# Device

## Description

A Device provides access to one or two character sequences. It may provide access to an input sequence, for reading, an output sequence, for writing, or both. The relationship between the two sequences, as well as the operations that may be performed on them, depends on the Device type.

Typically, Devices are class types which define one or more member functions `read`, `write` and `seek`. The concept Device is defined a bit more broadly, however, so that pre-existing types, such as standard streams and stream buffers, can be treated as models of Device, and so that Devices managing in-memory sequences can be optimized.

Each Device type has an associated character type and category. The character type is the type of the characters in the sequence or sequences controlled by instances of the Device. The category is a tag structure which the Iostreams library relies on to determine which operations the Device supports. Its function is similar to the `iterator_category` member of `std::iterator_traits`.[1]

There is one refinement of Device for each of the eight modes, and for each such refinement there is a corresponding refinement of Filter. In order to express this corresponce cleanly, it is helpful to include the requirements of the various refinements of Device in the definition of Device itself, qualified by category. The various refinements of Device can then be characterized exactly by the following definitions. For convenience, the requirements of the four most common Device refinements are also documented individually.

Concept | Definition
--- | ---
Source | Refinement of Device with mode convertible to `input`
Sink | Refinement of Device with mode convertible to `output`
BidirectionalDevice | Refinement of Device with mode convertible to `bidirectional`
SeekableDevice | Refinement of Device with mode convertible to `seekable`
SeekableSource | Refinement of Device with mode convertible to `input_seekable`
SeekableSink | Refinement of Device with mode convertible to `output_seekable`
BidirectionalSeekableDevice | Refinement of Device with mode convertible to `bidirectional_seekable`
DualSeekableDevice | Refinement of Device with mode convertible to `dual_seekable`

## Note

To be usable with the streams and stream buffers provided by the Boost Iostreams library, Devices must model Blocking.

## Standard Streams and Stream Buffers

Standard streams and stream buffers are models of Device. It is not possible to tell the exact mode of an arbitrary stream or stream buffer, so the Iostreams library must make an educated guess. (See Class Template `mode_of`.) Since this guess may not accurately reflect the behavior of a particular stream or stream buffer, the user is allowed some flexibility when adding a stream or stream buffer to a filter chain. In such cases, it is up to the user to ensure that the behavior of the stream or stream buffer is consistent with the mode of the chain.

## Refinement of

## Associated Types

Character type | The type of the characters in the controlled sequences
Category | A type convertible to `device_tag` and to a unique most-derived mode tag
Mode | The unique most-derived mode tag to which Category is convertible

## Notation

`D` | - A type which is a model of Device
`Ch` | - The character type of `D`
`dev` | - Object of type `D`
`s1` | - Object of type `Ch*`
`s2` | - Object of type `const Ch*`
`n` | - Object of type `std::streamsize`
`off` | - Object of type `stream_offset`
`way` | - Object of type `std::ios_base::seekdir`
`which` | - Object of type `std::ios_base::openmode`
`io` | - Alias for namespace `boost::iostreams`

## Valid Expressions — Typenames

Expression | Expression Type
--- | ---
`typename char_type_of<D>::type` | `typename` of the character type
`typename category_of<D>::type` | `typename` of the category

## Valid Expressions / Semantics — Input

Expression | Expression Type | Category Precondition | Semantics
--- | --- | --- | ---
    io::read(dev, s1, n)
| `std::streamsize` | Convertible to `input` but not to `direct_tag` | Reads up to `n` characters from the sequence controlled by `dev` into `s1`, returning the number of characters read, or `-1` to indicate end-of-sequence
    dev.input_sequence()
    std::pair<Ch*,Ch*>
| Convertible to `input` and to `direct_tag` | Returns a pair of pointers delimiting the sequence controlled by `dev`

## Valid Expressions / Semantics — Output

Expression | Expression Type | Category Precondition | Semantics
--- | --- | --- | ---
    io::write(dev, s2, n)
| `std::streamsize` | Convertible to `output` but not to `direct_tag` | Writes up to `n` characters from the sequence beginning at `s2` to the sequence controlled by `dev`, returning the number of characters written
    dev.output_sequence()
    std::pair<Ch*,Ch*>
| Convertible to `output` and to `direct_tag` | Returns a pair of pointers delimiting the sequence controlled by `dev`

## Valid Expressions / Semantics — Random-Access

Expression | Expression Type | Category Precondition | Semantics
--- | --- | --- | ---
    io::seek(dev, off, way)
| `std::streampos` | Convertible to `seekable` but not to `direct_tag` | Advances the read/write head by `off` characters, returning the new position, where the offset is calculated from:

- the start of the sequence if `way` is `ios_base::beg`
- the current position if `way` is `ios_base::cur`
- the end of the sequence if `way` is `ios_base::end`

    io::seek(dev, off, way, which)
| `std::streampos` | Convertible to `dual_seekable` or `bidirectional_seekable` but not to `direct_tag` | Advances the read head (if `which` is `std::ios_base::in`), the write head (if `which` is `std::ios_base::out`) or both heads (if `which` is `std::ios_base::in | std::ios_base::out`) by `off` characters, returning the new position, where the offset is calculated from

- the start of the sequence if `way` is `ios_base::beg`
- the current position if `way` is `ios_base::cur`
- the end of the sequence if `way` is `ios_base::end`

The result is undefined if `way` is `ios_base::cur` and `which` is `(std::ios_base::in | std::ios_base::out)`.

## Exceptions

Errors which occur during the execution of `read`, `write`, `seek`, `input_sequence` or `output_sequence` are indicated by throwing exceptions. Reaching the end of the input sequence is not an error, but attempting to write past the end of the output sequence is.

After an exception is thrown, a Device must be in a consistent state; further i/o operations may throw exceptions but must have well-defined behaviour.

## Models

See Source, Sink, BidirectionalDevice and SeekableDevice.

[1][ISO], 24.3.1. See Tag Dispatching for a discussion.

Revised 20 May, 2004

© Copyright Jonathan Turkanis, 2004

Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)