int);
foo(NULL); // calls int version instead of pointer version!
Is because of all that, in the new standard appears std::nullptr_t
If you don't want to wait for the new standard and want to use a nullptr, use at least a decent one like the proposed by Meyers (see jon.h comment).
Well I argue for not using 0 or NULL pointers at all whenever possible.
Using them will sooner or later lead to segmentation faults in your code. In my experience this, and pointers in gereral is one of the biggest source of bugs in C++
also, it leads to "if-not-null" statements all over your code. Much nicer if you can rely on always a valid state.
There is almost always a better alternative.
Setting a pointer to 0 is just not that clear. Especially if you come a language other than C++. This includes C as well as Javascript.
I recently delt with some code like so:
virtual void DrawTo(BITMAP *buffer) =0;
for pure virtual function for the first time. I thought it to be some magic jiberjash for a week. When I realized it was just basically setting the function pointer to a null (as virtual functions are just function pointers in most cases for C++) I kicked myself.
virtual void DrawTo(BITMAP *buffer) =null;
would have been less confusing than that basterdation without proper spacing to my new eyes. Actually, I am wondering why C++ doesn't employ lowercase null much like it employes lowercase false and true now.