为std::streampos保证是unsigned long long？
std::streampos
unsigned long long
如果不是这样,如何std::istream::seekg在大于4G的文件上正常工作？
std::istream::seekg
c++ types integer file unsigned-long-long-int
c++ ×1
file ×1
integer ×1
types ×1
unsigned-long-long-int ×1