31 Input/output library [input.output]

31.2 Iostreams requirements [iostreams.requirements]

31.2.2 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.271
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.272
271)271)
Typically long long.
272)272)
Most places where streamsize is used would use size_­t in ISO C, or ssize_­t in POSIX.