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-builtin compatible ref example in Python docs

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Jake Cobb 2018-04-13 11:14:51 -04:00 • committed by Jake Cobb
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@ -5329,10 +5329,18 @@ wheel size: 135019664
What has happened here is the garbage collector has collected the <tt>Bike</tt> instance as it doesn't think it is needed any more.
The proxy instance, <tt>wheel</tt>, contains a reference to memory that was deleted when the <tt>Bike</tt> instance was collected.
In order to prevent the garbage collector from collecting the <tt>Bike</tt> instance, a reference to the <tt>Bike</tt> must
be added to the <tt>wheel</tt> instance. You can do this by adding the reference when the <tt>getWheel()</tt> method
is called using the typemap-like <tt>%pythonappend</tt> directive (see <a href="#Python_nn42">36.6.2 Adding additional Python code</a>):
be added to the <tt>wheel</tt> instance.
</p>
<p>
You can do this by adding the reference when the <tt>getWheel()</tt> method
is called using one of two approaches:
</p>
<p>
The easier, but less optimized, way is to use the typemap-like <tt>%pythonappend</tt> directive
(see <a href="#Python_nn42">36.6.2 Adding additional Python code</a>):
</p>
<div class="code">
<pre>
@ -5349,6 +5357,41 @@ The code gets appended to the Python code generated for the
instance onto the <tt>Wheel</tt> proxy instance before it is returned to the
caller.
</p>
<p>
The second option, which performs better and is required if you use the
<tt>-builtin</tt> option, is to set the reference in the CPython implementation:
<div class="code">
<pre>
%fragment("extra_reference", "header") {
static PyObject *extra_reference() {
static PyObject *extra_reference_string = NULL;
if (!extra_reference_string)
extra_reference_string = SWIG_Python_str_FromChar("_extra_reference");
return extra_reference_string;
}
}
%extend Wheel {
%typemap(ret, fragment="extra_reference") Wheel& getWheel %{
// A reference to the parent class is added to ensure the underlying C++
// object is not deleted while the item is in use
PyObject_SetAttr($result, extra_reference(), $self);
%}
/* FYI: Alternative approach, but is possibly harder to understand, so suggest above
%typemap(out, fragment="extra_reference") Wheel& getWheel %{
$typemap(out, Wheel &)
// A reference to the parent class is added to ensure the underlying C++
// object is not deleted while the item is in use
PyObject_SetAttr($result, extra_reference(), $self);
%}
*/
}
</pre>
</div>
<H2><a name="Python_nn65">36.10 Docstring Features</a></H2>