Re: [boost] [mpl] annoying trivial bug in boost/mpl/print.hpp

From: Ian McCulloch (ianmcc_at_[hidden])
Date: 2006-10-20 08:28:51

Ian McCulloch wrote:

> 
> Hi,
> 
> In boost 1.33.1, using MPL with gcc -pedantic gives an annoying error,
> boost/mpl/print.hpp:62: error: comma at end of enumerator list
> The trivial fix is to remove the comma at the end of line 60 of that file,
> which I did for my downloaded copy of boost ages ago, then promptly forgot
> about it.  But I just checked the CVS and it seems that this bug is still
> present in both HEAD and RC_1_34_0.  It would be great if this could be
> fixed before 1.34.

To followup: this bug seems to only affect gcc 3.3.3, gcc 3.3.5 and later (I
also tested 3.4.5) are NOT affected.  

Test case:

#include <boost/mpl/print.hpp>
int main() {}

With GCC 3.3.3:

ianmcc_at_master:~> g++ --version
g++ (GCC) 3.3.3 (SuSE Linux)
Copyright (C) 2003 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

ianmcc_at_master:~> g++ -pedantic -I. test.cpp
In file included from test.cpp:1:
boost/mpl/print.hpp:62: error: comma at end of enumerator list

With GCC 3.3.5:

ianmcc_at_lxtc226 ~ $ g++ --version
g++ (GCC) 3.3.4 (pre 3.3.5 20040809)
Copyright (C) 2003 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

ianmcc_at_lxtc226 ~ $ g++ -pedantic test.cpp
ianmcc_at_lxtc226 ~ $ ./a.out

Nevertheless, C++98 appears to not allow a comma at the end of an
enumerator-list.  From A.6,

enum-specifier:
   enum itentifier_opt { enumerator-list_opt }

enumerator-list:
   enumerator-definition
   enumerator-list  ,  enumerator-definition

So it seems that the additional comma in the enumerator in
boost/mpl/print.hpp is technically not allowed, but for whatever reason gcc
removed the diagnostic sometime around version 3.3.4 or 3.3.5.  I only have
a 1997 working draft of the C standard but that could reveal why: it seems
that C allows the extra comma.  From 6.5.2.2:

enum-specified:
   enum identifier_opt { enumerator-list }
   enum identifier_opt { enumerator-list ,  }
   enum identifier

Although it seems to only affect one compiler, and then only in -pedantic
mode, it is a trivial patch and the current code does seem to be
non-standard c++.  Any reason not to apply?

--- print.hpp.old       2006-10-20 14:16:26.000000000 +0200
+++ print.hpp   2006-10-20 14:16:41.000000000 +0200
@@ -57,7 +57,7 @@
 # if defined(__EDG_VERSION__)
            aux::dependent_unsigned<T>::value > -1
 # else
-           sizeof(T) > -1,
+           sizeof(T) > -1
 # endif
         };
 #endif

Regards,
Ian McCulloch