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    <p>I don't know of any implementation in a major library, but it looked like an interesting problem so I wrote a basic implementation. I've only tested it with the test case I present here, so I don't recommend using it without further testing.</p> <p>The problem is a bit trickier than it looks because some of the "inner" containers may be empty and you have to skip over them. This means that advancing the <code>flattening_iterator</code> by one position may actually advance the iterator into the "outer" container by more than one position. Because of this, the <code>flattening_iterator</code> needs to know where the end of the outer range is so that it knows when it needs to stop.</p> <p>This implementation is a forward iterator. A bidirectional iterator would also need to keep track of the beginning of the outer range. The <code>flatten</code> function templates are used to make constructing <code>flattening_iterator</code>s a bit easier.</p> <pre><code>#include &lt;iterator&gt; // A forward 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 OuterIterator::value_type::iterator inner_iterator; typedef std::forward_iterator_tag iterator_category; typedef typename inner_iterator::value_type value_type; typedef typename inner_iterator::difference_type difference_type; typedef typename inner_iterator::pointer pointer; typedef typename inner_iterator::reference reference; flattening_iterator() { } flattening_iterator(outer_iterator it) : outer_it_(it), outer_end_(it) { } flattening_iterator(outer_iterator it, outer_iterator end) : outer_it_(it), outer_end_(end) { if (outer_it_ == outer_end_) { return; } inner_it_ = outer_it_-&gt;begin(); 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; } 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_ != 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(); } } outer_iterator outer_it_; outer_iterator outer_end_; inner_iterator inner_it_; }; template &lt;typename Iterator&gt; flattening_iterator&lt;Iterator&gt; flatten(Iterator it) { return flattening_iterator&lt;Iterator&gt;(it, it); } template &lt;typename Iterator&gt; flattening_iterator&lt;Iterator&gt; flatten(Iterator first, Iterator last) { return flattening_iterator&lt;Iterator&gt;(first, last); } </code></pre> <p>The following is a minimal test stub:</p> <pre><code>#include &lt;algorithm&gt; #include &lt;iostream&gt; #include &lt;set&gt; #include &lt;vector&gt; int main() { // Generate some test data: it looks like this: // { { 0, 1, 2, 3 }, { 4, 5, 6, 7 }, { 8, 9, 10, 11 } } 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++); } // Flatten the data and print all the elements: for (auto it(flatten(v.begin(), v.end())); it != v.end(); ++it) { std::cout &lt;&lt; *it &lt;&lt; ", "; } std::cout &lt;&lt; "\n"; // Or, since the standard library algorithms are awesome: std::copy(flatten(v.begin(), v.end()), flatten(v.end()), std::ostream_iterator&lt;int&gt;(std::cout, ", ")); } </code></pre> <p><em>Like I said at the beginning, I haven't tested this thoroughly. Let me know if you find any bugs and I'll be happy to correct them.</em></p>
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    1. COThanks a lot for taking the time to write that up. I haven't done much testing yet, but the only problem I've had is that gcc complains about "typedef typename OuterIterator" saying it should be "typedef OuterIterator".
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    2. CO@George: Ah, thanks. That was a copy-and-paste error combined with lax standards compliance in Visual C++ :-P.
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    3. CO@George: Heads-up: there was a bug in the `operator==` overload that caused it only to work when comparing against an end iterator; I've corrected it in an edit.
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