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<title>Error handling: 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-errors"></a>Error handling</h2></div></div></div>
<p>
      The error handling approach in C for Cairo has multiple
      elements:
    </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>
	  When a method on an object fails, the object is put into
	  an error state. Subsequent operations on the object do
	  nothing. The status of the object can be queried with
	  a function like <a class="link" href="cairo-cairo-t.html#cairo-status" title="cairo_status ()"><code class="function">status()</code></a>.
      </p></li>
<li class="listitem">
<p>
	  Constructors, rather than
	  returning <code class="constant">NULL</code> on out-of-memory failure,
	  return a special singleton object on which all
	  operations do nothing. Retrieving the status of the
	  singleton object returns <code class="constant">CAIRO_STATUS_NO_MEMORY</code>
	  </p>
<p class="remark"><em><span class="remark">
	    Is this going to apply to
	  <span class="type">cairo_surface_t</span> as well?
	</span></em></p>
<p class="remark"><em><span class="remark">
	  What about cairo_copy_path_data()? It's probably going to
	  have to return <code class="constant">NULL</code>.
	</span></em></p>
</li>
<li class="listitem"><p>
	  Errors propagate from object to object. Setting a pattern
	  in an out-of-memory state as the source of a
	  <span class="type">cairo_t</span> puts the type into an error state.
      </p></li>
</ul></div>
<p class="remark"><em><span class="remark">Much of the above is not yet implemented at the time of
      this writing</span></em></p>
<p>
      A language binding could copy the C approach, and for a 
      language without exceptions, this is likely the right thing
      to do. However, for a language with exceptions, exposing
      a completely different style of error handling for cairo
      would be strange. So, instead, status should be checked
      after every call to cairo, and exceptions thrown as necessary.
    </p>
<p>
      One problem that can arise with this, in languages
      where handling exceptions is mandatory (like Java), is that almost
      every cairo function can result in a status being set,
      usually because of an out-of-memory condition. This could make
      cairo hard to use. To resolve this problem, let's classify then
      cairo status codes:
    </p>
<pre class="programlisting">
/* Memory */      
CAIRO_STATUS_NO_MEMORY,

/* Programmer error */      
CAIRO_STATUS_INVALID_RESTORE
CAIRO_STATUS_INVALID_POP_GROUP
CAIRO_STATUS_NO_CURRENT_POINT
CAIRO_STATUS_INVALID_MATRIX
CAIRO_STATUS_NO_TARGET_SURFACE
CAIRO_STATUS_INVALID_STRING
CAIRO_STATUS_SURFACE_FINISHED
CAIRO_STATUS_BAD_NESTING

/* Language binding implementation */
CAIRO_STATUS_NULL_POINTER
CAIRO_STATUS_INVALID_PATH_DATA
CAIRO_STATUS_SURFACE_TYPE_MISMATCH

/* Other */      
CAIRO_STATUS_READ_ERROR
CAIRO_STATUS_WRITE_ERROR
</pre>
<p>
      If we look at these, the
      <code class="constant">CAIRO_STATUS_NO_MEMORY</code>
      should map to the native out-of-memory exception, which could
      happen at any point in any case. Most of the others indicate
      programmer error, and handling them in user code would be
      silly. These should be mapped into whatever the language uses
      for assertion failures, rather than errors that are normally
      handled. (In Java, a subclass of Error rather than Exception,
      perhaps.) And <code class="constant">CAIRO_STATUS_READ_ERROR</code>,
      and <code class="constant">CAIRO_STATUS_WRITE_ERROR</code> can occur
      only in very specific places. (In fact, as described
      in <a class="xref" href="bindings-streams.html" title="Streams and File I/O">the section called “Streams and File I/O”</a>, these errors may be
      mapped into the language's native I/O error types.)
      So, there really aren't exceptions that the programmer must
      handle at most points in the Cairo API.
    </p>
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