问题
Why doesn't C++ support unsigned double syntax?
回答1:
Because typical floating point formats don't support unsigned numbers. See, for instance, this list of IEEE 754 formats.
Adding a numerical format that isn't supported by common hardware just makes life difficult for compiler writers, and is probably not considered worth the effort.
回答2:
C++ doesn't support unsigned floating point types because most floating point hardware doesn't support unsigned floating point types. Some graphics cards do work with unsigned floating point, but it's generally internal, not really visible to a program or user.
回答3:
Unsigned integers gain an extra bit of precision and have slightly different bit-wise semantics to signed integers. Floats and doubles always reserve a bit for the sign (on most hardware) and have no bit-wise semantics, so there's no real benefit in having an unsigned real type.
来源：https://stackoverflow.com/questions/2523025/unsigned-double-in-c