boost/python/iterator.hpp

Introduction

<boost/python/iterator.hpp> provides types and functions for creating Python iterators from C++ Containers and Iterators. Note that if your class_ supports random-access iterators, implementing __getitem__ (also known as the Sequence Protocol) may serve you better than using this facility: Python will automatically create an iterator type for you (see iter()), and each access can be range-checked, leaving no possiblity of accessing through an invalidated C++ iterator.

Class template `iterator`

Instances of iterator<C,P> hold a reference to a callable Python object which, when invoked from Python, expects a single argument c convertible to C and creates a Python iterator that traverses [c.begin(), c.end()). The optional CallPolicies P can be used to control how elements are returned during iteration.

In the table below, c is an instance of Container.

Template Parameter	Requirements	Semantics	Default
Container	`[c.begin(),c.end()`) is a valid Iterator range.	The result will convert its argument to c and call c.begin() and c.end() to acquire iterators. To invoke Container's const `begin()` and `end()` functions, make it const.
NextPolicies	A default-constructible model of CallPolicies.	Applied to the resulting iterators' `next()` method.	An unspecified model of CallPolicies which always makes a copy of the result of deferencing the underlying C++ iterator

namespace boost { namespace python
  {
    template <class Container, class NextPolicies = unspecified>
    struct iterator : object
    {
      iterator();
    };
  }}

Class template iterator constructors

iterator()

Effects

Initializes its base class with the result of:

range<NextPolicies>(&iterators<Container>::begin, &iterators<Container>::end)

Postconditions

this->get() points to a Python callable object which creates a Python iterator as described above.

Rationale

Provides an easy way to create iterators for the common case where a C++ class being wrapped provides begin() and end().

Class template `iterators`

A utility class template which provides a way to reliably call its argument's begin() and end() member functions. Note that there is no portable way to take the address of a member function of a C++ standard library container, so iterators<> can be particularly helpful when wrapping them.

In the table below, x is an instance of C.

Required Valid Expression	Type
x.begin()	Convertible to C::const_iterator if C is a const type; convertible to C::iterator otherwise.
x.end()	Convertible to C::const_iterator if C is a const type; convertible to C::iterator otherwise.

namespace boost { namespace python
{
  template <class C>
  struct iterators
  {
      typedef typename C::const_iterator iterator;
      static iterator begin(C& x);
      static iterator end(C& x);
  };
}}

Class template iterators nested types

If C is a const type,

typedef typename C::const_iterator iterator;

Otherwise:

typedef typename C::iterator iterator;

Class template iterators static functions

static iterator begin(C&);

Returns: x.begin()

static iterator end(C&);

Returns: x.end()

Functions

template <class NextPolicies, class Target, class Accessor1, class Accessor2>
object range(Accessor1 start, Accessor2 finish);

template <class NextPolicies, class Accessor1, class Accessor2>
object range(Accessor1 start, Accessor2 finish);

template <class Accessor1, class Accessor2>
object range(Accessor1 start, Accessor2 finish);

Requires

NextPolicies is a default-constructible model of CallPolicies.

Effects

The first form creates a Python callable object which, when invoked, converts its argument to a Target object x, and creates a Python iterator which traverses [bind(start,_1)(x), bind(finish,_1)(x)), applying NextPolicies to the iterator's next() function. The second form is identical to the first, except that Target is deduced from Accessor1 as follows:

If Accessor1 is a function type, Target is the type of its first argument.
If Accessor1 is a data member pointer of the form R (T::*), Target is identical to T.
If Accessor1 is a member function pointer of the form R (T::*)(arguments...) cv-opt, where cv-opt is an optional cv-qualifier, Target is identical to T.

The third form is identical to the second, except that NextPolicies is an unspecified model of CallPolicies which always makes a copy of the result of deferencing the underlying C++ iterator

Rationale

The use of boost::bind() allows C++ iterators to be accessed through functions, member functions or data member pointers. Customization of NextPolicies (e.g. using return_internal_reference) is useful when it is expensive to copy sequence elements of a wrapped class type. Customization of Target is useful when Accessor1 is a function object, or when a base class of the intended target type would otherwise be deduced.

Example

#include <boost/python/module.hpp>
#include <boost/python/class.hpp>

#include <vector>

using namespace boost::python;
BOOST_PYTHON_MODULE(demo)
{
    class_<std::vector<double> >("dvec")
        .def("__iter__", iterator<std::vector<double> >())
        ;
}