[Chapter 7] 7.2.55 SelfLoader - Load Functions Only on Demand
Author: Larry Wall, Tom Christiansen & Randal L. Schwartz
Date: 1999-07-02T02:03:41Z

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| Chapter 7 The Standard Perl Library |

### 7.2.55 SelfLoader - Load Functions Only on Demand

    package GoodStuff;
    use SelfLoader;

    [initializing code]
    __DATA__
    sub {...};

This module is used for delayed loading of Perl functions that (unlike AutoLoader functions) are packaged within your script file. This gives the appearance of faster loading.

In the example above, SelfLoader tells its user (GoodStuff) that functions in the GoodStuff package are to be autoloaded from after the `__DATA__` token.

The `__DATA__` token tells Perl that the code for compilation is finished. Everything after the `__DATA__` token is available for reading via the filehandle `GoodStuff::DATA`, where GoodStuff is the name of the current package when the `__DATA__` token is reached. This token works just the same as `__END__` does in package main, except that data after `__END__` is retrievable only in package main, whereas data after `__DATA__` is retrievable in whatever the current package is.

Note that it is possible to have `__DATA__` tokens in the same package in multiple files, and that the last `__DATA__` token in a given package that is encountered by the compiler is the one accessible by the filehandle. That is, whenever the `__DATA__` token is parsed, any `DATA` filehandle previously open in the current package (opened in a different file, presumably) is closed so that the new one can be opened. (This also applies to `__END__` and the `main::DATA` filehandle: `main::DATA` is reopened whenever `__END__` is encountered, so any former association is lost.)

#### 7.2.55.1 SelfLoader autoloading

The SelfLoader will read from the `GoodStuff::DATA` filehandle to get definitions for functions placed after `__DATA__`, and then eval the requested subroutine the first time it's called. The costs are the one-time parsing of the data after `__DATA__`, and a load delay for the first call of any autoloaded function. The benefits are a speeded up compilation phase, with no need to load functions that are never used.

You can use `__END__` after `__DATA__`. The SelfLoader will stop reading from `DATA` if it encounters the `__END__` token, just as you might expect. If the `__END__` token is present, and is followed by the token `DATA`, then the SelfLoader leaves the `GoodStuff::DATA` filehandle open on the line after that token.

The SelfLoader exports the `AUTOLOAD` subroutine to the package using the SelfLoader, and this triggers the automatic loading of an undefined subroutine out of its `DATA` portion the first time that subroutine is called.

There is no advantage to putting subroutines that will always be called after the `__DATA__` token.

#### 7.2.55.2 Autoloading and file-scoped lexicals

A `my $pack_lexical` statement makes the variable `$pack_lexical` visible only up to the `__DATA__` token. That means that subroutines declared elsewhere cannot see lexical variables. Specifically, autoloaded functions cannot see such lexicals (this applies to both the SelfLoader and the Autoloader). The `use vars` pragma (see later in this chapter) provides a way to declare package-level globals that will be visible to autoloaded routines.

#### 7.2.55.3 SelfLoader and AutoLoader

The SelfLoader can replace the AutoLoader - just change `use AutoLoader` to `use SelfLoader`[13] and the `__END__` token to `__DATA__`.

> [13] Be aware, however, that the SelfLoader exports an `AUTOLOAD` function into your package. But if you have your own `AUTOLOAD` and are using the AutoLoader too, you probably know what you're doing.

There is no need to inherit from the SelfLoader.

The SelfLoader works similarly to the AutoLoader, but picks up the subroutine definitions from after the `__DATA__` instead of in the lib/auto/ directory. SelfLoader needs less maintenance at the time the module is installed, since there's no need to run AutoSplit. And it can run faster at load time because it doesn't need to keep opening and closing files to load subroutines. On the other hand, it can run slower because it needs to parse the code after the `__DATA__`. Details of the AutoLoader and another view of these distinctions can be found in that module's documentation.

#### 7.2.55.4 How to read DATA from your Perl program

(This section is only relevant if you want to use the `GoodStuff::DATA` together with the SelfLoader.)

The SelfLoader reads from wherever the current position of the `GoodStuff::DATA` filehandle is, until EOF or the `__END__` token. This means that if you want to use that filehandle (and only if you want to), you should either

- Put all your subroutine declarations immediately after the `__DATA__` token and put your own data after those declarations, using the `__END__` token to mark the end of subroutine declarations. You must also ensure that the SelfLoader first reads its stubs by calling `SelfLoader->load_stubs();`, or by using a function which is selfloaded; or

- You should read the `GoodStuff::DATA` filehandle first, leaving the handle open and positioned at the first line of subroutine declarations.

You could even conceivably do both.

#### 7.2.55.5 Classes and inherited methods

This section is only relevant if your module is a class, and has methods that could be inherited.

A subroutine stub (or forward declaration) looks like:

    sub stub;

That is, it is a subroutine declaration without the body of the subroutine. For modules that aren't classes, there is no real need for stubs as far as autoloading is concerned.

For modules that are classes, and need to handle inherited methods, stubs are needed to ensure that the method inheritance mechanism works properly. You can load the stubs into the module at require time, by adding the statement `SelfLoader->load_stubs();` to the module to do this.

The alternative is to put the stubs in before the `__DATA__` token before releasing the module, and for this purpose the Devel::SelfStubber module is available. However this does require the extra step of ensuring that the stubs are in the module. If you do this, we strongly recommended that you do it before releasing the module and not at install time.

#### 7.2.55.6 Multiple packages and fully qualified subroutine names

Subroutines in multiple packages within the same file are supported - but you should note that this requires exporting `SelfLoader::AUTOLOAD` to every package which requires it. This is done automatically by the SelfLoader when it first loads the subs into the cache, but you should really specify it in the initialization before the `__DATA__` by putting a `use SelfLoader` statement in each package.

Fully qualified subroutine names are also supported. For example:

    __DATA__
    sub foo::bar {23}
    package baz;
    sub dob {32}

will all be loaded correctly by the SelfLoader, and the SelfLoader will ensure that the packages "foo" and "baz" correctly have the `SelfLoader::AUTOLOAD` method when the data after `__DATA__` is first parsed.

See the discussion of autoloading in Chapter 5. Also see the AutoLoader module, a utility that handles modules that have been into a collection of files for autoloading.

7.2.54 SelectSaver - Save and Restore Selected Filehandle |  | 7.2.56 Shell - Run Shell Commands Transparently Within Perl