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Encode::Encoding(3)
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 as objects. The mapping of encoding name to object is via
  the %Encode::Encoding hash.  Though you can directly manipulate this hash,
  it is strongly encouraged to use this base class module and add encode()
  and decode() methods.

  Methods you should implement

  You are strongly encouraged to implement methods below, at least either
  encode() or decode().

  ->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.

  If you want your encoding to work with encoding pragma, you should also
  implement the method below.

  ->cat_decode($destination, $octets, $offset, $terminator [,$check])
      MUST decode $octets with $offset and concatenate it to $destination.
      Decoding will terminate when $terminator (a string) appears in output.
      $offset will be modified to the last $octets position at end of decode.
      Returns true if $terminator appears output, else returns false.

  Other methods defined in Encode::Encodings

  You do not have to override methods shown below unless you have to.

  ->name
      Predefined As:

	sub name  { return shift->{'Name'} }

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

  ->renew
      Predefined As:

	sub renew { return $_[0] }

      This method reconstructs the encoding object if necessary.  If you need
      to store the state during encoding, this is where you clone your
      object.  Here is an example:

	sub renew {
	    my $self = shift;
	    my $clone = bless { %$self } => ref($self);
	    $clone->{clone} = 1; # so the caller can see it
	    return $clone;
	}

      Since most encodings are stateless the default behavior is just return
      itself as shown above.

      PerlIO ALWAYS calls this method to make sure it has its own private
      encoding object.

  ->perlio_ok()
      Predefined As:

	sub perlio_ok {
	    eval{ require PerlIO::encoding };
	    return $@ ? 0 : 1;
	}

      If your encoding does not support PerlIO for some reasons, just;

       sub perlio_ok { 0 }

  ->needs_lines()
      Predefined As:

	sub needs_lines { 0 };

      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.

  Example: Encode::ROT13

    package Encode::ROT13;
    use strict;
    use base qw(Encode::Encoding);

    __PACKAGE__->Define('rot13');

    sub encode($$;$){
	my ($obj, $str, $chk) = @_;
	$str =~ tr/A-Za-z/N-ZA-Mn-za-m/;
	$_[1] = '' if $chk; # this is what in-place edit means
	return $str;
    }

    # Jr pna or ynml yvxr guvf;
    *decode = <!>encode;

    1;

Why the heck Encode API is different?
  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.

    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

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