concatenation does not work with BOOST_PP_COMMA() with g++

[mkraemer@…]

The concatenation of a token with a comma does not work with g++

Example:

cmd>cat test.cpp
#include <boost/preprocessor/cat.hpp>
#include <boost/preprocessor/punctuation/comma.hpp>
BOOST_PP_CAT(test,BOOST_PP_COMMA())

g++ v3.4.4

cmd>g++-3 --version
g++-3 (GCC) 3.4.4 (cygming special, gdc 0.12, using dmd 0.125)

cmd>g++-3 -E -P -I D:\Development\boost\1.40.0 test.cpp
test.cpp:3:35: macro "BOOST_PP_CAT_I" passed 3 arguments, but takes just 2
BOOST_PP_CAT_I;

g++ 4.3.2

cmd>g++-4 --version
g++-4 (GCC) 4.3.2 20080827 (beta) 2
cmd>g++-4 -E -P -I D:\Development\boost\1.40.0 test.cpp
test.cpp:3:35: error: macro "BOOST_PP_CAT_I" passed 3 arguments, but takes just 2
BOOST_PP_CAT_I;

Wheras MS VisualStudio 2005 works

cmd>cl /E -I D:\Development\boost\1.40.0 test.cpp
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 14.00.50727.762 for 80x86
Copyright (C) Microsoft Corporation.  All rights reserved.

...

#line 3 "test6.cpp"
test,

[Steven Watanabe]

I don't think that BOOST_PP_COMMA is intended to be used in this way. The example given in the documentation is

BOOST_PP_IF(1, BOOST_PP_COMMA, BOOST_PP_EMPTY)()

which does not depend on the order of macro expansion.

[Daniel James]

You can't concatenate with a comma in the C preprocessor since concatenation has to create a single token and test, is two tokens. The Visual C++ preprocessor is not following the standard when it allows it. There's not really any point in concatenating test and BOOST_PP_COMMA, since you can just write:

test BOOST_PP_COMMA()