what's wrong with lexical_cast? it would put the requirement of operator<< on the number type T
Actually my concern about using an operator<< was about performance, but on second thought there would be no performance hit doing like that, I think I was confusing with another use case of streaming operators. I will revise that, and will be happy to get back to lexical_cast if there's no problem.
Also take a look at the machinery we defined in boost/math/tools/constants.hpp, usage is a little clunky, for example: BOOST_DEFINE_MATH_CONSTANT(name, digits, extra-digits, exponent); after which you can use name<T>() to refer to the constant. Built in types simply return an appropriately defined numeric constant (so no lexical_cast overhead), while for UDT's the constant is defined as a string containing which gets lexical_cast'ed to the value. The remaining issue we haven't sorted out is thread safety: for UDT's the casted value is stored in a static variable for efficiency reasons, so only the first call results in a lexical_cast, but of course this isn't strictly thread safe (for built in types, there is no such issue). The "extra-digits" argument is required BTW because many compilers reject numeric constants with too many digits precision, so we put enough decimal places in "digits" to ensure 128-bit long double precision, and everything else (used only in extreme UDT cases) in "extra-digits". John.