Filter

Description

A Filter operates on the character sequence or sequences controlled by a Device, providing access to a filtered input sequence, output sequence or both. Informally, when a Filter f is used in conjunction with a Device d, data read from the input sequence of d is processed by f before being returned to the user, data written to the output sequence of d is first processed by f, and repositioning operations are processed by f before being conveyed to d.

Filters are class types which define one or more member functions get, put, read, write and seek; each function takes a reference to a Device as its first argument. Whenever a Filter is used to perform an i/o operation, a reference to the Device being filtered is passed to the Filter as a function argument.

Each Filter type has an associated character type and category. The character type is the type of the characters in the input and output sequences. The category is a tag structure which the Iostreams library relies on to determine which operations the Filter type supports. Its function is similar to the iterator_category member of std::iterator_traits.[1]

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

ConceptDefinition
InputFilter Refinement of Filter with mode convertible to input
OutputFilter Refinement of Filter with mode convertible to output
BidirectionalFilter Refinement of Filter with mode convertible to bidirectional
SeekableFilter Refinement of Filter with mode convertible to seekable
InputSeekableFilter Refinement of Filter with mode convertible to input_seekable
OutputSeekableFilter Refinement of Filter with mode convertible to output_seekable
BidirectionalSeekableFilter Refinement of Filter with mode convertible to bidirectional_seekable
DualSeekableFilter Refinement of Filter with mode convertible to dual_seekable

Refinement of

Associated Types

Character typeThe type of the characters in the filtered sequences
Category A type convertible to filter_tag and to a unique most-derived mode tag
Mode The unique most-derived mode tag to which Category is convertible

Notation

F- A type which is a model of Filter
D- A type which is a model of Device, with the same character type as F and with mode refining the mode of F
Ch- The character type of F
Tr- boost::iostreams::char_traits<Ch>
f- Object of type F
d- Object of type D
c- Object of type Ch
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

ExpressionExpression Type
typename char_type_of<F>::type typename of the character type
typename category_of<F>::type typename of the category

Valid Expressions / Semantics — Input

ExpressionExpression TypeCategory PreconditionSemantics
f.get(d)
Tr::int_type Convertible to input but not to multichar_tag Returns the next character in the input sequence controlled by f, Tr::eof() if the end of the sequence has been reached or Tr::would_block() if input is temporarily unavilable because a call to d has produced fewer characters than requested. The input sequence controlled by d may be accessed using io::get, io::read and io::putback.
f.read(d, s1, n)
std::streamsize
Convertible to input and to multichar_tag Reads up to n characters from the input sequence controlled by f into the buffer s1, returning the number of characters read or -1 to indicate end-of-sequence. A value less than n may be returned only at end-of-sequence or if input is temporarily unavilable because a call to d has produced fewer characters than requested. The input sequence controlled by d may be accessed using io::get, io::read and io::putback.

Valid Expressions / Semantics — Output

ExpressionExpression TypeCategory PreconditionSemantics
f.put(d, c)
bool Convertible to output but not to multichar_tag Attempts to writes the character c to the output sequence controlled by f, returning false if c cannot be consumed because a call to d has consumed fewer characters than requested. The output sequence controlled by d may be accessed using io::put and io::write.
f.write(d, s2, n)
std::streamsize
Convertible to output and to multichar_tag Writes up to n characters from the buffer s2 to the output sequence controlled by d, returning the number of characters written. A value less than n may be returned only if a call to d has consumed fewer characters than requested. The output sequence controlled by d may be accessed using io::put and io::write.

Valid Expressions / Semantics — Random-Access

ExpressionExpression TypeCategory PreconditionSemantics
f.seek(d, 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

The input sequence controlled by d may be accessed using io::seek and using io::write if the mode of F is convertible to output.

f.seek(d, 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 input sequence controlled by d may be accessed using io::seek, and using io::write if the mode of F is convertible to output.

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 get, put, read, write or seek 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 Filter must be in a consistent state; further i/o operations may throw exceptions but must have well-defined behaviour. Furthermore, unless it is Closable, it must be ready to begin processing a new character sequence.

Models

See InputFilter, OutputFilter, BidirectionalFilter and SeekableFilter.

Acknowledgments

The concept Filter was inspired by the inserters and extractors of [Kanze].


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