@#ifndef - what's it doing in Boost sources?

[walter@…]

Looking at the boost file:

http://www.boost.org/doc/libs/1_55_0b1/boost/type_traits/detail/is_mem_fun_pointer_impl.hpp

towards the end, there are these lines:

@#ifndef BOOST_TT_NO_ELLIPSIS_IN_FUNC_TESTING template <class R BOOST_PP_COMMA_IF(BOOST_PP_COUNTER) [...] struct is_function_ptr_helper<R (*)(BOOST_PP_ENUM_PARA [...] @#endif

What are the @ characters doing there? I can't find any reference to @ being an allowable character (at least in the C++98 Standard).

The @ characters also appear in is_function_ptr_helper.hpp

[John Maddock]

I didn't write that part, but from memory, they were chosen to deliberately be invalid code: the code there was used to generate content of the header, and is archived for posterity, but should otherwise never be used.