# Poco

# class TextEncoding

Library: Foundation
Package: Text
Header: Poco/TextEncoding.h

## Description

An abstract base class for implementing text encodings like UTF-8 or ISO 8859-1.

Subclasses must override the canonicalName(), isA(), characterMap() and convert() methods and need to be thread safe and stateless.

TextEncoding also provides static member functions for managing mappings from encoding names to TextEncoding objects.

## Inheritance

Known Derived Classes: DoubleByteEncoding, ISO8859_10Encoding, ISO8859_11Encoding, ISO8859_13Encoding, ISO8859_14Encoding, ISO8859_16Encoding, ISO8859_3Encoding, ISO8859_4Encoding, ISO8859_5Encoding, ISO8859_6Encoding, ISO8859_7Encoding, ISO8859_8Encoding, ISO8859_9Encoding, MacCentralEurRomanEncoding, MacChineseSimpEncoding, MacChineseTradEncoding, MacCyrillicEncoding, MacJapaneseEncoding, MacKoreanEncoding, MacRomanEncoding, Windows1253Encoding, Windows1254Encoding, Windows1255Encoding, Windows1256Encoding, Windows1257Encoding, Windows1258Encoding, Windows874Encoding, Windows932Encoding, Windows936Encoding, Windows949Encoding, Windows950Encoding, ASCIIEncoding, Latin1Encoding, Latin2Encoding, Latin9Encoding, UTF16Encoding, UTF32Encoding, UTF8Encoding, Windows1250Encoding, Windows1251Encoding, Windows1252Encoding

## Member Summary

Member Functions: add, byName, canonicalName, characterMap, convert, find, global, isA, manager, queryConvert, remove, sequenceLength

## Types Aliases

### CharacterMap

using CharacterMap = int[256];

The map[b] member gives information about byte sequences whose first byte is b. If map[b] is c where c is >= 0, then b by itself encodes the Unicode scalar value c. If map[b] is -1, then the byte sequence is malformed. If map[b] is -n, where n >= 2, then b is the first byte of an n-byte sequence that encodes a single Unicode scalar value. Byte sequences up to 4 bytes in length are supported.

### Ptr

using Ptr = SharedPtr < TextEncoding >;

## Enumerations

### Anonymous

MAX_SEQUENCE_LENGTH = 4

The maximum character byte sequence length supported.

## Destructor

### ~TextEncoding

virtual ~TextEncoding();

Destroys the encoding.

## Member Functions

### add

static void add(
TextEncoding::Ptr encoding
);

Adds the given TextEncoding to the table of text encodings, under the encoding's canonical name.

If an encoding with the given name is already registered, it is replaced.

### add

static void add(
TextEncoding::Ptr encoding,
const std::string & name
);

Adds the given TextEncoding to the table of text encodings, under the given name.

If an encoding with the given name is already registered, it is replaced.

### byName

static TextEncoding & byName(
const std::string & encodingName
);

Returns the TextEncoding object for the given encoding name.

Throws a NotFoundException if the encoding with given name is not available.

### canonicalName

virtual const char * canonicalName() const = 0;

Returns the canonical name of this encoding, e.g. "ISO-8859-1". Encoding name comparisons are case insensitive.

### characterMap

virtual const CharacterMap & characterMap() const = 0;

Returns the CharacterMap for the encoding. The CharacterMap should be kept in a static member. As characterMap() can be called frequently, it should be implemented in such a way that it just returns a static map. If the map is built at runtime, this should be done in the constructor.

### convert

virtual int convert(
const unsigned char * bytes
) const;

The convert function is used to convert multibyte sequences; bytes will point to a byte sequence of n bytes where sequenceLength(bytes, length) == -n, with length >= n.

The convert function must return the Unicode scalar value represented by this byte sequence or -1 if the byte sequence is malformed.

The default implementation returns (int) bytes[0].

### convert

virtual int convert(
int ch,
unsigned char * bytes,
int length
) const;

Transform the Unicode character ch into the encoding's byte sequence. The method returns the number of bytes used. The method must not use more than length characters. Bytes and length can also be null - in this case only the number of bytes required to represent ch is returned. If the character cannot be converted, 0 is returned and the byte sequence remains unchanged. The default implementation simply returns 0.

### find

static TextEncoding::Ptr find(
const std::string & encodingName
);

Returns a pointer to the TextEncoding object for the given encodingName, or nullptr if no such TextEncoding object exists.

### global

static TextEncoding::Ptr global(
TextEncoding::Ptr encoding
);

Sets global TextEncoding object.

This function sets the global encoding to the argument and returns a reference of the previous global encoding.

### global

static TextEncoding & global();

Return the current global TextEncoding object

### isA

virtual bool isA(
const std::string & encodingName
) const = 0;

Returns true if the given name is one of the names of this encoding. For example, the "ISO-8859-1" encoding is also known as "Latin-1".

Encoding name comparisons are case insensitive.

### queryConvert

virtual int queryConvert(
const unsigned char * bytes,
int length
) const;

The queryConvert function is used to convert single byte characters or multibyte sequences; bytes will point to a byte sequence of length bytes.

The queryConvert function must return the Unicode scalar value represented by this byte sequence or -1 if the byte sequence is malformed or -n where n is number of bytes requested for the sequence, if length is shorter than the sequence. The length of the sequence might not be determined by the first byte, in which case the conversion becomes an iterative process: First call with length == 1 might return -2, Then a second call with length == 2 might return -4 Eventually, the third call with length == 4 should return either a Unicode scalar value, or -1 if the byte sequence is malformed.

The default implementation returns (int) bytes[0].

### remove

static void remove(
const std::string & encodingName
);

Removes the encoding with the given name from the table of text encodings.

### sequenceLength

virtual int sequenceLength(
const unsigned char * bytes,
int length
) const;

The sequenceLength function is used to get the lenth of the sequence pointed by bytes. The length parameter should be greater or equal to the length of the sequence.

The sequenceLength function must return the length of the sequence represented by this byte sequence or a negative value -n if length is shorter than the sequence, where n is the number of byte requested to determine the length of the sequence. The length of the sequence might not be determined by the first byte, in which case the conversion becomes an iterative process as long as the result is negative: First call with length == 1 might return -2, Then a second call with length == 2 might return -4 Eventually, the third call with length == 4 should return 4. The default implementation returns 1.

### manager

static TextEncodingManager & manager();

Returns the TextEncodingManager.

## Variables

### GLOBAL

static const std::string GLOBAL;

Name of the global TextEncoding, which is the empty string.