---
name: "Graph::Reader"
section: "3pm"
platform: "linux"
package: "libgraph-readwrite-perl"
version: "2.10-1"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/Graph::Reader"
---

# Graph::Reader(3pm)

base class for Graph file format readers

## NAME

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

## SYNOPSIS

```
  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
  }
```

## DESCRIPTION

**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.

## METHODS

### 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.

## SUBCLASSING

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');
```

## SEE ALSO

- 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.

## REPOSITORY

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

## AUTHOR

Neil Bowers \<neil@bowers.com\>

## COPYRIGHT

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.