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<title>Memory management: Cairo: A Vector Graphics Library</title>
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<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="bindings-memory"></a>Memory management</h2></div></div></div>
<p>
      The objects in cairo can roughly be divided into two types:
      reference-counted, opaque types like
      <a class="link" href="cairo-cairo-surface-t.html#cairo-surface-t" title="cairo_surface_t"><span class="type">cairo_surface_t</span></a>
      and plain structures like
      <a class="link" href="cairo-text.html#cairo-glyph-t" title="cairo_glyph_t"><span class="type">cairo_glyph_t</span></a>.
      <a class="link" href="cairo-Paths.html#cairo-path-t" title="cairo_path_t"><span class="type">cairo_path_t</span></a>
      and 
      <a class="link" href="cairo-Paths.html#cairo-path-data-t" title="union cairo_path_data_t"><span class="type">cairo_path_data_t</span></a>
      are special cases and are treated separately in this appendix.
    </p>
<p>
      Refcounted opaque types all have a
      <code class="function">..._reference()</code>
      function to increase the refcount by one and a
      <code class="function">..._destroy()</code> to decrease the refcount
      by one. These should not be exposed to the user of the language
      binding, but rather used to implement memory management within
      the language binding. The simplest way to do memory management
      for a language binding is to treat the language binding object
      as a simple handle to the cairo object. The language binding
      object references the cairo object, and unreferences it when
      finalized. This is the recommended method, though there are
      a couple of caveats to be noted:
    </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>
	  Equality won't work as expected. You can have two language
	  objects for the same cairo and they won't necessarily
	  compare equal. If the language allows customizing the
	  equality operation, then this is fixable by comparing
	  the underlying pointers. It also can be fixed by creating
	  at most one language object per cairo object, and
	  uniquifying via a <em class="firstterm">pin table</em> (a hash
	  table that goes from cairo object to language object).
	  For <span class="type">cairo_surface_t</span> you can use also 
	  <a class="link" href="cairo-cairo-surface-t.html#cairo-surface-set-user-data" title="cairo_surface_set_user_data ()"><code class="function">cairo_surface_set_user_data()</code></a>
	  instead of a separate pin table.
	</p></li>
<li class="listitem">
<p>
	  Derivation from the language object doesn't work because
	  you can lose the language object while keeping the Cairo
	  object. Code like:
	</p>
<pre class="programlisting">
public class MySurface (ImageSurface) {
   public MySurface (width, height) {
      super (Format.ARGB32, width, height);
   }
   public int get42 () {
      return 42;	  
   }
}

   cr = Cairo(MySurface(width, height));
   surface = cr.getTarget();
</pre>
<p>
	  Can result in <code class="varname">surface</code> containing an
	  <code class="classname">ImageSurface</code> not a <code class="classname">MySurface</code>.
	  This is not easily fixable without creating memory leaks,
	  and it's probably best to simply forbid deriving from the
	  language objects.
	</p>
</li>
</ul></div>
<p>
      When a plain structure is used as a return value from cairo,
      this is done by passing it as a “out parameter”.
    </p>
<pre class="programlisting">
cairo_font_extents_t extents;      

cairo_font_extents (cr, &amp;extents);</pre>
<p>
      In a language binding, this should typically be treated
      as a return value:
    </p>
<pre class="programlisting">
FontExtents extents = cr.fontExtents ();</pre>
<p>
      A language binding has a choice in how it implements the
      language objects for plain structures. It can use a pure
      language object with fields corresponding to those of the C
      structure, and convert from and to the C structure when calling
      cairo functions or converting cairo return values. Or it
      can keep a pointer to the C structure internally and wrap
      it inside a language object much like occurs for refcounted
      objects. The choice should be invisible to the user: they should
      be able to imagine that it is implemented as a pure language
      object.
    </p>
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