30 Input/output library [input.output]

30.5 Iostreams base classes [iostreams.base]

30.5.2 Types [stream.types]

using streamoff = implementation-defined;
The type streamoff is a synonym for one of the signed basic integral types of sufficient size to represent the maximum possible file size for the operating system.291
using streamsize = implementation-defined;
The type streamsize is a synonym for one of the signed basic integral types.
It is used to represent the number of characters transferred in an I/O operation, or the size of I/O buffers.292
Typically long long.
streamsize is used in most places where ISO C would use size_­t.
Most of the uses of streamsize could use size_­t, except for the strstreambuf constructors, which require negative values.
It should probably be the signed type corresponding to size_­t (which is what Posix.
2 calls ssize_­t).