Exception::Class - A module that allows you to declare real exception classes in Perl


Exception::Class - A module that allows you to declare real exception classes in Perl


  use Exception::Class
      ( 'MyException',
        'AnotherException' =>
        { isa => 'MyException' },
        'YetAnotherException' =>
        { isa => 'AnotherException',
          description => 'These exceptions are related to IPC' },
        'ExceptionWithFields' =>
        { isa => 'YetAnotherException',
          fields => [ 'grandiosity', 'quixotic' ],
          alias => 'throw_fields',
  # try
  eval { MyException->throw( error => 'I feel funny.' ) };
  my $e;
  # catch
  if ( $e = Exception::Class->caught('MyException') )
     warn $e->error, "\n", $e->trace->as_string, "\n";
     warn join ' ',  $e->euid, $e->egid, $e->uid, $e->gid, $e->pid, $e->time;
  elsif ( $e = Exception::Class->caught('ExceptionWithFields') )
     $e->quixotic ? do_something_wacky() : do_something_sane();
     $e = Exception::Class->caught();
     ref $e ? $e->rethrow : die $e;
  # use an alias - without parens subroutine name is checked at
  # compile time
  throw_fields error => "No strawberry", grandiosity => "quite a bit";


Exception::Class allows you to declare exception hierarchies in your modules in a ``Java-esque'' manner.

It features a simple interface allowing programmers to 'declare' exception classes at compile time. It also has a base exception class, Exception::Class::Base, that can be easily extended.

It is designed to make structured exception handling simpler and better by encouraging people to use hierarchies of exceptions in their applications, as opposed to a single catch-all exception class.

This module does not implement any try/catch syntax. Please see the ``OTHER EXCEPTION MODULES (try/catch syntax)'' section for more information on how to get this syntax.


Importing Exception::Class allows you to automagically create Exception::Class::Base subclasses. You can also create subclasses via the traditional means of defining your own subclass with @ISA. These two methods may be easily combined, so that you could subclass an exception class defined via the automagic import, if you desired this.

The syntax for the magic declarations is as follows:

'MANDATORY CLASS NAME' => \%optional_hashref

The hashref may contain the following options:

The Exception::Class magic attempts to detect circular class hierarchies and will die if it finds one. It also detects missing links in a chain, for example if you declare Bar to be a subclass of Foo and never declare Foo.

Catching Exceptions

Exception::Class provides some syntactic sugar for catching exceptions in a safe manner:

 eval { ... }
 if ( my $e = Exception::Class->caught('My::Error') )

The caught() method takes a class name and returns an exception object if the last thrown exception is of the given class, or a subclass of that class. If it is not given any arguments, it simply returns $@.

You should always make a copy of the exception object, rather than using $@ directly. This is necessary because if your cleanup() function uses eval, or calls something which uses it, then $@ is overwritten. Copying the exception preserves it for the call to do_something_with_exception().

Exception objects also provide a caught method so you can write:

 if ( my $e = My::Error->caught() )

Uncatchable Exceptions

Internally, the caught() method will call isa() on the exception object. You could make an exception ``uncatchable'' by overriding isa() in that class like this:

 package Exception::Uncatchable;
 sub isa { shift->rethrow }

Of course, this only works if you always call Exception::Class->caught() after an eval.

Exception::Class::Base CLASS METHODS

Exception::Class::Base OBJECT METHODS


The Exception::Class::Base object is overloaded so that stringification produces a normal error message. It just calls the as_string method described above. This means that you can just print $@ after an eval and not worry about whether or not its an actual object. It also means an application or module could do this:

 $SIG{__DIE__} = sub { Exception::Class::Base->throw( error => join '', @_ ); };

and this would probably not break anything (unless someone was expecting a different type of exception object from die()).


By default, the as_string() method simply returns the value message or error param plus a stack trace, if the class's Trace() method returns a true value or show_trace was set when creating the exception.

However, once you add new fields to a subclass, you may want to include those fields in the stringified error.

Inside the as_string() method, the message (non-stack trace) portion of the error is generated by calling the full_message() method. This can be easily overridden. For example:

  sub full_message
      my $self = shift;
      my $msg = $self->message;
      $msg .= " and foo was " . $self->foo;
      return $msg;


If you're creating a complex system that throws lots of different types of exceptions, consider putting all the exception declarations in one place. For an app called Foo you might make a Foo::Exceptions module and use that in all your code. This module could just contain the code to make Exception::Class do its automagic class creation. Doing this allows you to more easily see what exceptions you have, and makes it easier to keep track of them.

This might look something like this:

  package Foo::Bar::Exceptions;
  use Exception::Class ( Foo::Bar::Exception::Senses =>
                        { description => 'sense-related exception' },
                         Foo::Bar::Exception::Smell =>
                         { isa => 'Foo::Bar::Exception::Senses',
                           fields => 'odor',
                           description => 'stinky!' },
                         Foo::Bar::Exception::Taste =>
                         { isa => 'Foo::Bar::Exception::Senses',
                           fields => [ 'taste', 'bitterness' ],
                           description => 'like, gag me with a spoon!' },
                         ... );

You may want to create a real module to subclass Exception::Class::Base as well, particularly if you want your exceptions to have more methods.

Subclassing Exception::Class::Base

As part of your usage of Exception::Class, you may want to create your own base exception class which subclasses Exception::Class::Base. You should feel free to subclass any of the methods documented above. For example, you may want to subclass new() to add additional information to your exception objects.

Exception::Class FUNCTIONS

The Exception::Class method offers one function, Classes(), which is not exported. This method returns a list of the classes that have been created by calling the Exception::Class import() method. Note that this is all the subclasses that have been created, so it may include subclasses created by things like CPAN modules, etc. Also note that if you simply define a subclass via the normal Perl method of setting @ISA or use base, then your subclass will not be included.

OTHER EXCEPTION MODULES (try/catch syntax)

If you are interested in adding try/catch/finally syntactic sugar to your code then I recommend you check out U. Arun Kumar's Error.pm module, which implements this syntax. It also includes its own base exception class, Error::Simple.

If you would prefer to use the Exception::Class::Base class included with this module, you'll have to add this to your code somewhere:

  push @Exception::Class::Base::ISA, 'Error'
      unless Exception::Class::Base->isa('Error');

It's a hack but apparently it works.


Please submit bugs to the CPAN RT system at http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Exception%3A%3AClass or via email at bug-exception-class@rt.cpan.org.


Dave Rolsky, <autarch@urth.org>


Copyright (c) 2000-2006 David Rolsky. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

The full text of the license can be found in the LICENSE file included with this module.

 Exception::Class - A module that allows you to declare real exception classes in Perl