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### Shared Container Iterator

Defined in header `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.

### 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.

#### 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 Container concept.

### 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.

`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:

    0,1,2,3,4,5,

Table 1.16. 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.

### Concepts

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

### Operations

The `shared_container_iterator` type implements the member functions and operators required of the 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)

#### 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.

### Example

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

`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.

#### 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));

### Example

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

`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.