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.
namespace boost {
template <typename Container>
class shared_container_iterator;
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);
}
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.
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.
#include "shared_container_iterator.hpp"
#include "boost/shared_ptr.hpp"
#include <algorithm>
#include <iostream>
#include <vector>
typedef boost::shared_container_iterator< std::vector<int> > iterator;
void set_range(iterator& i, iterator& end) {
boost::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,
| Parameter | Description |
|---|---|
| Container | The type of the container that we wish to iterate over. It must be a model of the Container concept. |
shared_container_iterator(Container::iterator const& it,
boost::shared_ptr<Container> const& container)
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.
#include "shared_container_iterator.hpp"
#include "boost/shared_ptr.hpp"
#include <algorithm>
#include <iterator>
#include <iostream>
#include <vector>
template <typename Iterator>
void print_range_nl (Iterator begin, Iterator end) {
typedef typename std::iterator_traits<Iterator>::value_type val;
std::copy(begin,end,std::ostream_iterator<val>(std::cout,","));
std::cout.put('\n');
}
int main() {
typedef boost::shared_ptr< std::vector<int> > ints_t;
{
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::make_shared_container_iterator(ints->begin(),ints),
boost::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.
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 equivalent to
std::make_pair(make_shared_container_iterator(container->begin(),container),
make_shared_container_iterator(container->end(),container));
#include "shared_container_iterator.hpp"
#include "boost/shared_ptr.hpp"
#include "boost/tuple/tuple.hpp" // for boost::tie
#include <algorithm> // for std::copy
#include <iostream>
#include <vector>
typedef boost::shared_container_iterator< std::vector<int> > iterator;
std::pair<iterator,iterator>
return_range() {
boost::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::make_shared_container_range(range);
}
int main() {
iterator i,end;
boost::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.
© Copyright 2003 The Trustees of Indiana University. Use, modification and distribution is subject to the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http:www.boost.org/LICENSE_1_0.txt)