Declaring options

The other major difference between LATEX 2.09 styles and LATEX2e packages and classes is in option handling. Packages and classes can now declare options and these can be specified by authors; for example, the |twocolumn| option is declared by the |article| class.

An option is declared as follows: 

   \DeclareOption{<option>}{<code>}
For example, the |dvips| option to the |graphics| package is implemented as: 
   \DeclareOption{dvips}{\input{dvips.def}}
This means that when an author writes ||, the file |dvips.def| is loaded. As another example, the |a4paper| option is declared in the |article| class to set the || and || lengths: 
   \DeclareOption{a4paper}{%
      \setlength{\paperheight}{297mm}%
      \setlength{\paperwidth}{210mm}%
   }
Sometimes a user will request an option which the class or package has not explicitly declared. By default this will produce a warning (for classes) or error (for packages); this behaviour can be altered as follows: 
   \DeclareOption*{<code>}
For example, to make the package |fred| produce a warning rather than an error for unknown options, you could specify: 
   \DeclareOption*{%
      \PackageWarning{fred}{Unknown option `\CurrentOption'}%
   }
Then, if an author writes ||, they will get a warning Package fred Warning: Unknown option `foo'. As another example, the |fontenc| package tries to load a file |<ENC>enc.def| whenever the ENC option is used. This can be done by writing: 
   \DeclareOption*{%
      \input{\CurrentOption enc.def}%
   }
It is possible to pass options on to another package or class, using the command |r | For example, to pass every unknown option on to the |article| class, you can use: 
   \DeclareOption*{%
      \PassOptionsToClass{\CurrentOption}{article}%
   }
If you do this then you should make sure you load the class at some later point, otherwise the options will never be processed!

So far, we have explained only how to declare options, not how to execute them. To process the options with which the file was called, you should use: 

   \ProcessOptions
This executes the code for each option that was both specified and declared (see Section [*] for details of how this is done).

For example, if the |jane| package file contains: 

   \DeclareOption{foo}{\typeout{Saw foo.}}
   \DeclareOption{baz}{\typeout{Saw baz.}}
   \DeclareOption*{\typeout{What's \CurrentOption?}}
   \ProcessOptions
and an author writes ||, then they will see the messages Saw foo. and What's bar?