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POCleaning up bidirectional iterator code
 

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  1. POCleaning up bidirectional iterator code
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    <p>I've modified <a href="https://stackoverflow.com/questions/3623082/flattening-iterator/3623597#3623597">James' flattening iterator</a> to act as a bidirectional iterator if possible, but I don't think my changes are very elegant (particularly relying on a bool to see if the inner iterator has been set). However, I can't seem to come up with a nicer solution. Does anyone have any ideas?</p> <pre><code>#include &lt;algorithm&gt; #include &lt;iostream&gt; #include &lt;set&gt; #include &lt;vector&gt; #include &lt;iterator&gt; #include &lt;type_traits&gt; // An iterator that "flattens" a container of containers. For example, // a vector&lt;vector&lt;int&gt;&gt; containing { { 1, 2, 3 }, { 4, 5, 6 } } is iterated as // a single range, { 1, 2, 3, 4, 5, 6 }. template &lt;typename OuterIterator&gt; class flattening_iterator { public: typedef OuterIterator outer_iterator; typedef typename std::iterator_traits&lt;outer_iterator&gt;::value_type::iterator inner_iterator; typedef typename std::iterator_traits&lt;outer_iterator&gt;::iterator_category outer_category; typedef typename std::iterator_traits&lt;inner_iterator&gt;::iterator_category inner_category; typedef typename std::common_type&lt;outer_category, inner_category&gt;::type common_category; typedef typename std::conditional&lt;std::is_same&lt;common_category, std::random_access_iterator_tag&gt;::value, std::bidirectional_iterator_tag, common_category&gt;::type iterator_category; typedef typename std::iterator_traits&lt;inner_iterator&gt;::value_type value_type; typedef typename std::iterator_traits&lt;inner_iterator&gt;::difference_type difference_type; typedef typename std::iterator_traits&lt;inner_iterator&gt;::pointer pointer; typedef typename std::iterator_traits&lt;inner_iterator&gt;::reference reference; flattening_iterator() { } flattening_iterator(outer_iterator it, outer_iterator begin, outer_iterator end) : outer_it_(it), outer_begin_(begin), outer_end_(end), inner_it_assigned_(false) { if (outer_begin_ == outer_end_) { return; } if (outer_it_ == outer_end_) { return; } inner_it_ = outer_it_-&gt;begin(); inner_it_assigned_ = true; advance_past_empty_inner_containers(); } reference operator*() const { return *inner_it_; } pointer operator-&gt;() const { return &amp;*inner_it_; } flattening_iterator&amp; operator++() { ++inner_it_; if (inner_it_ == outer_it_-&gt;end()) advance_past_empty_inner_containers(); return *this; } flattening_iterator operator++(int) { flattening_iterator it(*this); ++*this; return it; } flattening_iterator&amp; operator--() { if(!inner_it_assigned_) { if(outer_begin_ != outer_end_) { decrement_through_empty_inner_containers(); } return *this; } if(inner_it_ == outer_it_-&gt;begin()) { decrement_through_empty_inner_containers(); } else { --inner_it_; } return *this; } flattening_iterator operator--(int) { flattening_iterator it(*this); --*this; return it; } friend bool operator==(const flattening_iterator&amp; a, const flattening_iterator&amp; b) { if (a.outer_it_ != b.outer_it_) return false; if(a.outer_it_ != a.outer_end_ &amp;&amp; b.outer_it_ != b.outer_end_ &amp;&amp; a.inner_it_assigned_ == false &amp;&amp; b.inner_it_assigned_ == false) return true; if (a.outer_it_ != a.outer_end_ &amp;&amp; b.outer_it_ != b.outer_end_ &amp;&amp; a.inner_it_ != b.inner_it_) return false; return true; } friend bool operator!=(const flattening_iterator&amp; a, const flattening_iterator&amp; b) { return !(a == b); } private: void advance_past_empty_inner_containers() { while (outer_it_ != outer_end_ &amp;&amp; inner_it_ == outer_it_-&gt;end()) { ++outer_it_; if (outer_it_ != outer_end_) inner_it_ = outer_it_-&gt;begin(); } } void decrement_through_empty_inner_containers() { --outer_it_; while(outer_it_ != outer_begin_ &amp;&amp; outer_it_-&gt;begin() == outer_it_-&gt;end()) { --outer_it_; } if(outer_it_-&gt;begin() != outer_it_-&gt;end()) { inner_it_ = --outer_it_-&gt;end(); inner_it_assigned_ = true; } } outer_iterator outer_it_; outer_iterator outer_begin_; outer_iterator outer_end_; inner_iterator inner_it_; bool inner_it_assigned_; }; template &lt;typename Iterator&gt; flattening_iterator&lt;Iterator&gt; flatten(Iterator start, Iterator first, Iterator last) { return flattening_iterator&lt;Iterator&gt;(start, first, last); } template &lt;typename Iterator&gt; std::reverse_iterator&lt;flattening_iterator&lt;Iterator&gt;&gt; flatten_reverse(Iterator start, Iterator first, Iterator last) { return std::reverse_iterator&lt;flattening_iterator&lt;Iterator&gt;&gt;(flatten(start, first, last)); } int main() { std::vector&lt;std::vector&lt;int&gt;&gt; v(3); int i(0); for (auto it(v.begin()); it != v.end(); ++it) { it-&gt;push_back(i++); it-&gt;push_back(i++); it-&gt;push_back(i++); it-&gt;push_back(i++); } v.insert(v.begin(), std::vector&lt;int&gt;()); v.insert(v.begin(), std::vector&lt;int&gt;()); v.insert(v.begin() + 4, std::vector&lt;int&gt;()); v.push_back(std::vector&lt;int&gt;()); v.push_back(std::vector&lt;int&gt;()); for (auto it(flatten(v.begin(), v.begin(), v.end())), end = flatten(v.end(), v.begin(), v.end()); it != end; ++it) { std::cout &lt;&lt; *it &lt;&lt; ", "; } std::cout &lt;&lt; "\n"; for (auto it(flatten_reverse(v.end(), v.begin(), v.end())), end = flatten_reverse(v.begin(), v.begin(), v.end()); it != end; ++it) { std::cout &lt;&lt; *it &lt;&lt; ", "; } std::cout &lt;&lt; "\n"; std::vector&lt;std::vector&lt;int&gt;&gt; v2; for (auto it(flatten(v2.end(), v2.begin(), v2.end())), end = flatten(v2.begin(), v2.begin(), v2.end()); it != end; --it) { std::cout &lt;&lt; *it &lt;&lt; ", "; } std::cout &lt;&lt; "\n"; } </code></pre>
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    1. COI assume this is C++0x, looking at `std::vector<std::vector<int>>` *>>*?
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    2. CO@Kornel: Yes. I'm compiling using gcc 4.5 with -std=c++0x.
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