30 Input/output library [input.output]

30.5 Iostreams base classes [iostreams.base]

30.5.2 Types [stream.types]

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.290

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.291

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Typically long long.

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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).