BidirectionalDevice

Definition

An BidirectionalDevice is a Device whose mode refines bidirectional.

Description

An BidirectionalDevice provides read-access to a sequence of characters of a given type and write-access to a separate sequence of characters of the same type. An BidirectionalDevice may expose these sequences in three ways:^[1]

by defining member functions read and write , invoked indirectly by the Iostreams Library through the functions boost::iostreams::read and boost::iostreams::write;
by overloading or specializing boost::iostreams::read and boost::iostreams::write; or
by defining member functions input_sequence and output_sequence returning pairs of pointers delimiting the two sequences in their entirety.

The i/o mode of a BidirectionalDevice is bidirectional or bidirectional-seekable.

Example

A model of BidirectionalDevice can be defined as follows:

struct BidirectionalDevice {
    typedef char                char_type;
    typedef bidirectional_device_tag    category;
    std::streamsize read(char* s, std::streamsize n) 
        {
            // Reads up to n characters from the input 
            // sequence into the buffer s, returning the number 
            // of characters read. Returning a value less than n 
            // indicates end-of-sequence.
        }
    std::streamsize write(const char* s, std::streamsize n) 
        {
            // Write up to n characters from the buffer
            // s to the output sequence, returning the 
            // number of characters written
        }
};

Here category is a tag struct identifying the containing type as a model of BidirectionalDevice. When defining a new BidirectionalDevice, it suffices to use the tag bidirectional_device_tag. One can also derive from the helper classes device<bidirectional> or wdevice<bidirectional>.

Refinement of

Source, Sink.

Associated Types

Same as Device, with the following additional requirements:

Category A type convertible to device_tag and to bidirectional

Notation

`D`	- A type which is a model of BidirectionalDevice
`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`

Valid Expressions / Semantics

Same as Device, with the following additional requirements:

Expression	Expression Type	Category Precondition	Semantics
`boost::iostreams::read(dev, s1, n)`	`std::streamsize`	Not convertible to `direct_tag`	Reads up to `n` characters from the input sequence controlled by `dev` into `s1`, returning the number of characters read; returning a value less than n indicates end-of-sequence
`boost::iostreams::write(dev, s2, n)`	`std::streamsize`	Not convertible 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.input_sequence()`	`std::pair<Ch,Ch>`	Convertible to `direct_tag`	Returns a pair of pointers delimiting the input sequence controlled by `dev`
`dev.output_sequence()`	`std::pair<Ch,Ch>`	Convertible to `direct_tag`	Returns a pair of pointers delimiting the output sequence controlled by `dev`

Exceptions

Errors which occur during the execution of member functions read, write, 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, an BidirectionalDevice must be in a consistent state; further i/o operations may throw exceptions but must have well-defined behaviour.

Models

^[1]Strictly speaking, (i) and (ii) can be varied independently for input and output, so there are actually five possibilities.

Revised 02 Feb 2008

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)