What does implementation-dependant mean?

Implementation dependent means that the standard says nothing about some occurance in C/C++ and leaves the choice up to the people who create compilers and operating systems. It means that what you use on one system/compiler may not work the same way on another, but the behavior is well defined for that particular implementation. For example, whether the result of a right bitwise shift on a signed variable results in a logical or arithmetic shift is implementation defined.

-Prelude