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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="topics.indexing_support"></a><a class="link" href="indexing_support.html" title="Indexing support">Indexing support</a>
</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="indexing_support.html#topics.indexing_support.introduction">Introduction</a></span></dt>
<dt><span class="section"><a href="indexing_support.html#topics.indexing_support.the_indexing_interface">The
Indexing Interface</a></span></dt>
<dt><span class="section"><a href="indexing_support.html#topics.indexing_support.index_suite_sub_classes">index_suite
sub-classes</a></span></dt>
<dt><span class="section"><a href="indexing_support.html#topics.indexing_support.indexing_suite_class"><code class="computeroutput"><span class="identifier">indexing_suite</span></code> class</a></span></dt>
<dt><span class="section"><a href="indexing_support.html#topics.indexing_support.class_vector_indexing_suite">class
<code class="computeroutput"><span class="identifier">vector_indexing_suite</span></code></a></span></dt>
<dt><span class="section"><a href="indexing_support.html#topics.indexing_support.class_map_indexing_suite">class
<code class="computeroutput"><span class="identifier">map_indexing_suite</span></code></a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="topics.indexing_support.introduction"></a><a class="link" href="indexing_support.html#topics.indexing_support.introduction" title="Introduction">Introduction</a>
</h3></div></div></div>
<p>
Indexing is a <code class="computeroutput"><span class="identifier">Boost</span> <span class="identifier">Python</span></code>
facility for easy exportation of indexable C++ containers to Python. Indexable
containers are containers that allow random access through the <code class="computeroutput"><span class="keyword">operator</span><span class="special">[]</span></code>
(e.g. <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span></code>).
</p>
<p>
While <code class="computeroutput"><span class="identifier">Boost</span> <span class="identifier">Python</span></code>
has all the facilities needed to expose indexable C++ containers such as
the ubiquitous std::vector to Python, the procedure is not as straightforward
as we'd like it to be. Python containers do not map easily to C++ containers.
Emulating Python containers in C++ (see Python Reference Manual, <a href="http://www.python.org/doc/current/ref/sequence-types.html" target="_top">Emulating
container types</a>) using <code class="computeroutput"><span class="identifier">Boost</span><span class="special">.</span><span class="identifier">Python</span></code>
is non trivial. There are a lot of issues to consider before we can map
a C++ container to Python. These involve implementing wrapper functions
for the methods <code class="computeroutput"><span class="identifier">__len__</span></code>,
<code class="computeroutput"><span class="identifier">__getitem__</span></code>, <code class="computeroutput"><span class="identifier">__setitem__</span></code>, <code class="computeroutput"><span class="identifier">__delitem__</span></code>,
<code class="computeroutput"><span class="identifier">__iter__</span></code> and <code class="computeroutput"><span class="identifier">__contains__</span></code>.
</p>
<p>
The goals:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
Make indexable C++ containers behave exactly as one would expect a
Python container to behave.
</li>
<li class="listitem">
<p class="simpara">
Provide default reference semantics for container element indexing
(<code class="computeroutput"><span class="identifier">__getitem__</span></code>) such
that c[i] can be mutable. Require:
</p>
<pre class="programlisting"><span class="identifier">val</span> <span class="special">=</span> <span class="identifier">c</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span>
<span class="identifier">c</span><span class="special">[</span><span class="identifier">i</span><span class="special">].</span><span class="identifier">m</span><span class="special">()</span>
<span class="identifier">val</span> <span class="special">==</span> <span class="identifier">c</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span>
</pre>
<p class="simpara">
where m is a non-const (mutating) member function (method).
</p>
</li>
<li class="listitem">
Return safe references from <code class="computeroutput"><span class="identifier">__getitem__</span></code>
such that subsequent adds and deletes to and from the container will
not result in dangling references (will not crash Python).
</li>
<li class="listitem">
Support slice indexes.
</li>
<li class="listitem">
Accept Python container arguments (e.g. <code class="computeroutput"><span class="identifier">lists</span></code>,
<code class="computeroutput"><span class="identifier">tuples</span></code>) wherever appropriate.
</li>
<li class="listitem">
Allow for extensibility through re-definable policy classes.
</li>
<li class="listitem">
Provide predefined support for the most common STL and STL-like indexable
containers.
</li>
</ul></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="topics.indexing_support.the_indexing_interface"></a><a class="link" href="indexing_support.html#topics.indexing_support.the_indexing_interface" title="The Indexing Interface">The
Indexing Interface</a>
</h3></div></div></div>
<p>
The <code class="computeroutput"><span class="identifier">indexing_suite</span></code> class
is the base class for the management of C++ containers intended to be integrated
to Python. The objective is make a C++ container look and feel and behave
exactly as we'd expect a Python container. The class automatically wraps
these special Python methods (taken from the Python reference: Emulating
container types):
</p>
<div class="variablelist">
<p class="title"><b></b></p>
<dl class="variablelist">
<dt><span class="term">__len__(self)</span></dt>
<dd><p>
Called to implement the built-in function <code class="computeroutput"><span class="identifier">len</span><span class="special">()</span></code>. Should return the length of the
object, an integer <code class="computeroutput"><span class="special">&gt;=</span> <span class="number">0</span></code>. Also, an object that doesn't define
a <code class="computeroutput"><span class="identifier">__nonzero__</span><span class="special">()</span></code>
method and whose <code class="computeroutput"><span class="identifier">__len__</span><span class="special">()</span></code> method returns zero is considered
to be false in a Boolean context.
</p></dd>
<dt><span class="term">__getitem__(self, key)</span></dt>
<dd><p>
Called to implement evaluation of <code class="computeroutput"><span class="identifier">self</span><span class="special">[</span><span class="identifier">key</span><span class="special">]</span></code>. For sequence types, the accepted
keys should be integers and slice objects. Note that the special
interpretation of negative indexes (if the class wishes to emulate
a sequence type) is up to the <code class="computeroutput"><span class="identifier">__getitem__</span><span class="special">()</span></code> method. If key is of an inappropriate
type, <code class="computeroutput"><span class="identifier">TypeError</span></code> may
be raised; if of a value outside the set of indexes for the sequence
(after any special interpretation of negative values), IndexError
should be raised. [Note: for loops expect that an IndexError will
be raised for illegal indexes to allow proper detection of the end
of the sequence.]
</p></dd>
<dt><span class="term">__setitem__(self, key, value)</span></dt>
<dd><p>
Called to implement assignment to self[key]. Same note as for __getitem__().
This should only be implemented for mappings if the objects support
changes to the values for keys, or if new keys can be added, or for
sequences if elements can be replaced. The same exceptions should
be raised for improper key values as for the __getitem__() method.
</p></dd>
<dt><span class="term">__delitem__(self, key)</span></dt>
<dd><p>
Called to implement deletion of self[key]. Same note as for __getitem__().
This should only be implemented for mappings if the objects support
removal of keys, or for sequences if elements can be removed from
the sequence. The same exceptions should be raised for improper key
values as for the __getitem__() method.
</p></dd>
<dt><span class="term">__iter__(self)</span></dt>
<dd>
<p>
This method is called when an iterator is required for a container.
This method should return a new iterator object that can iterate
over all the objects in the container. For mappings, it should iterate
over the keys of the container, and should also be made available
as the method iterkeys().
</p>
<p>
Iterator objects also need to implement this method; they are required
to return themselves. For more information on iterator objects, see
<a href="https://docs.python.org/3/library/stdtypes.html#iterator-types" target="_top">Iterator
Types</a> in the <a href="https://docs.python.org/3/library/index.html" target="_top">Python
Library Reference</a>.
</p>
</dd>
<dt><span class="term">__contains__(self, item)</span></dt>
<dd><p>
Called to implement membership test operators. Should return true
if item is in self, false otherwise. For mapping objects, this should
consider the keys of the mapping rather than the values or the key-item
pairs.
</p></dd>
</dl>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="topics.indexing_support.index_suite_sub_classes"></a><a class="link" href="indexing_support.html#topics.indexing_support.index_suite_sub_classes" title="index_suite sub-classes">index_suite
sub-classes</a>
</h3></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="indexing_support.html#topics.indexing_support.index_suite_sub_classes.vector_index_suite">vector_index_suite</a></span></dt>
<dt><span class="section"><a href="indexing_support.html#topics.indexing_support.index_suite_sub_classes.map_index_suite">map_index_suite</a></span></dt>
</dl></div>
<p>
The <code class="computeroutput"><span class="identifier">indexing_suite</span></code> is not
meant to be used as is. A couple of policy functions must be supplied by
subclasses of <code class="computeroutput"><span class="identifier">indexing_suite</span></code>.
However, a set of indexing_suite subclasses for the standard indexable
STL containers will be provided, In most cases, we can simply use the available
predefined suites. In some cases, we can refine the predefined suites to
suit our needs.
</p>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="topics.indexing_support.index_suite_sub_classes.vector_index_suite"></a><a class="link" href="indexing_support.html#topics.indexing_support.index_suite_sub_classes.vector_index_suite" title="vector_index_suite">vector_index_suite</a>
</h4></div></div></div>
<p>
The <code class="computeroutput"><span class="identifier">vector_indexing_suite</span></code>
class is a predefined <code class="computeroutput"><span class="identifier">indexing_suite</span></code>
derived class designed to wrap <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span></code>
(and <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span></code>-like [i.e. a class with <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span></code> interface]) classes. It provides
all the policies required by the <code class="computeroutput"><span class="identifier">indexing_suite</span></code>.
</p>
<p>
Example usage:
</p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">X</span> <span class="special">{...};</span>
<span class="special">...</span>
<span class="identifier">class_</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">X</span><span class="special">&gt;</span> <span class="special">&gt;(</span><span class="string">"XVec"</span><span class="special">)</span>
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="identifier">vector_indexing_suite</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">X</span><span class="special">&gt;</span> <span class="special">&gt;())</span>
<span class="special">;</span>
</pre>
<p>
XVec is now a full-fledged Python container (see the example in full,
along with its python test).
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="topics.indexing_support.index_suite_sub_classes.map_index_suite"></a><a class="link" href="indexing_support.html#topics.indexing_support.index_suite_sub_classes.map_index_suite" title="map_index_suite">map_index_suite</a>
</h4></div></div></div>
<p>
The <code class="computeroutput"><span class="identifier">map_indexing_suite</span></code>
class is a predefined <code class="computeroutput"><span class="identifier">indexing_suite</span></code>
derived class designed to wrap <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span></code>
(and <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span></code>-like [i.e. a class with <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span></code> interface]) classes. It provides
all the policies required by the <code class="computeroutput"><span class="identifier">indexing_suite</span></code>.
</p>
<p>
Example usage:
</p>
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">X</span> <span class="special">{...};</span>
<span class="special">...</span>
<span class="identifier">class_</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special">&lt;</span><span class="identifier">X</span><span class="special">&gt;</span> <span class="special">&gt;(</span><span class="string">"XMap"</span><span class="special">)</span>
<span class="special">.</span><span class="identifier">def</span><span class="special">(</span><span class="identifier">map_indexing_suite</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special">&lt;</span><span class="identifier">X</span><span class="special">&gt;</span> <span class="special">&gt;())</span>
<span class="special">;</span>
</pre>
<p>
By default indexed elements are returned by proxy. This can be disabled
by supplying <code class="computeroutput"><span class="keyword">true</span></code> in the
<code class="computeroutput"><span class="identifier">NoProxy</span></code> template parameter.
XMap is now a full-fledged Python container (see the example in full,
along with its python test).
</p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="topics.indexing_support.indexing_suite_class"></a><a class="link" href="indexing_support.html#topics.indexing_support.indexing_suite_class" title="indexing_suite class"><code class="computeroutput"><span class="identifier">indexing_suite</span></code> class</a>
</h3></div></div></div>
<div class="toc"><dl class="toc"><dt><span class="section"><a href="indexing_support.html#topics.indexing_support.indexing_suite_class.derivedpolicies">DerivedPolicies</a></span></dt></dl></div>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Template Parameter
</p>
</th>
<th>
<p>
Requirements
</p>
</th>
<th>
<p>
Semantics
</p>
</th>
<th>
<p>
Default
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
Container
</p>
</td>
<td>
<p>
A class type
</p>
</td>
<td>
<p>
The container type to be wrapped to Python.
</p>
</td>
<td>
</td>
</tr>
<tr>
<td>
<p>
DerivedPolicies
</p>
</td>
<td>
<p>
A subclass of indexing_suite
</p>
</td>
<td>
<p>
Derived classes provide the policy hooks. See DerivedPolicies
below.
</p>
</td>
<td>
</td>
</tr>
<tr>
<td>
<p>
NoProxy
</p>
</td>
<td>
<p>
A boolean
</p>
</td>
<td>
<p>
By default indexed elements have Python reference semantics and
are returned by proxy. This can be disabled by supplying true
in the NoProxy template parameter.
</p>
</td>
<td>
<p>
false
</p>
</td>
</tr>
<tr>
<td>
<p>
NoSlice
</p>
</td>
<td>
<p>
A boolean
</p>
</td>
<td>
<p>
Do not allow slicing.
</p>
</td>
<td>
<p>
false
</p>
</td>
</tr>
<tr>
<td>
<p>
Data
</p>
</td>
<td>
</td>
<td>
<p>
The container's data type.
</p>
</td>
<td>
<p>
Container::value_type
</p>
</td>
</tr>
<tr>
<td>
<p>
Index
</p>
</td>
<td>
</td>
<td>
<p>
The container's index type.
</p>
</td>
<td>
<p>
Container::size_type
</p>
</td>
</tr>
<tr>
<td>
<p>
Key
</p>
</td>
<td>
</td>
<td>
<p>
The container's key type.
</p>
</td>
<td>
<p>
Container::value_type
</p>
</td>
</tr>
</tbody>
</table></div>
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">Container</span><span class="special">,</span>
<span class="keyword">class</span> <span class="identifier">DerivedPolicies</span><span class="special">,</span>
<span class="keyword">bool</span> <span class="identifier">NoProxy</span> <span class="special">=</span> <span class="keyword">false</span><span class="special">,</span>
<span class="keyword">bool</span> <span class="identifier">NoSlice</span> <span class="special">=</span> <span class="keyword">false</span><span class="special">,</span>
<span class="keyword">class</span> <span class="identifier">Data</span> <span class="special">=</span> <span class="keyword">typename</span> <span class="identifier">Container</span><span class="special">::</span><span class="identifier">value_type</span><span class="special">,</span>
<span class="keyword">class</span> <span class="identifier">Index</span> <span class="special">=</span> <span class="keyword">typename</span> <span class="identifier">Container</span><span class="special">::</span><span class="identifier">size_type</span><span class="special">,</span>
<span class="keyword">class</span> <span class="identifier">Key</span> <span class="special">=</span> <span class="keyword">typename</span> <span class="identifier">Container</span><span class="special">::</span><span class="identifier">value_type</span><span class="special">&gt;</span>
<span class="keyword">class</span> <span class="identifier">indexing_suite</span> <span class="special">:</span> <span class="identifier">unspecified</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="identifier">indexing_suite</span><span class="special">();</span> <span class="comment">// default constructor</span>
<span class="special">}</span>
</pre>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="topics.indexing_support.indexing_suite_class.derivedpolicies"></a><a class="link" href="indexing_support.html#topics.indexing_support.indexing_suite_class.derivedpolicies" title="DerivedPolicies">DerivedPolicies</a>
</h4></div></div></div>
<p>
Derived classes provide the hooks needed by the indexing_suite:
</p>
<pre class="programlisting"><span class="identifier">data_type</span><span class="special">&amp;</span>
<span class="identifier">get_item</span><span class="special">(</span><span class="identifier">Container</span><span class="special">&amp;</span> <span class="identifier">container</span><span class="special">,</span> <span class="identifier">index_type</span> <span class="identifier">i</span><span class="special">);</span>
<span class="keyword">static</span> <span class="identifier">object</span>
<span class="identifier">get_slice</span><span class="special">(</span><span class="identifier">Container</span><span class="special">&amp;</span> <span class="identifier">container</span><span class="special">,</span> <span class="identifier">index_type</span> <span class="identifier">from</span><span class="special">,</span> <span class="identifier">index_type</span> <span class="identifier">to</span><span class="special">);</span>
<span class="keyword">static</span> <span class="keyword">void</span>
<span class="identifier">set_item</span><span class="special">(</span><span class="identifier">Container</span><span class="special">&amp;</span> <span class="identifier">container</span><span class="special">,</span> <span class="identifier">index_type</span> <span class="identifier">i</span><span class="special">,</span> <span class="identifier">data_type</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">v</span><span class="special">);</span>
<span class="keyword">static</span> <span class="keyword">void</span>
<span class="identifier">set_slice</span><span class="special">(</span>
<span class="identifier">Container</span><span class="special">&amp;</span> <span class="identifier">container</span><span class="special">,</span> <span class="identifier">index_type</span> <span class="identifier">from</span><span class="special">,</span>
<span class="identifier">index_type</span> <span class="identifier">to</span><span class="special">,</span> <span class="identifier">data_type</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">v</span>
<span class="special">);</span>
<span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">Iter</span><span class="special">&gt;</span>
<span class="keyword">static</span> <span class="keyword">void</span>
<span class="identifier">set_slice</span><span class="special">(</span><span class="identifier">Container</span><span class="special">&amp;</span> <span class="identifier">container</span><span class="special">,</span> <span class="identifier">index_type</span> <span class="identifier">from</span><span class="special">,</span>
<span class="identifier">index_type</span> <span class="identifier">to</span><span class="special">,</span> <span class="identifier">Iter</span> <span class="identifier">first</span><span class="special">,</span> <span class="identifier">Iter</span> <span class="identifier">last</span>
<span class="special">);</span>
<span class="keyword">static</span> <span class="keyword">void</span>
<span class="identifier">delete_item</span><span class="special">(</span><span class="identifier">Container</span><span class="special">&amp;</span> <span class="identifier">container</span><span class="special">,</span> <span class="identifier">index_type</span> <span class="identifier">i</span><span class="special">);</span>
<span class="keyword">static</span> <span class="keyword">void</span>
<span class="identifier">delete_slice</span><span class="special">(</span><span class="identifier">Container</span><span class="special">&amp;</span> <span class="identifier">container</span><span class="special">,</span> <span class="identifier">index_type</span> <span class="identifier">from</span><span class="special">,</span> <span class="identifier">index_type</span> <span class="identifier">to</span><span class="special">);</span>
<span class="keyword">static</span> <span class="identifier">size_t</span>
<span class="identifier">size</span><span class="special">(</span><span class="identifier">Container</span><span class="special">&amp;</span> <span class="identifier">container</span><span class="special">);</span>
<span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">static</span> <span class="keyword">bool</span>
<span class="identifier">contains</span><span class="special">(</span><span class="identifier">Container</span><span class="special">&amp;</span> <span class="identifier">container</span><span class="special">,</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">val</span><span class="special">);</span>
<span class="keyword">static</span> <span class="identifier">index_type</span>
<span class="identifier">convert_index</span><span class="special">(</span><span class="identifier">Container</span><span class="special">&amp;</span> <span class="identifier">container</span><span class="special">,</span> <span class="identifier">PyObject</span><span class="special">*</span> <span class="identifier">i</span><span class="special">);</span>
<span class="keyword">static</span> <span class="identifier">index_type</span>
<span class="identifier">adjust_index</span><span class="special">(</span><span class="identifier">index_type</span> <span class="identifier">current</span><span class="special">,</span> <span class="identifier">index_type</span> <span class="identifier">from</span><span class="special">,</span>
<span class="identifier">index_type</span> <span class="identifier">to</span><span class="special">,</span> <span class="identifier">size_type</span> <span class="identifier">len</span><span class="special">);</span>
</pre>
<p>
Most of these policies are self explanatory. However, convert_index and
adjust_index deserve some explanation.
</p>
<p>
convert_index converts a Python index into a C++ index that the container
can handle. For instance, negative indexes in Python, by convention,
start counting from the right(e.g. C<span class="strikethrough">1</span>
indexes the rightmost element in C). convert_index should handle the
necessary conversion for the C++ container (e.g. convert -1 to C.size()-1).
convert_index should also be able to convert the type of the index (A
dynamic Python type) to the actual type that the C++ container expects.
</p>
<p>
When a container expands or contracts, held indexes to its elements must
be adjusted to follow the movement of data. For instance, if we erase
3 elements, starting from index 0 from a 5 element vector, what used
to be at index 4 will now be at index 1:
</p>
<pre class="programlisting"><span class="special">[</span><span class="identifier">a</span><span class="special">][</span><span class="identifier">b</span><span class="special">][</span><span class="identifier">c</span><span class="special">][</span><span class="identifier">d</span><span class="special">][</span><span class="identifier">e</span><span class="special">]</span> <span class="special">---&gt;</span> <span class="special">[</span><span class="identifier">d</span><span class="special">][</span><span class="identifier">e</span><span class="special">]</span>
<span class="special">^</span> <span class="special">^</span>
<span class="number">4</span> <span class="number">1</span>
</pre>
<p>
adjust_index takes care of the adjustment. Given a current index, the
function should return the adjusted index when data in the container
at index from..to is replaced by len elements.
</p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="topics.indexing_support.class_vector_indexing_suite"></a><a class="link" href="indexing_support.html#topics.indexing_support.class_vector_indexing_suite" title="class vector_indexing_suite">class
<code class="computeroutput"><span class="identifier">vector_indexing_suite</span></code></a>
</h3></div></div></div>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Template Parameter
</p>
</th>
<th>
<p>
Requirements
</p>
</th>
<th>
<p>
Semantics
</p>
</th>
<th>
<p>
Default
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
Container
</p>
</td>
<td>
<p>
A class type
</p>
</td>
<td>
<p>
The container type to be wrapped to Python.
</p>
</td>
<td>
</td>
</tr>
<tr>
<td>
<p>
NoProxy
</p>
</td>
<td>
<p>
A boolean
</p>
</td>
<td>
<p>
By default indexed elements have Python reference semantics and
are returned by proxy. This can be disabled by supplying true
in the NoProxy template parameter.
</p>
</td>
<td>
<p>
false
</p>
</td>
</tr>
<tr>
<td>
<p>
DerivedPolicies
</p>
</td>
<td>
<p>
A subclass of indexing_suite
</p>
</td>
<td>
<p>
The vector_indexing_suite may still be derived to further tweak
any of the predefined policies. Static polymorphism through CRTP
(James Coplien. "Curiously Recurring Template Pattern".
C++ Report, Feb. 1995) enables the base indexing_suite class
to call policy function of the most derived class
</p>
</td>
<td>
</td>
</tr>
</tbody>
</table></div>
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">Container</span><span class="special">,</span>
<span class="keyword">bool</span> <span class="identifier">NoProxy</span> <span class="special">=</span> <span class="keyword">false</span><span class="special">,</span>
<span class="keyword">class</span> <span class="identifier">DerivedPolicies</span> <span class="special">=</span> <span class="identifier">unspecified_default</span><span class="special">&gt;</span>
<span class="keyword">class</span> <span class="identifier">vector_indexing_suite</span> <span class="special">:</span> <span class="identifier">unspecified_base</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">Container</span><span class="special">::</span><span class="identifier">value_type</span> <span class="identifier">data_type</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">Container</span><span class="special">::</span><span class="identifier">value_type</span> <span class="identifier">key_type</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">Container</span><span class="special">::</span><span class="identifier">size_type</span> <span class="identifier">index_type</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">Container</span><span class="special">::</span><span class="identifier">size_type</span> <span class="identifier">size_type</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">Container</span><span class="special">::</span><span class="identifier">difference_type</span> <span class="identifier">difference_type</span><span class="special">;</span>
<span class="identifier">data_type</span><span class="special">&amp;</span>
<span class="identifier">get_item</span><span class="special">(</span><span class="identifier">Container</span><span class="special">&amp;</span> <span class="identifier">container</span><span class="special">,</span> <span class="identifier">index_type</span> <span class="identifier">i</span><span class="special">);</span>
<span class="keyword">static</span> <span class="identifier">object</span>
<span class="identifier">get_slice</span><span class="special">(</span><span class="identifier">Container</span><span class="special">&amp;</span> <span class="identifier">container</span><span class="special">,</span> <span class="identifier">index_type</span> <span class="identifier">from</span><span class="special">,</span> <span class="identifier">index_type</span> <span class="identifier">to</span><span class="special">);</span>
<span class="keyword">static</span> <span class="keyword">void</span>
<span class="identifier">set_item</span><span class="special">(</span><span class="identifier">Container</span><span class="special">&amp;</span> <span class="identifier">container</span><span class="special">,</span> <span class="identifier">index_type</span> <span class="identifier">i</span><span class="special">,</span> <span class="identifier">data_type</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">v</span><span class="special">);</span>
<span class="keyword">static</span> <span class="keyword">void</span>
<span class="identifier">set_slice</span><span class="special">(</span><span class="identifier">Container</span><span class="special">&amp;</span> <span class="identifier">container</span><span class="special">,</span> <span class="identifier">index_type</span> <span class="identifier">from</span><span class="special">,</span>
<span class="identifier">index_type</span> <span class="identifier">to</span><span class="special">,</span> <span class="identifier">data_type</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">v</span><span class="special">);</span>
<span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">Iter</span><span class="special">&gt;</span>
<span class="keyword">static</span> <span class="keyword">void</span>
<span class="identifier">set_slice</span><span class="special">(</span><span class="identifier">Container</span><span class="special">&amp;</span> <span class="identifier">container</span><span class="special">,</span> <span class="identifier">index_type</span> <span class="identifier">from</span><span class="special">,</span>
<span class="identifier">index_type</span> <span class="identifier">to</span><span class="special">,</span> <span class="identifier">Iter</span> <span class="identifier">first</span><span class="special">,</span> <span class="identifier">Iter</span> <span class="identifier">last</span><span class="special">);</span>
<span class="keyword">static</span> <span class="keyword">void</span>
<span class="identifier">delete_item</span><span class="special">(</span><span class="identifier">Container</span><span class="special">&amp;</span> <span class="identifier">container</span><span class="special">,</span> <span class="identifier">index_type</span> <span class="identifier">i</span><span class="special">);</span>
<span class="keyword">static</span> <span class="keyword">void</span>
<span class="identifier">delete_slice</span><span class="special">(</span><span class="identifier">Container</span><span class="special">&amp;</span> <span class="identifier">container</span><span class="special">,</span> <span class="identifier">index_type</span> <span class="identifier">from</span><span class="special">,</span> <span class="identifier">index_type</span> <span class="identifier">to</span><span class="special">);</span>
<span class="keyword">static</span> <span class="identifier">size_t</span>
<span class="identifier">size</span><span class="special">(</span><span class="identifier">Container</span><span class="special">&amp;</span> <span class="identifier">container</span><span class="special">);</span>
<span class="keyword">static</span> <span class="keyword">bool</span>
<span class="identifier">contains</span><span class="special">(</span><span class="identifier">Container</span><span class="special">&amp;</span> <span class="identifier">container</span><span class="special">,</span> <span class="identifier">key_type</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">key</span><span class="special">);</span>
<span class="keyword">static</span> <span class="identifier">index_type</span>
<span class="identifier">convert_index</span><span class="special">(</span><span class="identifier">Container</span><span class="special">&amp;</span> <span class="identifier">container</span><span class="special">,</span> <span class="identifier">PyObject</span><span class="special">*</span> <span class="identifier">i</span><span class="special">);</span>
<span class="keyword">static</span> <span class="identifier">index_type</span>
<span class="identifier">adjust_index</span><span class="special">(</span><span class="identifier">index_type</span> <span class="identifier">current</span><span class="special">,</span> <span class="identifier">index_type</span> <span class="identifier">from</span><span class="special">,</span>
<span class="identifier">index_type</span> <span class="identifier">to</span><span class="special">,</span> <span class="identifier">size_type</span> <span class="identifier">len</span><span class="special">);</span>
<span class="special">};</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="topics.indexing_support.class_map_indexing_suite"></a><a class="link" href="indexing_support.html#topics.indexing_support.class_map_indexing_suite" title="class map_indexing_suite">class
<code class="computeroutput"><span class="identifier">map_indexing_suite</span></code></a>
</h3></div></div></div>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Template Parameter
</p>
</th>
<th>
<p>
Requirements
</p>
</th>
<th>
<p>
Semantics
</p>
</th>
<th>
<p>
Default
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
Container
</p>
</td>
<td>
<p>
A class type
</p>
</td>
<td>
<p>
The container type to be wrapped to Python.
</p>
</td>
<td>
</td>
</tr>
<tr>
<td>
<p>
NoProxy
</p>
</td>
<td>
<p>
A boolean
</p>
</td>
<td>
<p>
By default indexed elements have Python reference semantics and
are returned by proxy. This can be disabled by supplying true
in the NoProxy template parameter.
</p>
</td>
<td>
<p>
false
</p>
</td>
</tr>
<tr>
<td>
<p>
DerivedPolicies
</p>
</td>
<td>
<p>
A subclass of indexing_suite
</p>
</td>
<td>
<p>
The vector_indexing_suite may still be derived to further tweak
any of the predefined policies. Static polymorphism through CRTP
(James Coplien. "Curiously Recurring Template Pattern".
C++ Report, Feb. 1995) enables the base indexing_suite class
to call policy function of the most derived class
</p>
</td>
<td>
</td>
</tr>
</tbody>
</table></div>
<pre class="programlisting"><span class="keyword">template</span> <span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">Container</span><span class="special">,</span>
<span class="keyword">bool</span> <span class="identifier">NoProxy</span> <span class="special">=</span> <span class="keyword">false</span><span class="special">,</span>
<span class="keyword">class</span> <span class="identifier">DerivedPolicies</span> <span class="special">=</span> <span class="identifier">unspecified_default</span><span class="special">&gt;</span>
<span class="keyword">class</span> <span class="identifier">map_indexing_suite</span> <span class="special">:</span> <span class="identifier">unspecified_base</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">Container</span><span class="special">::</span><span class="identifier">value_type</span> <span class="identifier">value_type</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">Container</span><span class="special">::</span><span class="identifier">value_type</span><span class="special">::</span><span class="identifier">second_type</span> <span class="identifier">data_type</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">Container</span><span class="special">::</span><span class="identifier">key_type</span> <span class="identifier">key_type</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">Container</span><span class="special">::</span><span class="identifier">key_type</span> <span class="identifier">index_type</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">Container</span><span class="special">::</span><span class="identifier">size_type</span> <span class="identifier">size_type</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="keyword">typename</span> <span class="identifier">Container</span><span class="special">::</span><span class="identifier">difference_type</span> <span class="identifier">difference_type</span><span class="special">;</span>
<span class="keyword">static</span> <span class="identifier">data_type</span><span class="special">&amp;</span>
<span class="identifier">get_item</span><span class="special">(</span><span class="identifier">Container</span><span class="special">&amp;</span> <span class="identifier">container</span><span class="special">,</span> <span class="identifier">index_type</span> <span class="identifier">i</span><span class="special">);</span>
<span class="keyword">static</span> <span class="keyword">void</span>
<span class="identifier">set_item</span><span class="special">(</span><span class="identifier">Container</span><span class="special">&amp;</span> <span class="identifier">container</span><span class="special">,</span> <span class="identifier">index_type</span> <span class="identifier">i</span><span class="special">,</span> <span class="identifier">data_type</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">v</span><span class="special">);</span>
<span class="keyword">static</span> <span class="keyword">void</span>
<span class="identifier">delete_item</span><span class="special">(</span><span class="identifier">Container</span><span class="special">&amp;</span> <span class="identifier">container</span><span class="special">,</span> <span class="identifier">index_type</span> <span class="identifier">i</span><span class="special">);</span>
<span class="keyword">static</span> <span class="identifier">size_t</span>
<span class="identifier">size</span><span class="special">(</span><span class="identifier">Container</span><span class="special">&amp;</span> <span class="identifier">container</span><span class="special">);</span>
<span class="keyword">static</span> <span class="keyword">bool</span>
<span class="identifier">contains</span><span class="special">(</span><span class="identifier">Container</span><span class="special">&amp;</span> <span class="identifier">container</span><span class="special">,</span> <span class="identifier">key_type</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">key</span><span class="special">);</span>
<span class="keyword">static</span> <span class="keyword">bool</span>
<span class="identifier">compare_index</span><span class="special">(</span><span class="identifier">Container</span><span class="special">&amp;</span> <span class="identifier">container</span><span class="special">,</span> <span class="identifier">index_type</span> <span class="identifier">a</span><span class="special">,</span> <span class="identifier">index_type</span> <span class="identifier">b</span><span class="special">);</span>
<span class="keyword">static</span> <span class="identifier">index_type</span>
<span class="identifier">convert_index</span><span class="special">(</span><span class="identifier">Container</span><span class="special">&amp;</span> <span class="identifier">container</span><span class="special">,</span> <span class="identifier">PyObject</span><span class="special">*</span> <span class="identifier">i</span><span class="special">);</span>
<span class="special">};</span>
</pre>
</div>
</div>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2002-2005, 2015 David Abrahams, Stefan Seefeld<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>
</p>
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<a name="topics.pickle_support"></a><a class="link" href="pickle_support.html" title="Pickle support">Pickle support</a>
</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="pickle_support.html#topics.pickle_support.introduction">Introduction</a></span></dt>
<dt><span class="section"><a href="pickle_support.html#topics.pickle_support.the_pickle_interface">The Pickle
Interface</a></span></dt>
<dt><span class="section"><a href="pickle_support.html#topics.pickle_support.example">Example</a></span></dt>
<dt><span class="section"><a href="pickle_support.html#topics.pickle_support.pitfall_and_safety_guard">Pitfall
and Safety Guard</a></span></dt>
<dt><span class="section"><a href="pickle_support.html#topics.pickle_support.practical_advice">Practical Advice</a></span></dt>
<dt><span class="section"><a href="pickle_support.html#topics.pickle_support.light_weight_alternative_pickle_">Light-weight
alternative: pickle support implemented in Python</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="topics.pickle_support.introduction"></a><a class="link" href="pickle_support.html#topics.pickle_support.introduction" title="Introduction">Introduction</a>
</h3></div></div></div>
<p>
Pickle is a Python module for object serialization, also known as persistence,
marshalling, or flattening.
</p>
<p>
It is often necessary to save and restore the contents of an object to
a file. One approach to this problem is to write a pair of functions that
read and write data from a file in a special format. A powerful alternative
approach is to use Python's pickle module. Exploiting Python's ability
for introspection, the pickle module recursively converts nearly arbitrary
Python objects into a stream of bytes that can be written to a file.
</p>
<p>
The Boost Python Library supports the pickle module through the interface
as described in detail in the <a href="https://docs.python.org/2/library/pickle.html" target="_top">Python
Library Reference for pickle</a>. This interface involves the special
methods <code class="computeroutput"><span class="identifier">__getinitargs__</span></code>,
<code class="computeroutput"><span class="identifier">__getstate__</span></code> and <code class="computeroutput"><span class="identifier">__setstate__</span></code> as described in the following.
Note that <code class="computeroutput"><span class="identifier">Boost</span><span class="special">.</span><span class="identifier">Python</span></code> is also fully compatible with
Python's cPickle module.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="topics.pickle_support.the_pickle_interface"></a><a class="link" href="pickle_support.html#topics.pickle_support.the_pickle_interface" title="The Pickle Interface">The Pickle
Interface</a>
</h3></div></div></div>
<p>
At the user level, the Boost.Python pickle interface involves three special
methods:
</p>
<div class="variablelist">
<p class="title"><b></b></p>
<dl class="variablelist">
<dt><span class="term">__getinitargs__</span></dt>
<dd>
<p>
When an instance of a Boost.Python extension class is pickled, the
pickler tests if the instance has a <code class="computeroutput"><span class="identifier">__getinitargs__</span></code>
method. This method must return a Python <code class="computeroutput"><span class="identifier">tuple</span></code>
(it is most convenient to use a <a class="link" href="../object_wrappers/boost_python_tuple_hpp.html#object_wrappers.boost_python_tuple_hpp.class_tuple" title="Class tuple"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">tuple</span></code></a>). When the instance
is restored by the unpickler, the contents of this tuple are used
as the arguments for the class constructor.
</p>
<p>
If <code class="computeroutput"><span class="identifier">__getinitargs__</span></code>
is not defined, <code class="computeroutput"><span class="identifier">pickle</span><span class="special">.</span><span class="identifier">load</span></code>
will call the constructor (<code class="computeroutput"><span class="identifier">__init__</span></code>)
without arguments; i.e., the object must be default-constructible.
</p>
</dd>
<dt><span class="term">__getstate__</span></dt>
<dd><p>
When an instance of a <code class="computeroutput"><span class="identifier">Boost</span><span class="special">.</span><span class="identifier">Python</span></code>
extension class is pickled, the pickler tests if the instance has
a <code class="computeroutput"><span class="identifier">__getstate__</span></code> method.
This method should return a Python object representing the state
of the instance.
</p></dd>
<dt><span class="term">__setstate__</span></dt>
<dd><p>
When an instance of a <code class="computeroutput"><span class="identifier">Boost</span><span class="special">.</span><span class="identifier">Python</span></code>
extension class is restored by the unpickler (<code class="computeroutput"><span class="identifier">pickle</span><span class="special">.</span><span class="identifier">load</span></code>),
it is first constructed using the result of <code class="computeroutput"><span class="identifier">__getinitargs__</span></code>
as arguments (see above). Subsequently the unpickler tests if the
new instance has a <code class="computeroutput"><span class="identifier">__setstate__</span></code>
method. If so, this method is called with the result of <code class="computeroutput"><span class="identifier">__getstate__</span></code> (a Python object)
as the argument.
</p></dd>
</dl>
</div>
<p>
The three special methods described above may be <code class="computeroutput"><span class="special">.</span><span class="identifier">def</span><span class="special">()</span></code>'ed
individually by the user. However, <code class="computeroutput"><span class="identifier">Boost</span><span class="special">.</span><span class="identifier">Python</span></code>
provides an easy to use high-level interface via the <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">pickle_suite</span></code>
class that also enforces consistency: <code class="computeroutput"><span class="identifier">__getstate__</span></code>
and <code class="computeroutput"><span class="identifier">__setstate__</span></code> must be
defined as pairs. Use of this interface is demonstrated by the following
examples.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="topics.pickle_support.example"></a><a class="link" href="pickle_support.html#topics.pickle_support.example" title="Example">Example</a>
</h3></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="pickle_support.html#topics.pickle_support.example.pickle1_cpp">pickle1.cpp</a></span></dt>
<dt><span class="section"><a href="pickle_support.html#topics.pickle_support.example.pickle2_cpp">pickle2.cpp</a></span></dt>
<dt><span class="section"><a href="pickle_support.html#topics.pickle_support.example.pickle3_cpp">pickle3.cpp</a></span></dt>
</dl></div>
<p>
There are three files in <code class="computeroutput"><span class="identifier">python</span><span class="special">/</span><span class="identifier">test</span></code>
that show how to provide pickle support.
</p>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="topics.pickle_support.example.pickle1_cpp"></a><a class="link" href="pickle_support.html#topics.pickle_support.example.pickle1_cpp" title="pickle1.cpp">pickle1.cpp</a>
</h4></div></div></div>
<p>
The C++ class in this example can be fully restored by passing the appropriate
argument to the constructor. Therefore it is sufficient to define the
pickle interface method <code class="computeroutput"><span class="identifier">__getinitargs__</span></code>.
This is done in the following way: Definition of the C++ pickle function:
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">world_pickle_suite</span> <span class="special">:</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">pickle_suite</span>
<span class="special">{</span>
<span class="keyword">static</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">tuple</span>
<span class="identifier">getinitargs</span><span class="special">(</span><span class="identifier">world</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">w</span><span class="special">)</span>
<span class="special">{</span>
<span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">make_tuple</span><span class="special">(</span><span class="identifier">w</span><span class="special">.</span><span class="identifier">get_country</span><span class="special">());</span>
<span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
Establishing the Python binding:
</p>
<pre class="programlisting"><span class="identifier">class_</span><span class="special">&lt;</span><span class="identifier">world</span><span class="special">&gt;(</span><span class="string">"world"</span><span class="special">,</span> <span class="identifier">args</span><span class="special">&lt;</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&amp;&gt;())</span>
<span class="comment">// ...</span>
<span class="special">.</span><span class="identifier">def_pickle</span><span class="special">(</span><span class="identifier">world_pickle_suite</span><span class="special">())</span>
<span class="comment">// ...</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="topics.pickle_support.example.pickle2_cpp"></a><a class="link" href="pickle_support.html#topics.pickle_support.example.pickle2_cpp" title="pickle2.cpp">pickle2.cpp</a>
</h4></div></div></div>
<p>
The C++ class in this example contains member data that cannot be restored
by any of the constructors. Therefore it is necessary to provide the
<code class="computeroutput"><span class="identifier">__getstate__</span></code>/<code class="computeroutput"><span class="identifier">__setstate__</span></code> pair of pickle interface
methods:
</p>
<p>
Definition of the C++ pickle functions:
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">world_pickle_suite</span> <span class="special">:</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">pickle_suite</span>
<span class="special">{</span>
<span class="keyword">static</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">tuple</span>
<span class="identifier">getinitargs</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">world</span><span class="special">&amp;</span> <span class="identifier">w</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// ...</span>
<span class="special">}</span>
<span class="keyword">static</span>
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">tuple</span>
<span class="identifier">getstate</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">world</span><span class="special">&amp;</span> <span class="identifier">w</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// ...</span>
<span class="special">}</span>
<span class="keyword">static</span>
<span class="keyword">void</span>
<span class="identifier">setstate</span><span class="special">(</span><span class="identifier">world</span><span class="special">&amp;</span> <span class="identifier">w</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">tuple</span> <span class="identifier">state</span><span class="special">)</span>
<span class="special">{</span>
<span class="comment">// ...</span>
<span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
Establishing the Python bindings for the entire suite:
</p>
<pre class="programlisting"><span class="identifier">class_</span><span class="special">&lt;</span><span class="identifier">world</span><span class="special">&gt;(</span><span class="string">"world"</span><span class="special">,</span> <span class="identifier">args</span><span class="special">&lt;</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&amp;&gt;())</span>
<span class="comment">// ...</span>
<span class="special">.</span><span class="identifier">def_pickle</span><span class="special">(</span><span class="identifier">world_pickle_suite</span><span class="special">())</span>
<span class="comment">// ...</span>
</pre>
<p>
For simplicity, the <code class="computeroutput"><span class="identifier">__dict__</span></code>
is not included in the result of <code class="computeroutput"><span class="identifier">__getstate__</span></code>.
This is not generally recommended, but a valid approach if it is anticipated
that the object's <code class="computeroutput"><span class="identifier">__dict__</span></code>
will always be empty. Note that the safety guard described below will
catch the cases where this assumption is violated.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="topics.pickle_support.example.pickle3_cpp"></a><a class="link" href="pickle_support.html#topics.pickle_support.example.pickle3_cpp" title="pickle3.cpp">pickle3.cpp</a>
</h4></div></div></div>
<p>
This example is similar to pickle2.cpp. However, the object's <code class="computeroutput"><span class="identifier">__dict__</span></code> is included in the result
of <code class="computeroutput"><span class="identifier">__getstate__</span></code>. This
requires a little more code but is unavoidable if the object's <code class="computeroutput"><span class="identifier">__dict__</span></code> is not always empty.
</p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="topics.pickle_support.pitfall_and_safety_guard"></a><a class="link" href="pickle_support.html#topics.pickle_support.pitfall_and_safety_guard" title="Pitfall and Safety Guard">Pitfall
and Safety Guard</a>
</h3></div></div></div>
<p>
The pickle protocol described above has an important pitfall that the end
user of a Boost.Python extension module might not be aware of:
</p>
<p>
<span class="bold"><strong><code class="computeroutput"><span class="identifier">__getstate__</span></code>
is defined and the instance's <code class="computeroutput"><span class="identifier">__dict__</span></code>
is not empty.</strong></span>
</p>
<p>
The author of a <code class="computeroutput"><span class="identifier">Boost</span><span class="special">.</span><span class="identifier">Python</span></code> extension class might provide
a <code class="computeroutput"><span class="identifier">__getstate__</span></code> method without
considering the possibilities that: * his class is used in Python as a
base class. Most likely the <code class="computeroutput"><span class="identifier">__dict__</span></code>
of instances of the derived class needs to be pickled in order to restore
the instances correctly. * the user adds items to the instance's <code class="computeroutput"><span class="identifier">__dict__</span></code> directly. Again, the <code class="computeroutput"><span class="identifier">__dict__</span></code> of the instance then needs to
be pickled.
</p>
<p>
To alert the user to this highly unobvious problem, a safety guard is provided.
If <code class="computeroutput"><span class="identifier">__getstate__</span></code> is defined
and the instance's <code class="computeroutput"><span class="identifier">__dict__</span></code>
is not empty, <code class="computeroutput"><span class="identifier">Boost</span><span class="special">.</span><span class="identifier">Python</span></code> tests if the class has an attribute
<code class="computeroutput"><span class="identifier">__getstate_manages_dict__</span></code>.
An exception is raised if this attribute is not defined:
</p>
<pre class="programlisting"><span class="identifier">RuntimeError</span><span class="special">:</span> <span class="identifier">Incomplete</span> <span class="identifier">pickle</span> <span class="identifier">support</span> <span class="special">(</span><span class="identifier">__getstate_manages_dict__</span> <span class="keyword">not</span> <span class="identifier">set</span><span class="special">)</span>
</pre>
<p>
To resolve this problem, it should first be established that the <code class="computeroutput"><span class="identifier">__getstate__</span></code> and <code class="computeroutput"><span class="identifier">__setstate__</span></code>
methods manage the instances's <code class="computeroutput"><span class="identifier">__dict__</span></code>
correctly. Note that this can be done either at the C++ or the Python level.
Finally, the safety guard should intentionally be overridden. E.g. in C++
(from pickle3.cpp):
</p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">world_pickle_suite</span> <span class="special">:</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">pickle_suite</span>
<span class="special">{</span>
<span class="comment">// ...</span>
<span class="keyword">static</span> <span class="keyword">bool</span> <span class="identifier">getstate_manages_dict</span><span class="special">()</span> <span class="special">{</span> <span class="keyword">return</span> <span class="keyword">true</span><span class="special">;</span> <span class="special">}</span>
<span class="special">};</span>
</pre>
<p>
Alternatively in Python:
</p>
<pre class="programlisting"><span class="identifier">import</span> <span class="identifier">your_bpl_module</span>
<span class="keyword">class</span> <span class="identifier">your_class</span><span class="special">(</span><span class="identifier">your_bpl_module</span><span class="special">.</span><span class="identifier">your_class</span><span class="special">):</span>
<span class="identifier">__getstate_manages_dict__</span> <span class="special">=</span> <span class="number">1</span>
<span class="identifier">def</span> <span class="identifier">__getstate__</span><span class="special">(</span><span class="identifier">self</span><span class="special">):</span>
<span class="preprocessor"># your</span> <span class="identifier">code</span> <span class="identifier">here</span>
<span class="identifier">def</span> <span class="identifier">__setstate__</span><span class="special">(</span><span class="identifier">self</span><span class="special">,</span> <span class="identifier">state</span><span class="special">):</span>
<span class="preprocessor"># your</span> <span class="identifier">code</span> <span class="identifier">here</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="topics.pickle_support.practical_advice"></a><a class="link" href="pickle_support.html#topics.pickle_support.practical_advice" title="Practical Advice">Practical Advice</a>
</h3></div></div></div>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
In <code class="computeroutput"><span class="identifier">Boost</span><span class="special">.</span><span class="identifier">Python</span></code> extension modules with many
extension classes, providing complete pickle support for all classes
would be a significant overhead. In general complete pickle support
should only be implemented for extension classes that will eventually
be pickled.
</li>
<li class="listitem">
Avoid using <code class="computeroutput"><span class="identifier">__getstate__</span></code>
if the instance can also be reconstructed by way of <code class="computeroutput"><span class="identifier">__getinitargs__</span></code>.
This automatically avoids the pitfall described above.
</li>
<li class="listitem">
If <code class="computeroutput"><span class="identifier">__getstate__</span></code> is
required, include the instance's <code class="computeroutput"><span class="identifier">__dict__</span></code>
in the Python object that is returned.
</li>
</ul></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="topics.pickle_support.light_weight_alternative_pickle_"></a><a class="link" href="pickle_support.html#topics.pickle_support.light_weight_alternative_pickle_" title="Light-weight alternative: pickle support implemented in Python">Light-weight
alternative: pickle support implemented in Python</a>
</h3></div></div></div>
<p>
The pickle4.cpp example demonstrates an alternative technique for implementing
pickle support. First we direct Boost.Python via the class_::enable_pickling()
member function to define only the basic attributes required for pickling:
</p>
<pre class="programlisting"><span class="identifier">class_</span><span class="special">&lt;</span><span class="identifier">world</span><span class="special">&gt;(</span><span class="string">"world"</span><span class="special">,</span> <span class="identifier">args</span><span class="special">&lt;</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&amp;&gt;())</span>
<span class="comment">// ...</span>
<span class="special">.</span><span class="identifier">enable_pickling</span><span class="special">()</span>
<span class="comment">// ...</span>
</pre>
<p>
This enables the standard Python pickle interface as described in the Python
documentation. By "injecting" a <code class="computeroutput"><span class="identifier">__getinitargs__</span></code>
method into the definition of the wrapped class we make all instances pickleable:
</p>
<pre class="programlisting"><span class="preprocessor"># import</span> <span class="identifier">the</span> <span class="identifier">wrapped</span> <span class="identifier">world</span> <span class="keyword">class</span>
<span class="identifier">from</span> <span class="identifier">pickle4_ext</span> <span class="identifier">import</span> <span class="identifier">world</span>
<span class="preprocessor"># definition</span> <span class="identifier">of</span> <span class="identifier">__getinitargs__</span>
<span class="identifier">def</span> <span class="identifier">world_getinitargs</span><span class="special">(</span><span class="identifier">self</span><span class="special">):</span>
<span class="keyword">return</span> <span class="special">(</span><span class="identifier">self</span><span class="special">.</span><span class="identifier">get_country</span><span class="special">(),)</span>
<span class="preprocessor"># now</span> <span class="identifier">inject</span> <span class="identifier">__getinitargs__</span> <span class="special">(</span><span class="identifier">Python</span> <span class="identifier">is</span> <span class="identifier">a</span> <span class="identifier">dynamic</span> <span class="identifier">language</span><span class="special">!)</span>
<span class="identifier">world</span><span class="special">.</span><span class="identifier">__getinitargs__</span> <span class="special">=</span> <span class="identifier">world_getinitargs</span>
</pre>
<p>
See also the tutorial section on injecting additional methods from Python.
</p>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright &#169; 2002-2005, 2015 David Abrahams, Stefan Seefeld<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>
</p>
</div></td>
</tr></table>
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