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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<title>
Indexing Support
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<h3>
<a href="../../../../index.htm"><img height="86" width="277"
alt="C++ Boost" src="../../../../boost.png" border=
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<h1 align="center">
<a href="../index.html">Boost.Python</a>
</h1>
<h2> Headers <boost/python/indexing/indexing_suite.hpp><br>
<boost/python/indexing/vector_indexing_suite.hpp></h2>
</td>
</tr>
</table>
<hr>
<h2>
Contents
</h2>
<dl class="page-index">
<dt>
<a href="#introduction">Introduction</a>
</dt>
<dt>
<a href="#interface">Interface</a>
</dt>
<dd>
<dl class="page-index">
<dt>
<a href="#indexing_suite">indexing_suite</a>
</dt>
<dt>
<a href="#indexing_suite_subclasses">indexing_suite
sub-classes</a>
</dt>
<dd>
<dl class="page-index">
<dt>
<a href="#vector_indexing_suite">vector_indexing_suite</a>
</dt>
</dl>
</dd>
</dl>
</dd>
</dl>
<dl>
<dt>
<a href="#indexing_suite_class">indexing_suite class</a>
</dt>
<dt>
<a href="#vector_indexing_suite_class">vector_indexing_suite
class</a>
</dt>
</dl>
<hr>
<h2>
<a name="introduction" id="introduction"></a>Introduction
</h2>
<p>
Indexing is a Boost Python facility for easy exportation of indexable
C++ containers to Python. Indexable containers are containers that
allow random access through the operator[] (e.g. std::vector).
</p>
<p>
While Boost Python 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">Emulating
container types</a>) using Boost Python 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
<strong>__len__</strong>, <strong>__getitem__</strong>,
<strong>__setitem__</strong>, <strong>__delitem__,</strong>
<strong>__iter__</strong> and <strong>__contains</strong>.
</p>
<p>
The goals:
</p>
<ul>
<li>
<div>
Make indexable C++ containers behave exactly as one would expect a
Python container to behave.
</div>
</li>
<li>
Provide default reference semantics for container element indexing
(<tt>__getitem__</tt>) such that <tt>c[i]</tt> can be mutable.
Require:
<div>
<pre>
val = c[i]
c[i].m()
val == c[i]
</pre>
</div>where <tt>m</tt> is a non-const (mutating) member function
(method).
</li>
<li>
Return safe references from <tt>__getitem__</tt> such that subsequent
adds and deletes to and from the container will not result in
dangling references (will not crash Python).
</li>
<li>
Support slice indexes.
</li>
<li>
Accept Python container arguments (e.g. lists, tuples) wherever
appropriate.
</li>
<li>
Allow for extensibility through re-definable policy classes.
</li>
<li>
Provide predefined support for the most common STL and STL like
indexable containers.
</li>
</ul>
<hr>
<h2> <a name="interface"></a>The Boost.Python Indexing Interface</h2>
<h3> <a name="indexing_suite"></a>indexing_suite [ Header <boost/python/indexing/indexing_suite.hpp>
]</h3>
<p>
The <tt>indexing_suite</tt> class is the base protocol 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: <a href=
"http://www.python.org/doc/current/ref/sequence-types.html">Emulating
container types</a>):
</p>
<dl>
<dd>
<dl>
<dt>
<b><a name="l2h-126"><tt class=
"method">__len__</tt></a></b>(<var>self</var>)
</dt>
<dd>
Called to implement the built-in function <tt class=
"function">len()</tt><a name="l2h-134"> </a> Should return
the length of the object, an integer <code>>=</code> 0. Also,
an object that doesn't define a <tt class=
"method">__nonzero__()</tt> method and whose <tt class=
"method">__len__()</tt> method returns zero is considered to be
false in a Boolean context. <a name="l2h-128"> </a>
</dd>
</dl>
<dl>
<dt>
<b><a name="l2h-129"><tt class=
"method">__getitem__</tt></a></b>(<var>self, key</var>)
</dt>
<dd>
Called to implement evaluation of
<code><var>self</var>[<var>key</var>]</code>. For sequence types,
the accepted keys should be integers and slice
objects.<a name="l2h-135"> </a> Note that the special
interpretation of negative indexes (if the class wishes to
emulate a sequence type) is up to the <tt class=
"method">__getitem__()</tt> method. If <var>key</var> is of
an inappropriate type, <tt class="exception">TypeError</tt>
may be raised; if of a value outside the set of indexes for
the sequence (after any special interpretation of negative
values), <tt class="exception">IndexError</tt> should be
raised. <span class="note"><b class="label">Note:</b>
<tt class="keyword">for</tt> loops expect that an <tt class=
"exception">IndexError</tt> will be raised for illegal
indexes to allow proper detection of the end of the
sequence.</span>
</dd>
</dl>
<dl>
<dt>
<b><a name="l2h-130"><tt class=
"method">__setitem__</tt></a></b>(<var>self, key, value</var>)
</dt>
<dd>
Called to implement assignment to
<code><var>self</var>[<var>key</var>]</code>. Same note as for
<tt class="method">__getitem__()</tt>. 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 <var>key</var> values as for the <tt class=
"method">__getitem__()</tt> method.
</dd>
</dl>
<dl>
<dt>
<b><a name="l2h-131"><tt class=
"method">__delitem__</tt></a></b>(<var>self, key</var>)
</dt>
<dd>
Called to implement deletion of
<code><var>self</var>[<var>key</var>]</code>. Same note as for
<tt class="method">__getitem__()</tt>. 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 <var>key</var>
values as for the <tt class="method">__getitem__()</tt> method.
</dd>
</dl>
<dl>
<dt>
<b><a name="l2h-132"><tt class=
"method">__iter__</tt></a></b>(<var>self</var>)
</dt>
<dd>
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 <tt class=
"method">iterkeys()</tt>.
<p>
Iterator objects also need to implement this method; they are
required to return themselves. For more information on iterator
objects, see ``<a class="ulink" href=
"http://www.python.org/doc/current/lib/typeiter.html">Iterator
Types</a>'' in the <em class="citetitle"><a href=
"http://www.python.org/doc/current/lib/lib.html" title=
"Python Library Reference">Python Library Reference</a></em>.
</p>
</dd>
</dl>
<dl>
<dt>
<b><a name="l2h-133"><tt class=
"method">__contains__</tt></a></b>(<var>self, item</var>)
</dt>
<dd>
Called to implement membership test operators. Should return true
if <var>item</var> is in <var>self</var>, false otherwise. For
mapping objects, this should consider the keys of the mapping
rather than the values or the key-item pairs.
</dd>
</dl>
</dd>
</dl>
<h3> <a name="indexing_suite_subclasses"></a>indexing_suite sub-classes</h3>
<p>
The <tt>indexing_suite</tt> is not meant to be used as is. A couple of
policy functions must be supplied by subclasses of
<tt>indexing_suite</tt>. However, a set of <tt>indexing_suite</tt>
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>
<h3> <a name="vector_indexing_suite"></a>vector_indexing_suite [ Header <boost/python/indexing/vector_indexing_suite.hpp>
] </h3>
<p>
The <tt>vector_indexing_suite</tt> class is a predefined
<tt>indexing_suite</tt> derived class designed to wrap
<tt>std::vector</tt> (and <tt>std::vector</tt> like [i.e. a class with
std::vector interface]) classes (currently, this is the only predefined
suite available). It provides all the policies required by the
<tt>indexing_suite</tt>.
</p>
<p>
Example usage:
</p>
<pre>
class X {...};
...
class_<std::vector<X> >("XVec")
.def(vector_indexing_suite<std::vector<X> >())
;
</pre>
<p>
<tt>XVec</tt> is now a full-fledged Python container (see the
<a href="../../test/vector_indexing_suite.cpp">example in full</a>,
along with its <a href="../../test/vector_indexing_suite.py">python
test</a>).
</p>
<hr>
<h2>
<a name="indexing_suite_class"></a>indexing_suite class
</h2>
<h3>
<br>
<tt>indexing_suite<<br>
class Container<br>
, class DerivedPolicies<font color="#007F00"><br>
</font></tt> <tt>,
bool NoProxy<br>
, class Element<br>
, class Key<br>
, class Index</tt>
</h3>
<table width="100%" border="1">
<tr>
<td>
<strong>Template Parameter</strong><br>
</td>
<td>
<strong>Requirements</strong>
</td>
<td>
<strong>Semantics</strong>
</td>
<td>
<strong>Default</strong>
</td>
</tr>
<tr>
<td>
<font color="#007F00"><tt>Container</tt></font>
</td>
<td>
A class type
</td>
<td>
The container type to be wrapped to Python.
</td>
<td>
</td>
</tr>
<tr>
<td>
<font color="#007F00"><tt>DerivedPolicies</tt></font>
</td>
<td>
A subclass of indexing_suite
</td>
<td>
Derived classes provide the policy hooks. See <a href=
"#DerivedPolicies">DerivedPolicies</a> below.
</td>
<td>
</td>
</tr>
<tr>
<td>
<font color="#007F00"><tt>NoProxy</tt></font>
</td>
<td>
A boolean
</td>
<td>
By default indexed elements have Python reference semantics and are
returned by proxy. This can be disabled by supplying
<strong>true</strong> in the <tt>NoProxy</tt> template parameter.
</td>
<td>
false
</td>
</tr>
<tr>
<td>
<font color="#007F00"><tt>Element</tt></font>
</td>
<td>
</td>
<td>
The container's element type.
</td>
<td>
<tt>Container::value_type</tt>
</td>
</tr>
<tr>
<td>
<font color="#007F00"><tt>Key</tt></font>
</td>
<td>
</td>
<td>
The container's key type.
</td>
<td>
<tt>Container::value_type</tt>
</td>
</tr>
<tr>
<td>
<font color="#007F00"><tt>Index</tt></font>
</td>
<td>
</td>
<td>
The container's index type.
</td>
<td>
<tt>Container::size_type</tt>
</td>
</tr>
</table>
<pre>
template <<br> class Container
, class DerivedPolicies
, bool NoProxy = false
, class Element = typename Container::value_type
, class Key = typename Container::value_type
, class Index = typename Container::size_type
><br> class indexing_suite
: unspecified
{
public:
indexing_suite(); // default constructor
}
</pre>
<h2>
<tt><a name="DerivedPolicies"></a>DerivedPolicies</tt>
</h2>
<dl>
<dd>
Derived classes provide the hooks needed by
the<tt>indexing_suite:</tt>
</dd>
</dl>
<pre>
static element_type&
get_item(Container& container, index_type i);
static object
get_slice(Container& container, index_type from, index_type to);
static void
set_item(Container& container, index_type i, element_type const& v);
static void
set_slice(
Container& container, index_type from,
index_type to, element_type const& v
);
template <class Iter>
static void<br> set_slice(Container& container, index_type from,
index_type to, Iter first, Iter last
);
static void
delete_item(Container& container, index_type i);
static void
delete_slice(Container& container, index_type from, index_type to);
static size_t
size(Container& container);
template <class T>
static bool
contains(Container& container, T const& val);
static index_type
convert_index(Container& container, PyObject* i);
static index_type
adjust_index(index_type current, index_type from,
index_type to, size_type len
);
</pre>
<blockquote>
<p>
Most of these policies are self explanatory. <tt>However,
<strong>convert_index</strong></tt> and
<tt><strong>adjust_index</strong></tt> deserve some explanation.
</p>
<p>
<strong><tt>convert_index</tt></strong> 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.
<tt>C[-1]</tt> indexes the rightmost element in <tt>C</tt>).
<strong><tt>convert_index</tt></strong> should handle the necessary
conversion for the C++ container (e.g. convert <tt>-1</tt> to
<tt>C.size()-1</tt>). <tt><strong>convert_index</strong></tt> 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>
[a][b][c][d][e] ---> [d][e]
^ ^
4 1
</pre>
<p>
<strong><tt>adjust_index</tt></strong> takes care of the adjustment.
Given a current index, the function should return the adjusted index
when data in the container at index <tt>from</tt>..<tt>to</tt> is
replaced by <tt>len</tt> elements.
</p>
</blockquote>
<div>
<hr>
<h2>
<a name="vector_indexing_suite_class"></a>vector_indexing_suite class
</h2>
<h3>
Class template <tt><br>
vector_indexing_suite<<br>
class <font color="#007F00">Container</font><br>
, bool <font color="#007F00">NoProxy</font><br>
, class <font color="#007F00">DerivedPolicies</font>></tt>
</h3>
<table width="100%" border="1">
<tr>
<td>
<strong>Template Parameter</strong><br>
</td>
<td>
<strong>Requirements</strong>
</td>
<td>
<strong>Semantics</strong>
</td>
<td>
<strong>Default</strong>
</td>
</tr>
<tr>
<td>
<font color="#007F00"><tt>Container</tt></font>
</td>
<td>
A class type
</td>
<td>
The container type to be wrapped to Python.
</td>
<td>
</td>
</tr>
<tr>
<td>
<font color="#007F00"><tt>NoProxy</tt></font>
</td>
<td>
A boolean
</td>
<td>
By default indexed elements have Python reference semantics and
are returned by proxy. This can be disabled by supplying
<strong>true</strong> in the <tt>NoProxy</tt> template parameter.
</td>
<td>
false
</td>
</tr>
<tr>
<td>
<font color="#007F00"><tt>DerivedPolicies</tt></font>
</td>
<td>
A subclass of indexing_suite
</td>
<td>
The <tt>vector_indexing_suite</tt> 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
<tt>indexing_suite</tt> class to call policy function of the most
derived class
</td>
<td>
</td>
</tr>
</table>
<pre>
template <<br> class Container,<br> bool NoProxy = false,<br> class DerivedPolicies = unspecified_default<br> class vector_indexing_suite<br> : public indexing_suite<Container, DerivedPolicies, NoProxy><br> {<br> public:<br><br> typedef typename Container::value_type element_type;<br> typedef typename Container::value_type key_type;<br> typedef typename Container::size_type index_type;<br> typedef typename Container::size_type size_type;<br> typedef typename Container::difference_type difference_type;<br> <br> static element_type&<br> get_item(Container& container, index_type i);
static object
get_slice(Container& container, index_type from, index_type to);
static void<br> set_item(Container& container, index_type i, element_type const& v);
static void
set_slice(Container& container, index_type from,
index_type to, element_type const& v);
template <class Iter><br> static void<br> set_slice(Container& container, index_type from,<br> index_type to, Iter first, Iter last);
static void
delete_item(Container& container, index_type i);
static void
delete_slice(Container& container, index_type from, index_type to);<br>
static size_t
size(Container& container);
static bool
contains(Container& container, key_type const& key);
static index_type
convert_index(Container& container, PyObject* i);
static index_type
adjust_index(index_type current, index_type from,
index_type to, size_type len);
};
</pre>
<hr>
© Copyright Joel de Guzman 2003. Permission to copy, use, modify,
sell and distribute this document is granted provided this copyright
notice appears in all copies. This document is provided "as is" without
express or implied warranty, and with no claim as to its suitability
for any purpose.
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