[boost] regex and g++ 3.4.2 problem

From: Maciej Sobczak (prog_at_[hidden])
Date: 2004-11-28 16:49:07

Hi,

$ g++ --version
g++ (GCC) 3.4.2 [FreeBSD] 20040728

(boost version 1.32)

When compiling a very simple example for the regex library, I've got the 
following error:

In file included from 
/usr/local/include/boost_1_32_0/boost/regex/v4/regex.hpp:99,
                  from /usr/local/include/boost_1_32_0/boost/regex.hpp:34,
                  from regex.cc:1:
/usr/local/include/boost_1_32_0/boost/regex/v4/instances.hpp:71: error: 
ISO C++ forbids the use of `extern' on explicit instantiations
*** Error code 1

A short investigation points to the following code in the instances.hpp 
file:

#elif (defined(BOOST_MSVC) && defined(_MSC_EXTENSIONS)) || defined(__GNUC__)

#  ifndef BOOST_REGEX_INSTANTIATE
#     define template extern template       // (*)
#  endif

#  ifdef BOOST_MSVC
#     pragma warning(push)
#     pragma warning(disable : 4251 4231 4660)
#  endif

template class BOOST_REGEX_DECL reg_expression< BOOST_REGEX_CHAR_T >;

IMHO (and rather guessing), the #define in (*) is performed 
unnecessarily for g++ 3.4.2, but is executed just because of too generic 
__GNUC__ a little bit higher.

As a temporary solution, I have just uncommented the following line in 
boost/regex/user.hpp:

#define BOOST_REGEX_NO_EXTERNAL_TEMPLATES

Now the simple program compiles and works fine. What influence will the 
above change in user.hpp have on bigger programs?

Is this a known problem?

-- 
Maciej Sobczak : http://www.msobczak.com/
Programming    : http://www.msobczak.com/prog/