python/doc/reference/iterator.qbk

[section boost/python/iterator.hpp]
[section Introduction]
<boost/python/iterator.hpp> provides types and functions for creating [@http://www.python.org/doc/current/lib/typeiter.html Python iterators] from C++ Containers and Iterators. Note that if your `class_` supports random-access iterators, implementing [@http://www.python.org/doc/current/ref/sequence-types.html#l2h-128 __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 [@http://www.python.org/doc/current/lib/built-in-funcs.html#l2h-35 `iter()`]), and each access can be range-checked, leaving no possiblity of accessing through an invalidated C++ iterator.
[endsect]
[section 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 [link concepts.callpolicies CallPolicies] `P` can be used to control how elements are returned during iteration.

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

[table
[[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 [link concepts.callpolicies CallPolicies].][Applied to the resulting iterators' `next()` method.][An unspecified model of [link concepts.callpolicies 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();
    };
  }}
``
[endsect]
[section Class template iterator constructors]
``iterator()``

[variablelist
[[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()`.]]
]
[endsect]
[section 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.
[table
[[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);
  };
}}
``
[endsect]
[section Class template iterators nested types]
If C is a const type,``typedef typename C::const_iterator iterator;``
Otherwise: ``typedef typename C::iterator iterator;``
[endsect]
[section Class template iterators static functions]
``static iterator begin(C&);``
[variablelist [[Returns][`x.begin()`]]]
``static iterator end(C&);``
[variablelist [[Returns][`x.end()`]]]
[endsect]
[section 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);
``
[variablelist
[[Requires][ NextPolicies is a default-constructible model of [link concepts.callpolicies 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 [link concepts.callpolicies 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 [link function_invocation_and_creation.models_of_callpolicies.boost_python_return_internal_ref.class_template_return_internal_r 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.]]
]
[endsect]
[section 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> >())
        ;
}
``
[endsect]
[endsect]