iterator/doc/quickbook/shared_container_iterator.qbk

[section:shared_container Shared Container Iterator]

Defined in header [@../../../boost/iterator/shared_container_iterator.hpp `boost/iterator/shared_container_iterator.hpp`].

The purpose of the shared container iterator is to attach the lifetime
of a container to the lifetime of its iterators. In other words, the
container will not be deleted until after all its iterators are
destroyed. The shared container iterator is typically used to
implement functions that return iterators over a range of objects that
only need to exist for the lifetime of the iterators. By returning a
pair of shared iterators from a function, the callee can return a
heap-allocated range of objects whose lifetime is automatically
managed.

The shared container iterator augments an iterator over a shared
container. It maintains a reference count on the shared container. If
only shared container iterators hold references to the container, the
container's lifetime will end when the last shared container iterator
over it is destroyed. In any case, the shared container is guaranteed
to persist beyond the lifetime of all the iterators. In all other
ways, the shared container iterator behaves the same as its base
iterator.

[h2 Synopsis]

  namespace boost {
  namespace iterators {
    template <typename Container>
    class shared_container_iterator;

    template <typename Container>
    shared_container_iterator<Container>
    make_shared_container_iterator(typename Container::iterator base,
      std::shared_ptr<Container> const& container);

    std::pair<
      typename shared_container_iterator<Container>,
      typename shared_container_iterator<Container>
    >
    make_shared_container_range(std::shared_ptr<Container> const& container);

    // Backward compatibility with boost::shared_ptr
    template <typename Container>
    shared_container_iterator<Container>
    make_shared_container_iterator(typename Container::iterator base,
      boost::shared_ptr<Container> const& container);

    std::pair<
      typename shared_container_iterator<Container>,
      typename shared_container_iterator<Container>
    >
    make_shared_container_range(boost::shared_ptr<Container> const& container);
  }
  }

[note `shared_container_iterator` and its factory functions support both
`std::shared_ptr` and `boost::shared_ptr` for a smart pointer that holds
a shared reference to the container. However, the support for `boost::shared_ptr`
comes at a cost of wrapping it in a `std::shared_ptr` internally. This means
that when constructing the iterator from a `boost::shared_ptr`, the construction
will have to allocate memory for `std::shared_ptr` shared state, which may
potentially fail. It is recommended to use `std::shared_ptr` directly.]

[section:shared_container_type The Shared Container Iterator Type]

  template <typename Container> class shared_container_iterator;

The class template `shared_container_iterator` is the shared container
iterator type. The `Container` template type argument must model the
[@http://www.sgi.com/tech/stl/Container.html Container] concept.

[h2 Example]

The following example illustrates how to create an iterator that
regulates the lifetime of a reference counted `std::vector`. Though the
original shared pointer `ints` ceases to exist after `set_range()`
returns, the `shared_counter_iterator` objects maintain references to
the underlying vector and thereby extend the container's lifetime.

[example_link shared_iterator_example1.cpp..`shared_iterator_example1.cpp`]:

  #include <boost/iterator/shared_container_iterator.hpp>
  #include <algorithm>
  #include <iostream>
  #include <vector>
  #include <memory>

  using iterator = boost::iterators::shared_container_iterator< std::vector<int> >;


  void set_range(iterator& i, iterator& end) {
    std::shared_ptr< std::vector<int> > ints(new std::vector<int>());

    ints->push_back(0);
    ints->push_back(1);
    ints->push_back(2);
    ints->push_back(3);
    ints->push_back(4);
    ints->push_back(5);

    i = iterator(ints->begin(), ints);
    end = iterator(ints->end(), ints);
  }


  int main() {
    iterator i, end;

    set_range(i, end);

    std::copy(i, end, std::ostream_iterator<int>(std::cout, ","));
    std::cout.put('\n');

    return 0;
  }

The output from this part is:

[pre
0,1,2,3,4,5,
]

[table Template Parameters
  [[Parameter][Description]]
  [[Container][The type of the container that we wish to iterate over. It must be a model of the Container concept.]]
]

[h2 Concepts]

The `shared_container_iterator` type models the same iterator concept
as the base iterator (`Container::iterator`).

[h2 Operations]

The `shared_container_iterator` type implements the member functions
and operators required of the
[@http://www.sgi.com/tech/stl/RandomAccessIterator.html Random Access
Iterator] concept, though only operations defined for the base
iterator will be valid. In addition it has the following constructors:

  shared_container_iterator(Container::iterator const& it,
                            std::shared_ptr<Container> const& container)

  // Backward compatibility with boost::shared_ptr
  shared_container_iterator(Container::iterator const& it,
                            boost::shared_ptr<Container> const& container)

[endsect]

[section:shared_container_object_generator The Shared Container Iterator Object Generator]

  template <typename Container>
  shared_container_iterator<Container>
  make_shared_container_iterator(Container::iterator base,
                                 std::shared_ptr<Container> const& container)

  // Backward compatibility with boost::shared_ptr
  template <typename Container>
  shared_container_iterator<Container>
  make_shared_container_iterator(Container::iterator base,
                                 boost::shared_ptr<Container> const& container)

This function provides an alternative to directly constructing a
`shared_container_iterator`. Using the object generator, a
`shared_container_iterator` can be created and passed to a function without
explicitly specifying its type.

[h2 Example]

This example, similar to the previous,
uses `make_shared_container_iterator()` to create the iterators.

[example_link shared_iterator_example2.cpp..`shared_iterator_example2.cpp`]:

  #include <boost/iterator/shared_container_iterator.hpp>
  #include <algorithm>
  #include <iterator>
  #include <iostream>
  #include <vector>
  #include <memory>


  template <typename Iterator>
  void print_range_nl(Iterator begin, Iterator end) {
    using val = typename std::iterator_traits<Iterator>::value_type;
    std::copy(begin, end, std::ostream_iterator<val>(std::cout, ","));
    std::cout.put('\n');
  }

  int main() {
    using ints_t = std::shared_ptr< std::vector<int> >;
    {
      ints_t ints(new std::vector<int>());

      ints->push_back(0);
      ints->push_back(1);
      ints->push_back(2);
      ints->push_back(3);
      ints->push_back(4);
      ints->push_back(5);

      print_range_nl(boost::iterators::make_shared_container_iterator(ints->begin(), ints),
                     boost::iterators::make_shared_container_iterator(ints->end(), ints));
    }

    return 0;
  }

Observe that the `shared_container_iterator` type is never explicitly
named. The output from this example is the same as the previous.

[endsect]

[section:shared_container_generator The Shared Container Iterator Range Generator]

  template <typename Container>
  std::pair<
    shared_container_iterator<Container>,
    shared_container_iterator<Container>
  >
  make_shared_container_range(std::shared_ptr<Container> const& container);

  // Backward compatibility with boost::shared_ptr
  template <typename Container>
  std::pair<
    shared_container_iterator<Container>,
    shared_container_iterator<Container>
  >
  make_shared_container_range(boost::shared_ptr<Container> const& container);

Class `shared_container_iterator` is meant primarily to return, using iterators,
a range of values that we can guarantee will be alive as long as the iterators are.
This is a convenience function to do just that. It is functionally equivalent to this:

  std::make_pair(make_shared_container_iterator(container->begin(), container),
                 make_shared_container_iterator(container->end(), container));

[h2 Example]

In the following example, a range of values is returned as a pair of `shared_container_iterator` objects.

[example_link shared_iterator_example3.cpp..`shared_iterator_example3.cpp`]:

  #include <boost/iterator/shared_container_iterator.hpp>
  #include <algorithm>     // for std::copy
  #include <iostream>
  #include <vector>
  #include <memory>
  #include <tuple>         // for std::tie


  using iterator = boost::iterators::shared_container_iterator< std::vector<int> >;

  std::pair<iterator, iterator>
  return_range() {
    std::shared_ptr< std::vector<int> > range(new std::vector<int>());
    range->push_back(0);
    range->push_back(1);
    range->push_back(2);
    range->push_back(3);
    range->push_back(4);
    range->push_back(5);
    return boost::iterators::make_shared_container_range(range);
  }

  int main() {
    iterator i,end;
    std::tie(i, end) = return_range();

    std::copy(i, end, std::ostream_iterator<int>(std::cout, ","));
    std::cout.put('\n');

    return 0;
  }

Though the range object only lives for the duration of the
`return_range` call, the reference counted `std::vector` will live
until `i` and `end` are both destroyed. The output from this example is
the same as the previous two.

[endsect]

[endsect]