=encoding utf8

=head1 NAME

Mail::Message::Head::Subset - subset of header information of a message

=head1 INHERITANCE

 Mail::Message::Head::Subset realizes a Mail::Message::Head::Complete

 Mail::Message::Head::Subset
   is a Mail::Message::Head
   is a Mail::Reporter

=head1 SYNOPSIS

  my $subset = Mail::Message::Head::Subset->new(...)
  $subset->isa('Mail::Message::Head')  # true
  $subset->guessBodySize               # integer or undef
  $subset->isDelayed                   # true

=head1 DESCRIPTION

Some types of folders contain an index file which lists a few lines of
information per messages.  Especially when it is costly to read header lines,
the index speeds-up access considerably.  For instance, the subjects of
all messages are often wanted, but waiting for a thousand messages of the
folder to be read may imply a thousand network reads (IMAP) or file
openings (MH)

When you access header fields which are not in the header subset, the whole
header has to be parsed (which may consume considerable time, depending on
the type of folder).

Extends L<"DESCRIPTION" in Mail::Message::Head|Mail::Message::Head/"DESCRIPTION">.

=head1 OVERLOADED

Extends L<"OVERLOADED" in Mail::Message::Head|Mail::Message::Head/"OVERLOADED">.

=over 4

=item overload: B<""> stringifaction

Inherited, see L<Mail::Message::Head/"OVERLOADED">

=item overload: B<bool> true/false

Inherited, see L<Mail::Message::Head/"OVERLOADED">

=back

=head1 METHODS

Extends L<"METHODS" in Mail::Message::Head|Mail::Message::Head/"METHODS">.

=head2 Constructors

Extends L<"Constructors" in Mail::Message::Head|Mail::Message::Head/"Constructors">.

=over 4

=item $class-E<gt>B<build>( [PAIR|$field]-LIST )

Inherited, see L<Mail::Message::Head/"Constructors">

=item $class-E<gt>B<new>(%options)

Inherited, see L<Mail::Message::Head/"Constructors">

=back

=head2 Attributes

Extends L<"Attributes" in Mail::Message::Head|Mail::Message::Head/"Attributes">.

=head2 The header

Extends L<"The header" in Mail::Message::Head|Mail::Message::Head/"The header">.

=over 4

=item $obj-E<gt>B<isDelayed>()

Inherited, see L<Mail::Message::Head/"The header">

=item $obj-E<gt>B<isEmpty>()

Inherited, see L<Mail::Message::Head/"The header">

=item $obj-E<gt>B<isModified>()

Inherited, see L<Mail::Message::Head/"The header">

=item $obj-E<gt>B<knownNames>()

Inherited, see L<Mail::Message::Head/"The header">

=item $obj-E<gt>B<message>( [$message] )

Inherited, see L<Mail::Message::Head/"The header">

=item $obj-E<gt>B<modified>( [BOOLEAN] )

Inherited, see L<Mail::Message::Head/"The header">

=item $obj-E<gt>B<orderedFields>()

Inherited, see L<Mail::Message::Head/"The header">

=back

=head2 Access to the header

Extends L<"Access to the header" in Mail::Message::Head|Mail::Message::Head/"Access to the header">.

=over 4

=item $obj-E<gt>B<count>($name)

Count the number of fields with this C<$name>.  If the C<$name> cannot be found,
the full header get loaded.  In case we find any C<$name> field, it is
decided we know all of them, and loading is not needed.

=item $obj-E<gt>B<get>( $name, [$index] )

Get the data which is related to the field with the C<$name>.  The case of the
characters in C<$name> does not matter.  When a C<$name> is used which is not known
yet, realization will take place.
Improves base, see L<Mail::Message::Head/"Access to the header">

=item $obj-E<gt>B<study>( $name, [$index] )

Inherited, see L<Mail::Message::Head/"Access to the header">

=back

=head2 About the body

Extends L<"About the body" in Mail::Message::Head|Mail::Message::Head/"About the body">.

=over 4

=item $obj-E<gt>B<guessBodySize>()

The body size is defined in the C<Content-Length> field.  However, this
field may not be known.  In that case, a guess is made based on the known
C<Lines> field.  When also that field is not known yet, C<undef> is returned.
Improves base, see L<Mail::Message::Head/"About the body">

=item $obj-E<gt>B<isMultipart>()

Inherited, see L<Mail::Message::Head/"About the body">

=back

=head2 Internals

Extends L<"Internals" in Mail::Message::Head|Mail::Message::Head/"Internals">.

=over 4

=item $obj-E<gt>B<addNoRealize>($field)

Inherited, see L<Mail::Message::Head/"Internals">

=item $obj-E<gt>B<addOrderedFields>($fields)

Inherited, see L<Mail::Message::Head/"Internals">

=item $obj-E<gt>B<fileLocation>()

Inherited, see L<Mail::Message::Head/"Internals">

=item $obj-E<gt>B<load>()

Inherited, see L<Mail::Message::Head/"Internals">

=item $obj-E<gt>B<moveLocation>($distance)

Inherited, see L<Mail::Message::Head/"Internals">

=item $obj-E<gt>B<read>($parser)

Inherited, see L<Mail::Message::Head/"Internals">

=item $obj-E<gt>B<setNoRealize>($field)

Inherited, see L<Mail::Message::Head/"Internals">

=back

=head2 Error handling

Extends L<"Error handling" in Mail::Message::Head|Mail::Message::Head/"Error handling">.

=over 4

=item $obj-E<gt>B<AUTOLOAD>()

Inherited, see L<Mail::Reporter/"Error handling">

=item $obj-E<gt>B<notImplemented>()

Inherited, see L<Mail::Reporter/"Error handling">

=back

=head2 Cleanup

Extends L<"Cleanup" in Mail::Message::Head|Mail::Message::Head/"Cleanup">.

=over 4

=item $obj-E<gt>B<DESTROY>()

Inherited, see L<Mail::Reporter/"Cleanup">

=back

=head1 DETAILS

Extends L<"DETAILS" in Mail::Message::Head|Mail::Message::Head/"DETAILS">.

=head1 DIAGNOSTICS

=over 4

=item Error: class $package does not implement method $method.

Fatal error: the specific C<$package> (or one of its superclasses) does not
implement this method where it should. This message means that some other
related classes do implement this method however the class at hand does
not.  Probably you should investigate this and probably inform the author
of the package.
Cast by C<notImplemented()>

=back

=head1 SEE ALSO

This module is part of Mail-Box version 4.01,
built on December 13, 2025. Website: F<http://perl.overmeer.net/CPAN/>

=head1 LICENSE

For contributors see file ChangeLog.

This software is copyright (c) 2001-2025 by Mark Overmeer.

This is free software; you can redistribute it and/or modify it under
the same terms as the Perl 5 programming language system itself.

