Graph::Reader - base class for Graph file format readers

       Neil Bowers <neil@bowers.com>

       Copyright (c) 2001-2012, Neil Bowers. All rights reserved.  Copyright (c)  2001,  Canon  Research  Centre
       Europe. All rights reserved.

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

perl v5.32.1                                       2021-09-19                                 Graph::Reader(3pm)

Graph::Reader is a base class for Graph file format readers.  A particular subclass of Graph::Reader will
       handle a specific file format, and generate a Graph, represented using Jarkko Hietaniemi's Graph class.

       You should never create an instance of this class yourself, it is only meant for subclassing. If you try
       to create an instance of Graph::Reader, the constructor will throw an exception.

new()
       Constructor - generate a new reader instance. This is a virtual method, or whatever the correct lingo is.
       You're not meant to call this on the base class, it is inherited by the subclasses. Ie if you do
       something like:

         $reader = Graph::Reader->new();

       It will throw an exception.

   read_graph()
       Read a graph from the specified file:

         $graph = $reader->read_graph($file);

       The $file argument can either be a filename, or a filehandle for a previously opened file.

       Graph::Reader - base class for Graph file format readers

       <https://github.com/neilb/Graph-ReadWrite>

       Graph
           Jarkko   Hietaniemi's   modules   for   representing  directed  graphs,  available  from  CPAN  under
           modules/by-module/Graph/

       Algorithms in Perl
           This O'Reilly book has a chapter on directed graphs, which is based around Jarkko's modules.

       Graph::Reader::XML
           A simple subclass of this class for reading a simple XML format for directed graphs.

       Graph::Writer
           A baseclass for Graph file format writers.

       To create your own graph format reader, create a module which subclasses Graph::Reader. For example,
       suppose DGF is a directed graph format - create a Graph::Reader::DGF module, with the following
       structure:

         package Graph::Reader::DGF;

         use Graph::Reader;
         use vars qw(@ISA);
         @ISA = qw(Graph::Reader);

         sub _read_graph
         {
           my $self  = shift;
           my $graph = shift;
           my $FILE  = shift;

           while (<$FILE>)
           {
           }

           return 1;
         }

         1;

       Note the leading underscore on the _read_graph() method.  The base class provides the public method, and
       invokes the private method which you're expected to provide, as above.

       If you want to perform additional initialisation at construction time, you can provide an _init() method,
       which will be invoked by the base class's constructor.  You should invoke the superclass's initialiser as
       well, as follows:

         sub _init
         {
           my $self = shift;

           $self->SUPER::_init();

           # your initialisation here
         }

       Someone can then use your class as follows:

         use Graph::Reader::DGF;

         $reader = Graph::Reader::DGF->new();
         $graph = $reader->read_graph('foo.dgf');

         package Graph::Reader::MyFormat;
         use Graph::Reader;
         use vars qw(@ISA);
         @ISA = qw(Graph::Reader);

         sub _read_graph
         {
           my ($self, $graph, $FILE) = @_;

           # read $FILE and populate $graph
         }