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Sub::Override - Perl extension for easily overriding subroutines




  use Sub::Override;

  sub foo { 'original sub' };
  print foo(); # prints 'original sub'

  my $override = Sub::Override->new( foo => sub { 'overridden sub' } );
  print foo(); # prints 'overridden sub'
  print foo(); # prints 'original sub'


The Problem

Sometimes subroutines need to be overridden. In fact, your author does this frequently for tests. Particularly when testing, using a Mock Object can be overkill when all you want to do is override one tiny, little function.

Overriding a subroutine is often done with syntax similar to the following.

   local *Some::sub = sub {'some behavior'};
   # do something
 # original subroutine behavior restored

This has a few problems.

   local *Get::some_feild = { 'some behavior' };
   # do something

In the above example, not only have we probably misspelled the subroutine name, but even if there had been a subroutine with that name, we haven't overridden it. These two bugs can be subtle to detect.

Further, if we're attempting to localize the effect by placing this code in a block, the entire construct is cumbersome.

Hook::LexWrap also allows us to override sub behavior, but I can never remember the exact syntax.

An easier way to replace subroutines

Instead, Sub::Override allows the programmer to simply name the sub to replace and to supply a sub to replace it with.

  my $override = Sub::Override->new('Some::sub', sub {'new data'});

  # which is equivalent to:
  my $override = Sub::Override->new;
  $override->replace('Some::sub', sub { 'new data' });

You can replace multiple subroutines, if needed:

  $override->replace('Some::sub1', sub { 'new data1' });
  $override->replace('Some::sub2', sub { 'new data2' });
  $override->replace('Some::sub3', sub { 'new data3' });

If replacing the subroutine succeeds, the object is returned. This allows the programmer to chain the calls, if this style of programming is preferred:

  $override->replace('Some::sub1', sub { 'new data1' })
           ->replace('Some::sub2', sub { 'new data2' })
           ->replace('Some::sub3', sub { 'new data3' });

If the subroutine has a prototype, the new subroutine should be declared with same prototype as original one:

  $override->replace('Some::sub_with_proto', sub ($$) { ($_[0], $_ [1]) });

A subroutine may be replaced as many times as desired. This is most useful when testing how code behaves with multiple conditions.

  $override->replace('Some::thing', sub { 0 });
  is($object->foo, 'wibble', 'wibble is returned if Some::thing is false');

  $override->replace('Some::thing', sub { 1 });
  is($object->foo, 'puppies', 'puppies are returned if Some::thing is true');

Wrapping a subroutine

There may be times when you want to 'conditionally' replace a subroutine - for example, to override the original subroutine only if certain args are passed. For this you can specify 'wrap' instead of 'replace'. Wrap is identical to replace, except the original subroutine is passed as the first arg to your new subroutine. You can call the original sub via 'shift->(@_)':

    sub {
      my ($old_sub, @args) = @_;
      return 1 if $args[0];
      return $old_sub->(@args);

Restoring subroutines

If the object falls out of scope, the original subs are restored. However, if you need to restore a subroutine early, just use the restore() method:

  my $override = Sub::Override->new('Some::sub', sub {'new data'});
  # do stuff

Which is somewhat equivalent to:

    my $override = Sub::Override->new('Some::sub', sub {'new data'});
    # do stuff

If you have overridden more than one subroutine with an override object, you will have to explicitly name the subroutine you wish to restore:


Note restore() will always restore the original behavior of the subroutine no matter how many times you have overridden it.

Which package is the subroutine in?

Ordinarily, you want to fully qualify the subroutine by including the package name. However, failure to fully qualify the subroutine name will assume the current package.

  package Foo;
  use Sub::Override;
  sub foo { 23 };
  my $override = Sub::Override->new( foo => sub { 42 } ); # assumes Foo::foo
  print foo(); # prints 42
  print foo(); # prints 23



  my $sub = Sub::Override->new;
  my $sub = Sub::Override->new($sub_name, $sub_ref);

Creates a new Sub::Override instance. Optionally, you may override a subroutine while creating a new object.


 $sub->replace($sub_name, $sub_body);

Temporarily replaces a subroutine with another subroutine. Returns the instance, so chaining the method is allowed:

 $sub->replace($sub_name, $sub_body)
     ->replace($another_sub, $another_body);

This method will croak if the subroutine to be replaced does not exist.


 my $sub = Sub::Override->new;
 $sub->override($sub_name, $sub_body);

override is an alternate name for replace. They are the same method.



Restores the previous behavior of the subroutine. This will happen automatically if the Sub::Override object falls out of scope.


None by default.


If you need to override the same sub several times do not create a new Sub::Override object, but instead always reuse the existing one and call replace on it. Creating a new object to override the same sub will result in weird behavior.

 # Do not do this!
 my $sub_first = Sub::Override->new( 'Foo:bar' => sub { 'first' } );
 my $sub_second = Sub::Override->new( 'Foo::bar' => sub { 'second' } );

 # Do not do this either!
 my $sub = Sub::Override->new( 'Foo::bar' => sub { 'first' } );
 $sub = Sub::Override->new( 'Foo::bar' => sub { 'second' } );

Both of those usages could result in of your subs being lost, depending on the order in which you restore them.

Instead, call replace on the existing $sub.

 my $sub = Sub::Override->new( 'Foo::bar' => sub { 'first' } );
 $sub->replace( 'Foo::bar' => sub { 'second' } );


Probably. Tell me about 'em.



Curtis "Ovid" Poe, <ovid [at] cpan [dot] org>


Copyright (C) 2004-2013 by Curtis "Ovid" Poe

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself, either Perl version 5.8.2 or, at your option, any later version of Perl 5 you may have available.