NAME

Encode::Encoding - Encode Implementation Base Class

SYNOPSIS

  package Encode::MyEncoding;
  use base qw(Encode::Encoding);

  __PACKAGE__->Define(qw(myCanonical myAlias));

DESCRIPTION

As mentioned in Encode, encodings are (in the current implementation at least) defined by objects. The mapping of encoding name to object is via the %encodings hash.

The values of the hash can currently be either strings or objects. The string form may go away in the future. The string form occurs when encodings() has scanned @INC for loadable encodings but has not actually loaded the encoding in question. This is because the current "loading" process is all Perl and a bit slow.

Once an encoding is loaded, the value of the hash is the object which implements the encoding. The object should provide the following interface:

->name

MUST return the string representing the canonical name of the encoding.

->new_sequence

This is a placeholder for encodings with state. It should return an object which implements this interface. All current implementations return the original object.

->encode($string,$check)

MUST return the octet sequence representing $string.

• If $check is true, it SHOULD modify $string in place to remove the converted part (i.e. the whole string unless there is an error). If perlio_ok() is true, SHOULD becomes MUST.

• If an error occurs, it SHOULD return the octet sequence for the fragment of string that has been converted and modify $string in-place to remove the converted part leaving it starting with the problem fragment. If perlio_ok() is true, SHOULD becomes MUST.

• If $check is is false then encode MUST make a "best effort" to convert the string - for example, by using a replacement character.

->decode($octets,$check)

MUST return the string that $octets represents.

• If $check is true, it SHOULD modify $octets in place to remove the converted part (i.e. the whole sequence unless there is an error). If perlio_ok() is true, SHOULD becomes MUST.

• If an error occurs, it SHOULD return the fragment of string that has been converted and modify $octets in-place to remove the converted part leaving it starting with the problem fragment. If perlio_ok() is true, SHOULD becomes MUST.

• If $check is false then decode should make a "best effort" to convert the string - for example by using Unicode's "\x{FFFD}" as a replacement character.

->perlio_ok()

If you want your encoding to work with PerlIO, you MUST define this method so that it returns 1 when PerlIO is enabled. Here is an example;

 sub perlio_ok { exists $INC{"PerlIO/encoding.pm"} }

By default, this method is defined as follows;

 sub perlio_ok { 0 }

->needs_lines()

If your encoding can work with PerlIO but needs line buffering, you MUST define this method so it returns true. 7bit ISO-2022 encodings are one example that needs this. When this method is missing, false is assumed.

It should be noted that the $check behaviour is different from the outer public API. The logic is that the "unchecked" case is useful when the encoding is part of a stream which may be reporting errors (e.g. STDERR). In such cases, it is desirable to get everything through somehow without causing additional errors which obscure the original one. Also, the encoding is best placed to know what the correct replacement character is, so if that is the desired behaviour then letting low level code do it is the most efficient.

By contrast, if $check is true, the scheme above allows the encoding to do as much as it can and tell the layer above how much that was. What is lacking at present is a mechanism to report what went wrong. The most likely interface will be an additional method call to the object, or perhaps (to avoid forcing per-stream objects on otherwise stateless encodings) an additional parameter.

It is also highly desirable that encoding classes inherit from Encode::Encoding as a base class. This allows that class to define additional behaviour for all encoding objects. For example, built-in Unicode, UCS-2, and UTF-8 classes use

  package Encode::MyEncoding;
  use base qw(Encode::Encoding);

  __PACKAGE__->Define(qw(myCanonical myAlias));

to create an object with bless {Name => ...}, $class, and call define_encoding. They inherit their name method from Encode::Encoding.

Compiled Encodings

For the sake of speed and efficiency, most of the encodings are now supported via a compiled form: XS modules generated from UCM files. Encode provides the enc2xs tool to achieve that. Please see enc2xs for more details.

SEE ALSO

perlmod, enc2xs