<page xmlns="http://projectmallard.org/1.0/"
      xmlns:its="http://www.w3.org/2005/11/its"
      type="topic"
      id="namespacing">

  <info>
    <link type="guide" xref="index#maintainer-guidelines"/>

    <credit type="author copyright">
      <name>Philip Withnall</name>
      <email its:translate="no">philip.withnall@collabora.co.uk</email>
      <years>2015</years>
    </credit>

    <include href="cc-by-sa-3-0.xml" xmlns="http://www.w3.org/2001/XInclude"/>

    <desc>
      Avoiding symbol conflicts between libraries by namespacing all APIs
    </desc>
  </info>

  <title>Namespacing</title>

  <synopsis>
    <title>Summary</title>

    <p>
      If a library is namespaced correctly, it can define types and methods in
      its API which have the same names as those in another library, and a
      program can use both without conflicts. This is achieved by prefixing all
      types and method names with a namespace unique to the library.
    </p>
  </synopsis>

  <section id="gobject">
    <title>GObject APIs</title>

    <p>
      Consistent and complete namespacing of symbols (functions and types) and
      files is important for two key reasons:
    </p>
    <list type="numbered">
      <item><p>
        Establishing a convention which means developers have to learn fewer
        symbol names to use the library — they can guess them reliably instead.
      </p></item>
      <item><p>
        Ensuring symbols from two projects do not conflict if included in the
        same file.
      </p></item>
    </list>

    <p>
      The second point is important — imagine what would happen if every project
      exported a function called <code>create_object()</code>. The headers
      defining them could not be included in the same file, and even if that
      were overcome, the programmer would not know which project each function
      comes from. Namespacing eliminates these problems by using a unique,
      consistent prefix for every symbol and filename in a project, grouping
      symbols into their projects and separating them from others.
    </p>

    <p>
      The conventions below should be used for namespacing all symbols. They are
      <link href="https://developer.gnome.org/gobject/stable/gtype-conventions.html">
      used in all GLib-based projects</link>, so should be familiar to a lot of
      developers:
    </p>
    <list>
      <item><p>
        Functions should use <code>lower_case_with_underscores</code>.
      </p></item>
      <item><p>
        Structures, types and objects should use
        <code>CamelCaseWithoutUnderscores</code>.
      </p></item>
      <item><p>
        Macros and constants should use
        <code>UPPER_CASE_WITH_UNDERSCORES</code>.
      </p></item>
      <item><p>
        All symbols should be prefixed with a short (2–4 characters) version of
        the namespace. This is shortened purely for ease of typing, but should
        still be unique.
      </p></item>
      <item><p>
        All methods of a class should also be prefixed with the class name.
      </p></item>
    </list>

    <p>
      Additionally, public headers should be included from a subdirectory,
      effectively namespacing the header files. For example, instead of
      <code>#include &lt;abc.h&gt;</code>, a project should allow its users to
      use <code>#include &lt;namespace/ns-abc.h&gt;</code>.
    </p>

    <p>
      For example, for a project called ‘Walbottle’, the short namespace ‘Wbl’
      would be chosen. If it has a ‘schema’ class and a ‘writer’ class, it
      would install headers:
    </p>
    <list>
      <item><p>
        <file><var>$(includedir)</var>/walbottle-<var>$API_MAJOR</var>/walbottle/wbl-schema.h</file>
      </p></item>
      <item><p>
        <file><var>$(includedir)</var>/walbottle-<var>$API_MAJOR</var>/walbottle/wbl-writer.h</file>
      </p></item>
    </list>

    <p>
      (The use of <var>$API_MAJOR</var> above is for
      <link xref="parallel-installability">parallel installability</link>.)
    </p>

    <p>
      For the schema class, the following symbols would be exported (amongst
      others), following GObject conventions:
    </p>
    <list>
      <item><p><code>WblSchema</code> structure</p></item>
      <item><p><code>WblSchemaClass</code> structure</p></item>
      <item><p><code>WBL_TYPE_SCHEMA</code> macro</p></item>
      <item><p><code>WBL_IS_SCHEMA</code> macro</p></item>
      <item><p><code>wbl_schema_get_type</code> function</p></item>
      <item><p><code>wbl_schema_new</code> function</p></item>
      <item><p><code>wbl_schema_load_from_data</code> function</p></item>
    </list>
  </section>
</page>
