stl_interfaces/example/back_insert_iterator.cpp

//[ back_insert_iterator
// Copyright (C) 2019 T. Zachary Laine
//
// Distributed under 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)
#include <boost/iterator_facade/iterator_facade.hpp>

#include <algorithm>
#include <vector>

#include <cassert>


// This is an output iterator that holds a reference to a container, and calls
// push_back() on that container when it is written to, just like
// std::back_insert_iterator.  This is not a lot less code to write than the
// implementation of std::back_insert_iterator, but it demonstrates that
// iterator_facade is flexible to handle this odd kind of iterator.
template<typename Container>
struct back_insert_iterator : boost::iterator_facade::iterator_facade<
                                  back_insert_iterator<Container>,
                                  std::output_iterator_tag,
                                  typename Container::value_type,
                                  back_insert_iterator<Container> &>
{
    // Construct from a container.
    explicit back_insert_iterator(Container & c) : c_(std::addressof(c)) {}

    // When writing to *this, copy the into the container via push_back().
    back_insert_iterator & operator=(typename Container::value_type const & v)
    {
        c_->push_back(v);
        return *this;
    }
    // When writing to *this, move the into the container via push_back().
    back_insert_iterator & operator=(typename Container::value_type && v)
    {
        c_->push_back(std::move(v));
        return *this;
    }

private:
    friend boost::iterator_facade::access;
    // Dereferencing *this just returns a reference to *this, so that the
    // expression *it = value uses the operator=() overloads above.
    back_insert_iterator & dereference() { return *this; }
    // There is nowhere to go in next(), so do nothing.
    void next() {}

    Container * c_;
};

// A convenience function that creates a back_insert_iterator<Container>.
template<typename Container>
back_insert_iterator<Container> back_inserter(Container & c)
{
    return back_insert_iterator<Container>(c);
}


int main()
{
    std::vector<int> ints = {{0, 1, 2, 3, 4, 5, 6, 7, 8, 9}};
    std::vector<int> ints_copy;
    std::copy(ints.begin(), ints.end(), ::back_inserter(ints_copy));
    assert(ints_copy == ints);
}
//]