Issue 0017.22: Does the rescan of a macro invocation also perform token pasting?

This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.

Authors: Derek M. Jones, WG14
Date: 1992-12-10
Reference document: X3J11/90-056
Submitted against: C90
Status: Fixed
Fixed in: C90 TC1
Converted from: dr.htm, dr_017.html

Gluing during rescan

Reference: subclause 6.8.3.3, page 90. Does the rescan of a macro invocation also perform gluing?

#define hash_hash # ## #
 #define mkstr(a) # a
 #define in_between(a) mkstr(a)
 #define join(c, d) in_between(c hash_hash d)

 char p[2] = join(x, y);

Is the above legal? Does join expand to "xy" or "x ## y"?

It all depends on the wording in subclause 6.8.3.3 on page 90, lines 39-40. Does the wording “... before the replacement list is reexamined ...” mean before being reexamined for the first time only, or before being reexamined on every rescan?

This rather perverse macro expansion is only made possible because the constraints on the use of # refer to function-like macros only. If this constraint were extended to cover object-like macros the whole question goes away.

Dave Prosser says that the intent was to produce "x ## y". My reading is that the result should be "xy". I cannot see any rule that says a created ## should not be processed appropriately. The standard does say in subclause 6.8.3.3, page 90, line 40 “... each instance of a ## ...”

The reason I ask if the above is legal is that the order of evaluation of # and ## is not defined. Thus if # is performed first the result is very different than if ## goes first.

Comment from WG14 on 1997-09-23:

Correction

Add to subclause 6.8.3.3, page 90:

Example

#define hash_hash # ## #
 #define mkstr(a) # a
 #define in_between(a) mkstr(a)
 #define join(c, d) in_between(c hash_hash d)
 char p[] = join(x, y); /* equivalent to char p[] = "x ## y";*/

The expansion produces, at various stages:

join(x, y)
 in_between(x hash_hash y)
 in_between(x ## y)
 mkstr(x ## y)
 "x ## y"

In other words, expanding hash_hash produces a new token, consisting of two adjacent sharp signs, but this new token is not the catenation operator.