Note: This answer applies to the C language, not C++.
Null Pointers
The integer constant literal 0 has different meanings depending upon the context in which it's used. In all cases, it is still an integer constant with the value 0, it is just described in different ways.
If a pointer is being compared to the constant literal 0, then this is a check to see if the pointer is a null pointer. This 0 is then referred to as a null pointer constant. The C standard defines that 0 cast to the type void * is both a null pointer and a null pointer constant.
Additionally, to help readability, the macro NULL is provided in the header file stddef.h. Depending upon your compiler it might be possible to #undef NULL and redefine it to something wacky.
Therefore, here are some valid ways to check for a null pointer:
if (pointer == NULL)
NULL is defined to compare equal to a null pointer. It is implementation defined what the actual definition of NULL is, as long as it is a valid null pointer constant.
if (pointer == 0)
0 is another representation of the null pointer constant.
if (!pointer)
This if statement implicitly checks "is not 0", so we reverse that to mean "is 0".
The following are INVALID ways to check for a null pointer:
int mynull = 0;
<some code>
if (pointer == mynull)
To the compiler this is not a check for a null pointer, but an equality check on two variables. This might work if mynull never changes in the code and the compiler optimizations constant fold the 0 into the if statement, but this is not guaranteed and the compiler has to produce at least one diagnostic message (warning or error) according to the C Standard.
Note that the value of a null pointer in the C language does not matter on the underlying architecture. If the underlying architecture has a null pointer value defined as address 0xDEADBEEF, then it is up to the compiler to sort this mess out.
As such, even on this funny architecture, the following ways are still valid ways to check for a null pointer:
if (!pointer)
if (pointer == NULL)
if (pointer == 0)
The following are INVALID ways to check for a null pointer:
#define MYNULL (void *) 0xDEADBEEF
if (pointer == MYNULL)
if (pointer == 0xDEADBEEF)
as these are seen by a compiler as normal comparisons.
Null Characters
'\0' is defined to be a null character - that is a character with all bits set to zero. '\0' is (like all character literals) an integer constant, in this case with the value zero. So '\0' is completely equivalent to an unadorned 0 integer constant - the only difference is in the intent that it conveys to a human reader ("I'm using this as a null character.").
'\0' has nothing to do with pointers. However, you may see something similar to this code:
if (!*char_pointer)
checks if the char pointer is pointing at a null character.
if (*char_pointer)
checks if the char pointer is pointing at a non-null character.
Don't get these confused with null pointers. Just because the bit representation is the same, and this allows for some convenient cross over cases, they are not really the same thing.
References
See Question 5.3 of the comp.lang.c FAQ for more. See this pdf for the C standard. Check out sections 6.3.2.3 Pointers, paragraph 3.
Difference between NUL(for string) vs NULL(for pointer) in c++
Trying to understand NULL pointers
c++ - Using NULL with C's char* strings - Stack Overflow
NULL Define in c & c++ differs?! | Handmade Network
Note: This answer applies to the C language, not C++.
Null Pointers
The integer constant literal 0 has different meanings depending upon the context in which it's used. In all cases, it is still an integer constant with the value 0, it is just described in different ways.
If a pointer is being compared to the constant literal 0, then this is a check to see if the pointer is a null pointer. This 0 is then referred to as a null pointer constant. The C standard defines that 0 cast to the type void * is both a null pointer and a null pointer constant.
Additionally, to help readability, the macro NULL is provided in the header file stddef.h. Depending upon your compiler it might be possible to #undef NULL and redefine it to something wacky.
Therefore, here are some valid ways to check for a null pointer:
if (pointer == NULL)
NULL is defined to compare equal to a null pointer. It is implementation defined what the actual definition of NULL is, as long as it is a valid null pointer constant.
if (pointer == 0)
0 is another representation of the null pointer constant.
if (!pointer)
This if statement implicitly checks "is not 0", so we reverse that to mean "is 0".
The following are INVALID ways to check for a null pointer:
int mynull = 0;
<some code>
if (pointer == mynull)
To the compiler this is not a check for a null pointer, but an equality check on two variables. This might work if mynull never changes in the code and the compiler optimizations constant fold the 0 into the if statement, but this is not guaranteed and the compiler has to produce at least one diagnostic message (warning or error) according to the C Standard.
Note that the value of a null pointer in the C language does not matter on the underlying architecture. If the underlying architecture has a null pointer value defined as address 0xDEADBEEF, then it is up to the compiler to sort this mess out.
As such, even on this funny architecture, the following ways are still valid ways to check for a null pointer:
if (!pointer)
if (pointer == NULL)
if (pointer == 0)
The following are INVALID ways to check for a null pointer:
#define MYNULL (void *) 0xDEADBEEF
if (pointer == MYNULL)
if (pointer == 0xDEADBEEF)
as these are seen by a compiler as normal comparisons.
Null Characters
'\0' is defined to be a null character - that is a character with all bits set to zero. '\0' is (like all character literals) an integer constant, in this case with the value zero. So '\0' is completely equivalent to an unadorned 0 integer constant - the only difference is in the intent that it conveys to a human reader ("I'm using this as a null character.").
'\0' has nothing to do with pointers. However, you may see something similar to this code:
if (!*char_pointer)
checks if the char pointer is pointing at a null character.
if (*char_pointer)
checks if the char pointer is pointing at a non-null character.
Don't get these confused with null pointers. Just because the bit representation is the same, and this allows for some convenient cross over cases, they are not really the same thing.
References
See Question 5.3 of the comp.lang.c FAQ for more. See this pdf for the C standard. Check out sections 6.3.2.3 Pointers, paragraph 3.
It appears that a number of people misunderstand what the differences between NULL, '\0' and 0 are. So, to explain, and in attempt to avoid repeating things said earlier:
A constant expression of type int with the value 0, or an expression of this type, cast to type void * is a null pointer constant, which if converted to a pointer becomes a null pointer. It is guaranteed by the standard to compare unequal to any pointer to any object or function.
NULL is a macro, defined in as a null pointer constant.
\0 is a construction used to represent the null character, used to terminate a string.
A null character is a byte which has all its bits set to 0.
Whats the difference between in NUL and NULL in c++ these both are not same.
Hello all again,
I have another stupid question here lol, so I'm trying to wrap my head around NULL. Im currently under the impression that NULL is a built in constant that has a value of zero, but what does that actually mean? When would it be appropriate to use null? If someone could explain it in layman's terms that would be super helpful!
Does that mean that according to the standard it is legal to use the NULL constant as the terminator? (OP)
str1[3] = NULL;
Sometimes. Further: does it always properly cause a character array to form a string without concerns?
First, it looks wrong. Akin to int z = 0.0;. Yes it is legal well defined code, but unnecessarily draws attention to itself.
In practice, I have always used
NULLinstead of'\0'(OP)
I doubt you will find any modern style guide or group of coders endorsing that. NULL is best reserved for pointer contexts.1
These are 2 common and well understood alternatives.
str1[3] = '\0';
str1[3] = 0;
strings in C are null-terminated (OP)
The C spec consistently uses null character, not just null.
The macros are
NULLwhich expands to an implementation-defined null pointer constant; and ... C11 §7.19 3
OK, now what is a null pointer constant?
An integer constant expression with the value 0, or such an expression cast to type
void *, is called a null pointer constant. §6.3.2.3 5
If the null pointer constant is a void* then we have something like
str1[3] = (void*) 0;
The above can warn about converting a pointer to a char. This is something best avoided.
Will it be the same in case of C++? (OP)
Yes, the above applies. (Aside: str1[3] = 0 may warn.) Further, NULL is less preferred than nullptr. So NULL is rarely the best to use in C++ in any context.
1Note: @Joshua reports a style that matches OP's in 1995 Turbo C 4.5
The bottom line is that in C/C++, NULL is for pointers and is not the same as the null character, despite the fact that both are defined as zero. You might use NULL as the null character and get away with it depending on the context and platform, but to be correct, use '\0'. This is described in both standards:
C specifies that the macro
NULLis defined as a macro in<stddef.h>which "expands to an implementation-defined null pointer constant" (Section 7.17.3), which is itself defined as "an integer constant expression with the value 0, or such an expression cast to typevoid *" (Section 6.3.2.3.3).The null character is defined in section 5.2.1.2: "A byte with all bits set to 0, called the null character, shall exist in the basic execution character set; it is used to terminate a character string." That same section explains that
\0will be the representation of this null character.C++ makes the same distinctions. From section 4.10.1 of the C++ standard: "A null pointer constant is an integer literal (2.13.2) with value zero or a prvalue of type
std::nullptr_t." In section 2.3.3, it describes the as "null character (respectively, null wide character), whose value is 0". Section C.5.2 further confirms that C++ respectsNULLas a standard macro imported from the C Standard Library.
The 0xCACACACA generated by Visual Studio is usually there for un-initialized pointers, for precisely this reason. When you see this, you know something is wrong. If it initialized it to 0, it can very well be expected behavior. This is only done in debug node, with full optimization the value of an uninitialized pointer would be just garbage.
And yes, NULL pointers don't have a value of 0 per say. 0 is just a literal. The actual value can be different. But the compiler is smart enough to abstract that away, so even if you have a NULL pointer whose value isn't really 0 (as in the number 0), comparing it to 0 or NULL would still yield true. It's better to think in terms of nullptr really.
This question previously appeared on the comp.lang.c newsgroup. You can read the archive with Google Groups
In that thread, Paul Sand quoted another source, "Portable C" by H. Rabinowitz and Chaim Schaap, as follows:
Certain Prime computers use a value different from all-bits-0 to encode the null pointer. Also, some large Honeywell-Bull machines use the bit pattern 06000 to encode the null pointer. On such machines, the assignment of
0to a pointer yields the special bit pattern that designates the null pointer.Similarly,
(char *)0yields the special bit pattern that designates a null pointer.
Where you'd commonly see non-null pointers is when working with physical memory addresses (that is, when there is no MMU or in kernel mode bypassing the MMU) and the machine has memory-mapped I/O at or near address 0. You want a null pointer to be way out in no-man's land, so that if you offset it (e.g. structure member access via a pointer) you won't get any useful address.
For your specific question, only an integral constant expression with value 0 is interpreted as a null pointer. So
char* p = (char*)i;
does not portably make p a null pointer (i.e. the Standard makes no such guarantee, but your particular compiler may).