## Target types

The first thing we did in the intruduction was declaring a new target type:

    import type ;
    type.register VERBATIM : verbatim ;

The type is the most important property of a target. Boost.Build can automatically generate necessary build actions only because you specify the desired type (using the different main target rules), and because Boost.Build can guess the type of sources from their extensions.

The first two parameters for the `type.register` rule are the name of new type and the list of extensions associated with it. A file with an extension from the list will have the given target type. In the case where a target of the declared type is generated from other sources, the first specified extension will be used.

Sometimes you want to change the suffix used for generated targets depending on build properties, such as toolset. For example, some compiler uses extension `elf` for executable files. You can use the `type.set-generated-target-suffix` rule:

    type.set-generated-target-suffix EXE : <toolset>elf : elf ;

A new target type can be inherited from an existing one.

    type.register PLUGIN : : SHARED_LIB ;

The above code defines a new type derived from `SHARED_LIB`. Initially, the new type inherits all the properties of the base type - in particular generators and suffix. Typically, you'll change the new type in some way. For example, using `type.set-generated-target-suffix` you can set the suffix for the new type. Or you can write special a generator for the new type. For example, it can generate additional metainformation for the plugin. In either way, the `PLUGIN` type can be used whenever `SHARED_LIB` can. For example, you can directly link plugins to an application.

A type can be defined as "main", in which case Boost.Build will automatically declare a main target rule for building targets of that type. More details can be found later.

### Scanners

Sometimes, a file can refer to other files via some include mechanism. To make Boost.Build track dependencies to the included files, you need to provide a scanner. The primary limitation is that only one scanner can be assigned to a target type.

First, we need to declare a new class for the scanner:

    class verbatim-scanner : common-scanner
    {
        rule pattern ( )
        {
            return "//###include[ ]*\"([^\"]*)\"" ;
        }
    }

All the complex logic is in the `common-scanner` class, and you only need to override the method that returns the regular expression to be used for scanning. The parentheses in the regular expression indicate which part of the string is the name of the included file. Only the first parenthesized group in the regular expression will be recognized; if you can't express everything you want that way, you can return multiple regular expressions, each of which contains a parenthesized group to be matched.

After that, we need to register our scanner class:

    scanner.register verbatim-scanner : include ;

The value of the second parameter, in this case `include`, specifies the properties that contain the list of paths that should be searched for the included files.

Finally, we assign the new scanner to the `VERBATIM` target type:

    type.set-scanner VERBATIM : verbatim-scanner ;

That's enough for scanning include dependencies.