[boost] ATTN: All Developers Interested in Boost.Preprocessor

From: Vesa Karvonen (vesa_karvonen_at_[hidden])
Date: 2003-04-30 04:22:10

[posted and e-mailed]

Hi,

I have one proposal that could make it significantly easier to generate code 
using the preprocessor. One of the biggest problems in using the 
preprocessor for generating C or C++ code is that the preprocessor 
essentially requires
1. parenthesis to be balanced, and
2. commas to be parenthesized,
in order for them to be passed as macro parameters. Consider the following 
example:

  #define ID(x) x
  ID( (         )
  ID(   a , b   )
  ID(         ) )

The macro expansion of the above preprocessor code does not produce the 
intended result:

  ( a , b )

Although there are tricky ways of avoiding some of these limitations, it 
makes it significantly more difficult to perform extensive code generation 
with the preprocessor. My proposal is trivial and should be equally trivial 
to implement in a C or C++ compiler.

The basic idea is that the keywords '__l_paren__', '__r_paren__' and 
'__comma__' could be used in place of '(', ')' and ',', respectively. The 
above example would now become:

  #define ID(x) x
  ID( __l_paren__                           )
  ID(             a __comma__ b             )
  ID(                           __r_paren__ )

and it would expand into:

   __l_paren__ a __comma__ b __r_paren__

which would be recognized in translation phases after macro replacement as 
equivalent to the token sequence:

   ( a , b )

This trivial extension would make it an order of magnitude easier to 
generate C++ code using the C preprocessor.

A trivial way to put this into standardeze would be to add a translation 
phase after macro replacement that would convert these alternative tokens to 
their usual representation.

-Vesa Karvonen

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