Defect Report #258

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Submitter: UK C Panel
Submission Date: 2001-09-07
Source: Clive D.W. Feather <clive@demon.net>
Version: 1.1
Date: 2002-03-06
Subject: ordering of "defined" and macro replacement

Problem

Consider the code:

   #define repeat(x) x && x    // Line 1 
   #if repeat(defined fred)    // Line 2 

and the code:

   #define forget(x) 0         // Line 3 
   #if forget(defined fred)    // Line 4 

6.10.1#3 says:

[#3] Prior to evaluation, macro invocations in the list of preprocessing tokens that will become the controlling constant expression are replaced (except for those macro names modified by the defined unary operator), just as in normal text. If the token defined is generated as a result of this replacement process or use of the defined unary operator does not match one of the two specified forms prior to macro replacement, the behavior is undefined.

Does line 2 "generate" a defined operator ? Is line 4 strictly conforming code, or does the fact that macro expansion "forgets" the defined operator cause a problem ?

The restriction was clearly intended to make code like the following undefined:

    #define jim defined
    #if jim loves sheila

I would guess that the original intention was that any defined X pair in the original source worked correctly. The proposed change would resolve this.

In addition, given the order of events, it is unsuitable to say that a defined X expression is "evaluated". Rather it should be described as a textual substitution.

Suggested Technical Corrigendum

Change 6.10.1#1 to read:

[...]
      defined identifier
or
      defined ( identifier )
which are replaced by the token 1 if the identifier is currently
[...]
subject identifier), or the token 0 if it is not.
and #3 to read:
[#3] Prior to evaluation, the list of preprocessing tokens that will become the controlling constant expression is examined. Firstly all expressions using the defined operator are replaced as described above, and then macro invocations are replaced, just as in normal text. If the token defined appears in the list after the replacement process, or the use of the defined unary operator does not match one of the two specified forms prior to macro replacement, the behavior is undefined. After all [...]

Proposed Committee Response

The standard does not clearly specify what happens in this case, so portable programs should not use these sorts of constructs.


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