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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 { templateContainer> class shared_container_iterator; template Container> shared_container_iterator make_shared_container_iterator(typename Container::iterator base, boost::shared_ptr const& container); std::pair< typename shared_container_iterator , typename shared_container_iterator > make_shared_container_range(boost::shared_ptr const& container); }
templateThe class template shared_container_iterator is the shared container iterator type. The Container template type argument must model the Container concept.class shared_container_iterator;
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" #includeThe output from this part is:#include #include 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; }
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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_ptrconst& container)
templateThis 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.shared_container_iterator make_shared_container_iterator(Container::iterator base, boost::shared_ptr const& container)
#include "shared_container_iterator.hpp" #include "boost/shared_ptr.hpp" #includeObserve that the shared_container_iterator type is never explicitly named. The output from this example is the same as the previous.#include #include #include template <typename Iterator> void print_range_nl (Iterator begin, Iterator end) { typedef typename std::iterator_traits ::value_type val; std::copy(begin,end,std::ostream_iterator (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; }
templateClass 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 tostd::pair< shared_container_iterator , shared_container_iterator > make_shared_container_range(boost::shared_ptr const& container);
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 #includeThough 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.// for std::copy #include#include typedef boost::shared_container_iterator< std::vector<int> > iterator; std::pair 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; }
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