31 Input/output library [input.output]

31.2 Iostreams requirements [iostreams.requirements]

31.2.2 Types [stream.types]

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using streamoff = implementation-defined;
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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.265
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using streamsize = implementation-defined;
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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.266
265)265)
Typically long long.
266)266)
Most places where streamsize is used would use size_t in ISO C, or ssize_t in POSIX.