...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
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
).
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, Emulating
container types) 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 __len__
,
__getitem__
, __setitem__
, __delitem__
,
__iter__
and __contains__
.
The goals:
Provide default reference semantics for container element indexing
(__getitem__
) such
that c[i] can be mutable. Require:
val = c[i] c[i].m() val == c[i]
where m is a non-const (mutating) member function (method).
__getitem__
such that subsequent adds and deletes to and from the container will
not result in dangling references (will not crash Python).
lists
,
tuples
) wherever appropriate.
The indexing_suite
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):
Called to implement the built-in function len()
. Should return the length of the
object, an integer >= 0
. Also, an object that doesn't define
a __nonzero__()
method and whose __len__()
method returns zero is considered
to be false in a Boolean context.
Called to implement evaluation of self[key]
. 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 __getitem__()
method. If key is of an inappropriate
type, TypeError
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.]
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.
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.
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().
Iterator objects also need to implement this method; they are required to return themselves. For more information on iterator objects, see Iterator Types in the Python Library Reference.
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.
The indexing_suite
is not
meant to be used as is. A couple of policy functions must be supplied by
subclasses of indexing_suite
.
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.
The vector_indexing_suite
class is a predefined indexing_suite
derived class designed to wrap std::vector
(and std::vector
-like [i.e. a class with std::vector
interface]) classes. It provides
all the policies required by the indexing_suite
.
Example usage:
class X {...}; ... class_<std::vector<X> >("XVec") .def(vector_indexing_suite<std::vector<X> >()) ;
XVec is now a full-fledged Python container (see the example in full, along with its python test).
The map_indexing_suite
class is a predefined indexing_suite
derived class designed to wrap std::map
(and std::map
-like [i.e. a class with std::map
interface]) classes. It provides
all the policies required by the indexing_suite
.
Example usage:
class X {...}; ... class_<std::map<X> >("XMap") .def(map_indexing_suite<std::map<X> >()) ;
By default indexed elements are returned by proxy. This can be disabled
by supplying true
in the
NoProxy
template parameter.
XMap is now a full-fledged Python container (see the example in full,
along with its python test).
Template Parameter |
Requirements |
Semantics |
Default |
---|---|---|---|
Container |
A class type |
The container type to be wrapped to Python. |
|
DerivedPolicies |
A subclass of indexing_suite |
Derived classes provide the policy hooks. See DerivedPolicies below. |
|
NoProxy |
A boolean |
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. |
false |
NoSlice |
A boolean |
Do not allow slicing. |
false |
Data |
The container's data type. |
Container::value_type |
|
Index |
The container's index type. |
Container::size_type |
|
Key |
The container's key type. |
Container::value_type |
template <class Container, class DerivedPolicies, bool NoProxy = false, bool NoSlice = false, class Data = typename Container::value_type, class Index = typename Container::size_type, class Key = typename Container::value_type> class indexing_suite : unspecified { public: indexing_suite(); // default constructor }
Derived classes provide the hooks needed by the indexing_suite:
data_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, data_type const& v); static void set_slice( Container& container, index_type from, index_type to, data_type const& v ); template <class Iter> static void 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);
Most of these policies are self explanatory. However, convert_index and adjust_index deserve some explanation.
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. C1 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.
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:
[a][b][c][d][e] ---> [d][e] ^ ^ 4 1
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.
Template Parameter |
Requirements |
Semantics |
Default |
---|---|---|---|
Container |
A class type |
The container type to be wrapped to Python. |
|
NoProxy |
A boolean |
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. |
false |
DerivedPolicies |
A subclass of indexing_suite |
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 |
template <class Container, bool NoProxy = false, class DerivedPolicies = unspecified_default> class vector_indexing_suite : unspecified_base { public: typedef typename Container::value_type data_type; typedef typename Container::value_type key_type; typedef typename Container::size_type index_type; typedef typename Container::size_type size_type; typedef typename Container::difference_type difference_type; data_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, data_type const& v); static void set_slice(Container& container, index_type from, index_type to, data_type const& v); template <class Iter> static void 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); 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); };
Template Parameter |
Requirements |
Semantics |
Default |
---|---|---|---|
Container |
A class type |
The container type to be wrapped to Python. |
|
NoProxy |
A boolean |
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. |
false |
DerivedPolicies |
A subclass of indexing_suite |
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 |
template <class Container, bool NoProxy = false, class DerivedPolicies = unspecified_default> class map_indexing_suite : unspecified_base { public: typedef typename Container::value_type value_type; typedef typename Container::value_type::second_type data_type; typedef typename Container::key_type key_type; typedef typename Container::key_type index_type; typedef typename Container::size_type size_type; typedef typename Container::difference_type difference_type; static data_type& get_item(Container& container, index_type i); static void set_item(Container& container, index_type i, data_type const& v); static void delete_item(Container& container, index_type i); static size_t size(Container& container); static bool contains(Container& container, key_type const& key); static bool compare_index(Container& container, index_type a, index_type b); static index_type convert_index(Container& container, PyObject* i); };