In the C++ Standard Library the value std::numeric_limits<T>::max() is specified as a function. Further properties of a specific type are given as constants (likestd::numeric_limits<T>::is_signed). All constants that are of type T are given as functions, whereas all other constants are given as, well, constant values.
What's the rationale behind that?
To expand on Neil's remark, std::numeric_limit<T> is available for any number type including floating point numbers, and if you dig through the comp.lang.c++ thread, you'll see the mention that it might not be possible to define the static variables for floating point values.
So, for consistency they decided to put both integral and floating points behind methods.
It will change with C++0x, so there's hope.
来源：https://stackoverflow.com/questions/2766272/why-is-stdnumeric-limitstmax-a-function