Hi!

I'm proposing some refactoring of the specification of the preprocessor
in the C Committee (WG14). The committee wanted me to forward the
proposal to the C++ Committee too, just to make sure that you're aware
of it, and in case you have any reasons to disagree.

I assume you'll agree, as the only syntactic difference between the C
preprocessor and the C++ one is modules, and for the specification of
modules, you did it exactly as I'm proposing now, so my expectation
would be that WG21 is fine with this proposal.

Essentially, it splits the huge Syntax clause from the preprocessor
introduction, and puts each part under its own subclause.

Please let me know, in any case. Find the proposal below (plain text).


Have a lovely day!
Alex

---
Name
 alx-0014r6 - Refactor syntax of preprocessing directives
Category
 Cosmetic refactor; readability; editorial;
Authors
 Alejandro Colomar <alx_at_[hidden]>
History
 <https://www.alejandro-colomar.es/src/alx/alx/wg14/alx-0014.git/>
 r4 (2025-08-29):
 -  Split from alx-0014.
 r5 (2025-08-31):
 -  Move alx-0014 to a separate proposal: alx-0065.
 r6 (2025-08-31):
 -  Merge the split proposals into a single on with subsections.
Principles
 -  Keep the language small and simple
 -  Facilitate interoperability
 -  Codify existing practice to address evident deficiencies
Rationale
 Editorial change.  This moves text around for better
 organization and readability.  No semantic changes.  This makes
 it so that any future proposals to the preprocessor will be
 easier to apply.
 The proposal is split in subsections, separated by '---', for
 better readability.
 This proposal applies as is to the C++ latest draft, N5014, only
 changing section numbers and their titles.
 This change has prior art in C++, as they've done the same exact
 thing with their pp-import directive.  I guess they haven't done
 it with the other directives for compatibility with us, so let's
 help them and do it everywhere.
    Interaction with other proposals
 This blocks alx-0003 ("Add directives #def and #enddef")
 This blocks alx-0013 ("Prohibit non-directives (other than ID directives)").
---
Name
 alx-0014A - Refactor syntax of include directives
Proposed wording
 Based on C N3550.
    6.10.1  Preprocessing directives :: General  (C++: 15.1  :: Preamble)
 @@ Syntax, p1  (C++: no 'Syntax' subtitle)
  control-line:
 - <b># include</b> pp-tokens new-line
 + include-directive
  pp-import (C++ only)
   <b># embed</b> pp-tokens new-line
   <b># define</b> identifier replacement-list new-line
   <b># define</b> identifier lparen identifier-list(opt) <b>)</b> replacement-list new-line
   <b># define</b> identifier lparen <b>... )</b> replacement-list new-line
   <b># define</b> identifier lparen identifier-list <b>, ... )</b> replacement-list new-line
   <b># undef</b> identifier new-line
   <b># line</b> pp-tokens new-line
   <b># error</b> pp-tokens(opt) new-line
   <b># warning</b> pp-tokens(opt) new-line
   <b># pragma</b> pp-tokens(opt) new-line
   <b>#</b> new-line
    6.10.3  Source file inclusion  (C++: 15.3)
 ## Add 'Syntax' before 'Constraints'  (C++: No subtitles)
 @@ Syntax, new p after title
 +include-directive:
 + <b># include</b> pp-tokens new-line
---
Name
 alx-0014B - Refactor syntax of embed directives
Proposed wording
 Based on N3550.
    6.10.1  Preprocessing directives :: General
 @@ Syntax, p1
  control-line:
   <b># include</b> pp-tokens new-line
  pp-import (C++ only)
 - <b># embed</b> pp-tokens new-line
 + embed-directive
   <b># define</b> identifier replacement-list new-line
   <b># define</b> identifier lparen identifier-list(opt) <b>)</b> replacement-list new-line
   <b># define</b> identifier lparen <b>... )</b> replacement-list new-line
   <b># define</b> identifier lparen identifier-list <b>, ... )</b> replacement-list new-line
   <b># undef</b> identifier new-line
   <b># line</b> pp-tokens new-line
   <b># error</b> pp-tokens(opt) new-line
   <b># warning</b> pp-tokens(opt) new-line
   <b># pragma</b> pp-tokens(opt) new-line
   <b>#</b> new-line
 ...
 -pp-parameter:
 - pp-parameter-name pp-parameter-clause(opt)
 -
 -pp-parameter-name:
 - pp-standard-parameter
 - pp-prefixed-parameter
 -
 -pp-standard-parameter:
 - identifier
 -
 -pp-prefixed-parameter
 - identifier <b>::</b> identifier
 -
 -pp-parameter-clause:
 - <b>(</b> pp-balanced-token-sequence(opt) <b>)</b>
 -
 -pp-balanced-token-sequence:
 - pp-balanced-token
 - pp-balanced-token-sequence pp-balanced-token
 -
 -pp-balanced-token:
 - <b>(</b> pp-balanced-token-sequence(opt) <b>)</b>
 - <b>[</b> pp-balanced-token-sequence(opt) <b>]</b>
 - <b>{</b> pp-balanced-token-sequence(opt) <b>}</b>
 - <r>any pp-token other than a parenthesis, a bracket, or a brace</r>
 -
 -embed-parameter-sequence:
 - pp-parameter
 - embed-parameter-sequence pp-parameter
    6.10.4.1  Binary resource inclusion :: #embed preprocessing directive
 ## Add 'Syntax' before 'Description'
 @@ Syntax, new p after title
 +embed-directive:
 + <b># embed</b> pp-tokens new-line
 +
 +pp-parameter:
 + pp-parameter-name pp-parameter-clause(opt)
 +
 +pp-parameter-name:
 + pp-standard-parameter
 + pp-prefixed-parameter
 +
 +pp-standard-parameter:
 + identifier
 +
 +pp-prefixed-parameter
 + identifier <b>::</b> identifier
 +
 +pp-parameter-clause:
 + <b>(</b> pp-balanced-token-sequence(opt) <b>)</b>
 +
 +pp-balanced-token-sequence:
 + pp-balanced-token
 + pp-balanced-token-sequence pp-balanced-token
 +
 +pp-balanced-token:
 + <b>(</b> pp-balanced-token-sequence(opt) <b>)</b>
 + <b>[</b> pp-balanced-token-sequence(opt) <b>]</b>
 + <b>{</b> pp-balanced-token-sequence(opt) <b>}</b>
 + <r>any pp-token other than a parenthesis, a bracket, or a brace</r>
 +
 +embed-parameter-sequence:
 + pp-parameter
 + embed-parameter-sequence pp-parameter
---
Name
 alx-0014C - Refactor syntax of macros
Proposed wording
 Based on N3550.
    6.10.1  Preprocessing directives :: General
 @@ Syntax, p1
  control-line:
   <b># include</b> pp-tokens new-line
   <b># embed</b> pp-tokens new-line
 - <b># define</b> identifier replacement-list new-line
 - <b># define</b> identifier lparen identifier-list(opt) <b>)</b> replacement-list new-line
 - <b># define</b> identifier lparen <b>... )</b> replacement-list new-line
 - <b># define</b> identifier lparen identifier-list <b>, ... )</b> replacement-list new-line
 + define-directive
 - <b># undef</b> identifier new-line
 + undef-directive
   <b># line</b> pp-tokens new-line
   <b># error</b> pp-tokens(opt) new-line
   <b># warning</b> pp-tokens(opt) new-line
   <b># pragma</b> pp-tokens(opt) new-line
   <b>#</b> new-line
 ...
 -lparen:
 - a <b>(</b> character not immediately preceded by white space
 -
 -replacement-list:
 - pp-tokens(opt)
 ...
 -identifier-list:
 - identifier
 - identifier-list <b>,</b> identifier
    6.10.5.1  Macro replacement :: General
 ## Add 'Syntax' before 'Constraints'
 @@ Syntax, new p after title
 +define-directive:
 + <b># define</b> identifier replacement-list new-line
 + <b># define</b> identifier lparen identifier-list(opt) <b>)</b> replacement-list new-line
 + <b># define</b> identifier lparen <b>... )</b> replacement-list new-line
 + <b># define</b> identifier lparen identifier-list <b>, ... )</b> replacement-list new-line
 +
 +undef-directive:
 + <b># undef</b> identifier new-line
 +
 +lparen:
 + a <b>(</b> character not immediately preceded by white space
 +
 +replacement-list:
 + pp-tokens(opt)
 +
 +identifier-list:
 + identifier
 + identifier-list <b>,</b> identifier
---
Name
 alx-0014D - Refactor syntax of conditional directives
Proposed wording
 Based on N3550.
    6.10.1  Preprocessing directives :: General
 @@ Syntax, p1
 -if-section:
 - if-group elif-groups(opt) else-group(opt) endif-line
 -
 -if-group:
 - <b># if</b> constant-expression newline group(opt)
 - <b># ifdef</b> identifier new-line group(opt)
 - <b># ifndef</b> identifier new-line group(opt)
 -
 -elif-groups:
 - elif-group
 - elif-groups elif-group
 -
 -elif-group:
 - <b># elif</b> constant-expression new-line group(opt)
 - <b># elifdef</b> identifier new-line group(opt)
 - <b># elifndef</b> identifier new-line group(opt)
 -
 -else-group:
 - <b># else</b> new-line group(opt)
 -
 -endif-line:
 - <b># endif</b> new-line
    6.10.2  Conditional inclusion
 @@ Syntax, p1
 +if-section:
 + if-group elif-groups(opt) else-group(opt) endif-line
 +
 +if-group:
 + <b># if</b> constant-expression newline group(opt)
 + <b># ifdef</b> identifier new-line group(opt)
 + <b># ifndef</b> identifier new-line group(opt)
 +
 +elif-groups:
 + elif-group
 + elif-groups elif-group
 +
 +elif-group:
 + <b># elif</b> constant-expression new-line group(opt)
 + <b># elifdef</b> identifier new-line group(opt)
 + <b># elifndef</b> identifier new-line group(opt)
 +
 +else-group:
 + <b># else</b> new-line group(opt)
 +
 +endif-line:
 + <b># endif</b> new-line
 +
 ...
---
Name
 alx-0014E - Refactor syntax of line directives
Proposed wording
 Based on N3550.
    6.10.1  Preprocessing directives :: General
 @@ Syntax, p1
  control-line:
   <b># include</b> pp-tokens new-line
   <b># embed</b> pp-tokens new-line
   <b># define</b> identifier replacement-list new-line
   <b># define</b> identifier lparen identifier-list(opt) <b>)</b> replacement-list new-line
   <b># define</b> identifier lparen <b>... )</b> replacement-list new-line
   <b># define</b> identifier lparen identifier-list <b>, ... )</b> replacement-list new-line
   <b># undef</b> identifier new-line
 - <b># line</b> pp-tokens new-line
 + line-directive
   <b># error</b> pp-tokens(opt) new-line
   <b># warning</b> pp-tokens(opt) new-line
   <b># pragma</b> pp-tokens(opt) new-line
   <b>#</b> new-line
    6.10.6  Line control
 ## Add 'Syntax' before 'Constraints'
 @@ Syntax, new p after title
 +line-directive:
 + <b># line</b> pp-tokens new-line
---
Name
 alx-0014F - Refactor syntax of diagnostic directives
Proposed wording
 Based on N3550.
    6.10.1  Preprocessing directives :: General
 @@ Syntax, p1
  control-line:
   <b># include</b> pp-tokens new-line
   <b># embed</b> pp-tokens new-line
   <b># define</b> identifier replacement-list new-line
   <b># define</b> identifier lparen identifier-list(opt) <b>)</b> replacement-list new-line
   <b># define</b> identifier lparen <b>... )</b> replacement-list new-line
   <b># define</b> identifier lparen identifier-list <b>, ... )</b> replacement-list new-line
   <b># undef</b> identifier new-line
   <b># line</b> pp-tokens new-line
 - <b># error</b> pp-tokens(opt) new-line
 - <b># warning</b> pp-tokens(opt) new-line
 + diagnostic-directive
   <b># pragma</b> pp-tokens(opt) new-line
   <b>#</b> new-line
    6.10.7  Diagnostic directives
 ## Add 'Syntax' before 'Constraints'
 @@ Syntax, new p after title
 +diagnostic-directive:
 + <b># error</b> pp-tokens(opt) new-line
 + <b># warning</b> pp-tokens(opt) new-line
---
Name
 alx-0014G - Refactor syntax of pragma directives
Proposed wording
 Based on N3550.
    6.10.1  Preprocessing directives :: General
 @@ Syntax, p1
  control-line:
   <b># include</b> pp-tokens new-line
   <b># embed</b> pp-tokens new-line
   <b># define</b> identifier replacement-list new-line
   <b># define</b> identifier lparen identifier-list(opt) <b>)</b> replacement-list new-line
   <b># define</b> identifier lparen <b>... )</b> replacement-list new-line
   <b># define</b> identifier lparen identifier-list <b>, ... )</b> replacement-list new-line
   <b># undef</b> identifier new-line
   <b># line</b> pp-tokens new-line
   <b># error</b> pp-tokens(opt) new-line
   <b># warning</b> pp-tokens(opt) new-line
 - <b># pragma</b> pp-tokens(opt) new-line
 + pragma-directive
   <b>#</b> new-line
    6.10.8  Pragma directive
 ## Add 'Syntax' before 'Constraints'
 @@ Syntax, new p after title
 +pragma-directive:
 + <b># pragma</b> pp-tokens(opt) new-line
---
Name
 alx-0014H - Refactor syntax of null directives
Proposed wording
 Based on N3550.
    6.10.1  Preprocessing directives :: General
 @@ Syntax, p1
  control-line:
   <b># include</b> pp-tokens new-line
   <b># embed</b> pp-tokens new-line
   <b># define</b> identifier replacement-list new-line
   <b># define</b> identifier lparen identifier-list(opt) <b>)</b> replacement-list new-line
   <b># define</b> identifier lparen <b>... )</b> replacement-list new-line
   <b># define</b> identifier lparen identifier-list <b>, ... )</b> replacement-list new-line
   <b># undef</b> identifier new-line
   <b># line</b> pp-tokens new-line
   <b># error</b> pp-tokens(opt) new-line
   <b># warning</b> pp-tokens(opt) new-line
   <b># pragma</b> pp-tokens(opt) new-line
 - <b>#</b> new-line
 + null-directive
    6.10.9  Null directive
 ## Add 'Syntax' before 'Constraints'
 @@ Syntax, new p after title
 +null-directive:
 + <b>#</b> new-line
-- 
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