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RAII in perl

Suppose you have matching start() and end() functions. You want to ensure that each start() is always matched with its corresponding end(), without having to explicitly pepper your code with calls to that function. Here’s a good way to do it in perl — create a guard object:

package Scoper;
sub new {
  my $class = shift; bless({ func => shift },$class);
  my $self = shift; $self->{func}->();

Here’s an example of its use:

  my $s = Scoper->new(sub { end(); });
  [... do something...]
[at this point, end() has been called, even if a die() occurred]

The idea is simply to use DESTROY to perform whatever the cleanup operation is. Once the $s object goes out of scope, it’ll be deleted by perl’s GC, in the process of which, calling $s->DESTROY(). In other words, it’s using the GC for its own ends.

Unlike an eval { } block to catch die()s, this will even be called if exit() or POSIX::exit() is called. (POSIX::_exit(), however, skips DESTROY.)

This is a pretty old C++ pattern — Resource Acquisition Is Initialization. C++’s auto_ptr template class is the best-known example in that language. Here’s a article on its use in perl, from last year, mostly regarding the CPAN module Object::Destroyer. To be honest, though, it’s 6 lines of code — not sure if that warrants a CPAN module! ;)

RAII is used in SpamAssassin, in the Mail::SpamAssassin::Util::ScopedTimer class.