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## Bringing Together a BoostBook Document

### Linking in BoostBook

How one links to another element in BoostBook depends greatly on the nature of the element linked and how the link should appear. There are three general linking elements: <xref>, <link>, and <ulink>. Additionally, there are linking elements for referencing specific types of entities, such as classes (<classname>), functions (<functionname>), or libraries (<libraryname>).

The <xref> element references elements that have an `id` attribute and a title. The actual link text is composed from title and type of the element referenced. To link to a particular ID, create an <xref> element with the `linkend` attribute set to the ID of the intended target. For instance, this section's ID is `boostbook.linking`, so we create a reference it to with `<xref linkend="boostbook.linking"/>`, which will look like this in the text: the section called “Linking in BoostBook”.

The <link> element references an ID in the same way as <xref>, except that <link> does not generate any text for the link, so text must be supplied within the element. For instance, we can again link to this chapter but this time specify our own text with `<link linkend="boostbook.linking">like this</link>`. This markup will result in a link to this chapter that looks like this.

The <ulink> element references a URL that is outside of the DocBook document. The `url` attribute contains the URL to link to, and the element data provides the link text.For instance, we can link to the the Boost web site with `<ulink url="http://www.boost.org">Boost</ulink>,` which appears in the document like this: Boost.

In BoostBook, <ulink> supports a custom url schema for linking to files within the boost distribution. This is formed by setting the `url` attribute to `boost:` followed by the file's path. For example, we can link to the flyweight library with `<ulink url="boost:/libs/flyweight/index.html">Boost.Flyweight</ulink>`, which will appear like this: Boost.Flyweight. This schema is only supported for BoostBook <ulink> elements. It isn't available for any other elements or in Docbook.

The <classname>, <functionname>, <methodname>, and <libraryname> link to classes, functions, methods, and libraries, respectively. The text of each element gives both the name of the element to link to and the link text. For instance, we can link to the Function library with `<libraryname>Function</libraryname>`, which results in the following: Function. In cases where the displayed text is different from the actual name, the `alt` attribute can be specified. For instance, the following XML element references the `boost::function` class template but displays the text `function`: `<classname alt="boost::function">function</classname>`.