lexical_cast is using math::changesign with long double regardless of whether BOOST_MATH_NO_LONG_DOUBLE_MATH_FUNCTIONS is defined or not. This is causing Boost.Test build problems (see here http://tinyurl.com/af5453h ).
I'm pretty sure this is a bug in lexical_cast, so I've opened a ticket: https://svn.boost.org/trac/boost/ticket/8162
So, portable way of changing sign is just to multiply on -1 ?
Doesn't work for NaN's if you care about that (probably not in this case?)
And math::isnan I shall change to value != value ?
That doesn't always work, actually it quite often doesn't work, especially in release mode.
And math::isinf to something like value >= numeric_limits<>::max?
Make that fabs(value) > numeric_limits<>::max and it should work - unless of course the system has no (reliable) long double math functions in which case fabs(double) may be called rather than a true fabs(long double), and you may get spurious overflow (finite long double treated as non-finite double). Fun isn't it? :-( John.