void DoSomeThing(CHAR parm[])
{
}
int main()
{
DoSomeThing(NULL);
}
Is the passing NULL for array parameter allowed in C/C++?
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5@VladLazarenko: I think you missed the distinction between 'possible' and 'allowed'sehe– sehe2011-09-28 18:49:07 +00:00Commented Sep 28, 2011 at 18:49
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4"Try it" is a good first shot, but especially in the realm of C and C++ with its hundreds of compiler-specific language extensions, he who wants portability has to ask the language lawyers. Not that this is a particular great question.user395760– user3957602011-09-28 18:50:40 +00:00Commented Sep 28, 2011 at 18:50
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2@reader: No, that's why we have Specifications. Just because it compiles doesn't mean it's allowed.John Dibling– John Dibling2011-09-28 18:51:02 +00:00Commented Sep 28, 2011 at 18:51
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1@JohnDibling -- And just because it's allowed doesn't mean it compiles.Hot Licks– Hot Licks2011-09-28 18:55:46 +00:00Commented Sep 28, 2011 at 18:55
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1@JohnDibling I think compilers allow things or reject things, specifications tell how things should be. You can't compile a program with a specification. If the question was is is a valid use then we would definitely look at the specification. Look at this post: lists.whatwg.org/pipermail/whatwg-whatwg.org/2009-June/… The specification writer updated the specification because although things should be according to specifications but it is not the case in the real world.reader_1000– reader_10002011-09-28 19:02:12 +00:00Commented Sep 28, 2011 at 19:02
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4 Answers
What you cannot do is pass an array by value. The signature of the function
void f( char array[] );
is transformed into:
void f( char *array );
and you can always pass NULL as argument to a function taking a pointer.
You can however pass an array by reference in C++, and in that case you will not be able to pass a NULL pointer (or any other pointer):
void g( char (&array)[ 10 ] );
Note that the size of the array is part of the type, and thus part of the signature, which means that g will only accept lvalue-expressions of type array of 10 characters
answered Sep 28, 2011 at 18:50
David Rodríguez - dribeas
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Short answer is yes, you can pass NULL in this instance (at least for C, and I think the same is true for C++).
There are two reasons for this. First, in the context of a function parameter declaration, the declarations T a[] and T a[N] are synonymous with T *a; IOW, despite the array notation, a is declared as a pointer to T, rather than an array of T. From the C language standard:
6.7.5.3 Function declarators (including prototypes)
...
7 A declaration of a parameter as ‘‘array of type’’ shall be adjusted to ‘‘qualified pointer to type’’, where the type qualifiers (if any) are those specified within the[and]of the array type derivation. If the keywordstaticalso appears within the[and]of the array type derivation, then for each call to the function, the value of the corresponding actual argument shall provide access to the first element of an array with at least as many elements as specified by the size expression.
The second reason is that when an expression of array type appears in most contexts (such as in a function call), the type of that expression is implicitly converted ("decays") to a pointer type, so what actually gets passed to the function is a pointer value, not an array:
6.3.2.1 Lvalues, arrays, and function designators
...
3 Except when it is the operand of thesizeofoperator or the unary&operator, or is a string literal used to initialize an array, an expression that has type ‘‘array of type’’ is converted to an expression with type ‘‘pointer to type’’ that points to the initial element of the array object and is not an lvalue. If the array object has register storage class, the behavior is undefined.
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First off, what you're passing to the function is a pointer and not an array. The array notation used for the parm parameter is just syntactic sugar for CHAR *parm.
So yes, you can pass a NULL pointer.
answered Sep 28, 2011 at 18:49
Blagovest Buyukliev
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The only issue would be if DoSomeThing were overloaded, in which case the compiler might not be able to distinguish which function to call. But casting to the appropriate type would theoretically fix that.
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