This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.
Authors: Terence David Carroll, WG14
Date: 1992-12-10
Reference document: X3J11/90-011
Submitted against: C90
Status: Closed
Cross-references: 0153
Converted from: dr.htm, dr_003.html
Subclause 6.8.3: Empty arguments to function-like macros I would like to make a request for clarification and a request for a stronger statement of standardization. Given
#define macro( xx ) xx
macro()
is this a constraint violation of subclause 6.8.3 Constraints paragraph 4:
The number of arguments in an invocation of a function-like macro shall agree with the number of parameters in the macro definition, ...
or is this an undefined, implementation-dependent program - subclause 6.8.3, Semantics paragraph 5:
If (before argument substitution) any argument consists of no preprocessing tokens, the behavior is undefined.
In connection with the above I would request that the Committee make a much stronger statement as to whether empty arguments are to be treated as valid arguments or are to be treated as errors. They can have their uses, but if that is recognized then it should be standardized; if not, it should not be allowed.
Comment from WG14 on 1997-09-23:
If one takes the general case, empty arguments in invocations of function-like macros are easy to recognize:
#define f(a,b,c) whatever
f(,,)
These empty arguments all have “shadows” that cause the sentence you reference in subclause 6.8.3 (page 90, lines 4-5) to be clearly in effect.
The only uncertain case is one in which an empty argument in an invocation of a one-parameter function-like macro mimics a “no arguments” invocation. (It should also be noted that the definition of a macro argument from clause 3 does not preclude an empty sequence.) Thus the standard is clear that the behavior is undefined in the example from your request. If an implementation does not choose to allow empty arguments, a diagnostic will probably be emitted; otherwise, the invocation will most likely be replaced by a preprocessing token sequence in which the parameter is replaced with no tokens. But because the standard does not define this, other than as a common extension, there are no guarantees.