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');

Injecting a subroutine¶

If you want to inject a subroutine into a package, you can use the inject() method. This is identical to replace(), except that it requires that the subroutine does not exist:

  $override->inject('Some::sub', sub {'new data'});

This is useful if you want to add a subroutine to a package that doesn't already have it.

If you attempt to inject a subroutine that already exists, an exception will be thrown.

  $override->inject('Some::sub', sub {'new data'}); # works
  $override->inject('Some::sub', sub {'new data'}); # throws an exception

Calling restore() or allowing the $override to go out of scope will remove the injected subroutine.

  $override->inject('Some::sub', sub {'new data'});
  $override->restore('Some::sub'); # removes the injected subroutine

Inheriting a subroutine¶

Similar to 'inject', 'inherit' will only allow you to create a new subroutine on a child object that inherits the routine from the parent, and doesn't exist in the child:

  package Parent;
  sub foo {}
  sub bar {}
  package Child;
  use parent 'Parent';
  sub foo {}

'Inherit' will allow you to set up a new 'Child::bar' subroutine since it is inherited from Parent. Attempting to 'inherit' 'Child::foo' will result in an exception being thrown since 'foo' already exists in Child. Similarly, attempting to 'inherit' new subroutine 'something' in Child will also result in an exception since it doesn't exist in Parent and won't be inherited by Child.

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->(@_)':

  $override->wrap('Some::sub',
    sub {
      my ($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
  $override->restore;

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:

  $override->restore('This::sub');

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
  $override->restore;
  print foo(); # prints 23

new¶

  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.

replace¶

 $sub->replace($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.

override¶

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

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

inject¶

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

Temporarily injects a subroutine into a package. Returns the instance, so chaining the method is allowed:

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

inherit¶

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

Checks that the subroutine exists in a parent class, but not in the current class, and injects it into the current class to inherit the parent's version.

restore¶

 $sub->restore($sub_name);

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

wrap¶

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

Temporarily wraps a subroutine with another subroutine. The original subroutine is passed as the first arg to the new subroutine.