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hexcoder-

opened on Oct 3, 2017

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While analyzing issue #60, I found that

#define x # #
x

gives

test.c:1: syntax error: failed to expand 'x', Invalid ## usage when expanding 'x'.

but should give
# #

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1. danmar commented on Nov 5, 2017

   

   danmar

   on Nov 5, 2017

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   but should give

   hmm.. how do you know that is the expected result. The # is a preprocessor token and should be handled by the preprocessor as far as I know. What use is there in a real program?
2. hexcoder- commented on Nov 5, 2017

   

   hexcoder-

   on Nov 5, 2017

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   Yes, '#' should be handled by the preprocessor as does '##'. But the semantics between both of them are different.
   -> # and ## operators

   '# #' should not be silently transformed into '##'.

   The 'real use' came from issue #60 while analysing the stack trace.
   For reference cpp of gcc gives this

   >cat demo.cpp
   #define a # #
   a
   
   >cpp -E demo.cpp
   # 1 "<stdin>"
   # 1 "<built-in>"
   # 1 "<command-line>"
   # 31 "<command-line>"
   # 1 "/usr/include/stdc-predef.h" 1 3 4
   # 32 "<command-line>" 2
   # 1 "<stdin>"
   
    # #
   

   My intention was like this:
   I created this new issue in order to simplify #60.
   The problem in #60 might be a consequence of misparsing '# #'.
   Since the misparsing happens first, I found it worthwhile to concentrate on this.
3. amai2012 commented on Nov 7, 2017

   

   amai2012

   on Nov 7, 2017

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   MSVC2012 also gives # #
4. danmar commented on Nov 11, 2017

   

   danmar

   on Nov 11, 2017

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   ok
5. asmwarrior commented on Jan 20, 2018

   

   asmwarrior

   on Jan 20, 2018

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   I see a very nice header only simple C Preprocessor here:glampert/parse-utils: C++ utilities for lexing and parsing of script languages and configuration files., which is a GPLv3 project, and I just comment out the line:

    preprocessor.hpp | 16 ++++++++--------
    1 file changed, 8 insertions(+), 8 deletions(-)
   
   diff --git a/preprocessor.hpp b/preprocessor.hpp
   index 6973cc3..3d721ba 100644
   --- a/preprocessor.hpp
   +++ b/preprocessor.hpp
   @@ -3022,14 +3022,14 @@ bool preprocessor::expand_macro_and_append(const int macro_index, std::string *
                    const auto & first_tok = m_macro_tokens[macro.first_body_token];
                    const auto & last_tok  = m_macro_tokens[macro.first_body_token + macro.body_token_count - 1];
    
   -                if (lexer::is_punctuation_token(first_tok, lexer::punctuation_id::preprocessor))
   -                {
   -                    return error("\'#\' cannot appear at start of macro expansion!");
   -                }
   -                if (lexer::is_punctuation_token(last_tok, lexer::punctuation_id::preprocessor))
   -                {
   -                    return error("\'#\' cannot appear at end of macro expansion!");
   -                }
   +//                if (lexer::is_punctuation_token(first_tok, lexer::punctuation_id::preprocessor))
   +//                {
   +//                    return error("\'#\' cannot appear at start of macro expansion!");
   +//                }
   +//                if (lexer::is_punctuation_token(last_tok, lexer::punctuation_id::preprocessor))
   +//                {
   +//                    return error("\'#\' cannot appear at end of macro expansion!");
   +//                }
    
                    if (lexer::is_punctuation_token(first_tok, lexer::punctuation_id::preprocessor_merge))
                    {
   
   

   Then I see the parsed result are: # #

   I use this code to run the test: parse-utils/test_eval_macros.cpp at master · glampert/parse-utils
6. danmar commented on Jan 20, 2018

   

   danmar

   on Jan 20, 2018

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   thanks for pointing out that preprocessor. For information.. a small and simple preprocessor could easily be integrated into cppcheck if it gets good enough and we see that it's actively maintained.
7. danmar

   self-assigned this

   on Jun 30, 2020
8. danmar

   added a commit that references this issue on Jun 30, 2020

   Better handling of standalone # in macros (#108)

   546d085
9. danmar

   closed this as completedon Jun 30, 2020

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