Macro name suppression in rescanned replacement text
15.6.5 [cpp.rescan]
Bjarne Stroustrup

Created on 2001-01-18.00:00:00 last changed 2 weeks ago


Date: 2022-06-15.00:00:00

Additional notes (June, 2022):

It is unclear whether the phrasing "following the macro's replacement list" in the suggested resolution covers this case:

  #define f(x) b
  #define b(x) x(1)

  f(1)(f) // the result is 1
Date: 2004-09-10.00:00:00

It is not clear from the Standard what the result of the following example should be:

#define NIL(xxx) xxx
#define G_0(arg) NIL(G_1)(arg)
#define G_1(arg) NIL(arg)

The relevant text from the Standard is found in 15.6.5 [cpp.rescan] paragraph 2:

If the name of the macro being replaced is found during this scan of the replacement list (not including the rest of the source file's preprocessing tokens), it is not replaced. Further, if any nested replacements encounter the name of the macro being replaced, it is not replaced. These nonreplaced macro name preprocessing tokens are no longer available for further replacement even if they are later (re)examined in contexts in which that macro name preprocessing token would otherwise have been replaced.

The sequence of expansion of G0(42) is as follows:


The question is whether the use of NIL in the last line of this sequence qualifies for non-replacement under the cited text. If it does, the result will be NIL(42). If it does not, the result will be simply 42.

The original intent of the J11 committee in this text was that the result should be 42, as demonstrated by the original pseudo-code description of the replacement algorithm provided by Dave Prosser, its author. The English description, however, omits some of the subtleties of the pseudo-code and thus arguably gives an incorrect answer for this case.

Suggested resolution (Mike Miller): Replace the cited paragraph with the following:

As long as the scan involves only preprocessing tokens from a given macro's replacement list, or tokens resulting from a replacement of those tokens, an occurrence of the macro's name will not result in further replacement, even if it is later (re)examined in contexts in which that macro name preprocessing token would otherwise have been replaced.

Once the scan reaches the preprocessing token following a macro's replacement list — including as part of the argument list for that or another macro — the macro's name is once again available for replacement. [Example:

    #define NIL(xxx) xxx
    #define G_0(arg) NIL(G_1)(arg)
    #define G_1(arg) NIL(arg)
    G_0(42)                         // result is 42, not NIL(42)

The reason that NIL(42) is replaced is that (42) comes from outside the replacement list of NIL(G_1), hence the occurrence of NIL within the replacement list for NIL(G_1) (via the replacement of G_1(42)) is not marked as nonreplaceable. —end example]

(Note: The resolution of this issue must be coordinated with J11/WG14.)

Notes (via Tom Plum) from April, 2004 WG14 Meeting:

Back in the 1980's it was understood by several WG14 people that there were tiny differences between the "non-replacement" verbiage and the attempts to produce pseudo-code. The committee's decision was that no realistic programs "in the wild" would venture into this area, and trying to reduce the uncertainties is not worth the risk of changing conformance status of implementations or programs.

Date User Action Args
2022-06-11 20:21:09adminsetmessages: + msg6851
2001-01-18 00:00:00admincreate