Typical autoload systems intercept procedure calls to undefined subroutines.
For example, IDL includes a primitive path searcher, and Perl allows individual modules to determine how and whether autoloading should occur.
The Unix shell may be said to consist almost entirely of an autoloader, as its main job is to search a path of directories to load and execute command files.
In PHP 5, autoload functionality is triggered when referencing an undefined class.
One or more autoload functions—implemented as the __autoload magic function or any function registered to the SPL autoload stack—is called and given the opportunity to define the class, usually by loading the file it is defined in.