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    <p>If i had to do this in the confines of your proposed usage (maintain a key-map for quick counter updates, then dump a sorted result), I would likely use an unordered map and a pointer vector. With this I'm assuming the primary reason you want some indexed-key soluton in the first place is to make data processing significantly quicker when updating counts.</p> <p>In other words, you're looking to get a good speed-bump out of code that does this:</p> <pre><code>++product[ id ]; // increment counter for product 'id' in our keyed-container. </code></pre> <p>But still be able to report output sorted not on id, but rather on the accumulated count of each id. That being said, the following, though a little dense, will do <em>exactly</em> that:</p> <pre><code>#include &lt;iostream&gt; #include &lt;sstream&gt; #include &lt;algorithm&gt; #include &lt;vector&gt; #include &lt;ctime&gt; #include &lt;unordered_map&gt; #include &lt;iterator&gt; #include &lt;iomanip&gt; using namespace std; int main(int argc, char *argv[]) { typedef std::unordered_map&lt;int, int&gt; Products; Products product; // fill with random data for our test. std::srand((unsigned)time(0)); for (int i=1;i&lt;20;++i) { product[i] = rand() % 50 + 1; cout &lt;&lt; setw(3) &lt;&lt; i &lt;&lt; " ==&gt; " &lt;&lt; product[i] &lt;&lt; endl; } cout &lt;&lt; endl; // now setup a one-shot sort. we're using a vector of // pointers to our map value type std::vector&lt;const Products::value_type*&gt; data; data.reserve(product.size()); std::transform(product.begin(), product.end(), std::back_inserter(data), [](const Products::value_type&amp; obj) { return std::addressof(obj); }); // sort the vector by value (second) std::stable_sort(data.begin(), data.end(), [](const Products::value_type* left, const Products::value_type* right) { return left-&gt;second &lt; right-&gt;second; }); // results are in the vector as pointers. the original map is unchanged. for (auto ptr : data) cout &lt;&lt; setw(3) &lt;&lt; ptr-&gt;first &lt;&lt; " ==&gt; " &lt;&lt; ptr-&gt;second &lt;&lt; endl; return EXIT_SUCCESS; }; </code></pre> <p><strong>Sample Run</strong></p> <pre><code> 1 ==&gt; 42 2 ==&gt; 18 3 ==&gt; 35 4 ==&gt; 1 5 ==&gt; 20 6 ==&gt; 25 7 ==&gt; 29 8 ==&gt; 9 9 ==&gt; 13 10 ==&gt; 15 11 ==&gt; 6 12 ==&gt; 46 13 ==&gt; 32 14 ==&gt; 28 15 ==&gt; 12 16 ==&gt; 42 17 ==&gt; 46 18 ==&gt; 43 19 ==&gt; 28 20 ==&gt; 37 4 ==&gt; 1 11 ==&gt; 6 8 ==&gt; 9 15 ==&gt; 12 9 ==&gt; 13 10 ==&gt; 15 2 ==&gt; 18 5 ==&gt; 20 6 ==&gt; 25 14 ==&gt; 28 19 ==&gt; 28 7 ==&gt; 29 13 ==&gt; 32 3 ==&gt; 35 20 ==&gt; 37 1 ==&gt; 42 16 ==&gt; 42 18 ==&gt; 43 12 ==&gt; 46 17 ==&gt; 46 </code></pre> <p>I've used this method in the past because it is swimmingly-efficient for more complex structures that are expensive to copy into temporary containers for sorting. That the ending pointer-vector references the <em>real</em> data in the map is a nicety that, while probably overkill for this specific problem, certainly has reaps of benefits as a general solution.</p> <p>That being said, if all you want is an int-to-int dump, sorted on second-int rather than your map key, this will likewise do the trick, though it does replicate data out of your container to accomplish the end-goal:</p> <pre><code>#include &lt;iostream&gt; #include &lt;sstream&gt; #include &lt;algorithm&gt; #include &lt;vector&gt; #include &lt;ctime&gt; #include &lt;unordered_map&gt; #include &lt;iterator&gt; #include &lt;iomanip&gt; using namespace std; int main(int argc, char *argv[]) { typedef std::unordered_map&lt;int, int&gt; Products; Products product; // fill with random data for our test. std::srand((unsigned)time(0)); for (int i=1;i&lt;20;++i) { product[i] = rand() % 50 + 1; cout &lt;&lt; setw(3) &lt;&lt; i &lt;&lt; " ==&gt; " &lt;&lt; product[i] &lt;&lt; endl; } cout &lt;&lt; endl; // copy the values from the map to a sort bed. std::vector&lt;std::pair&lt;int,int&gt;&gt; vals; std::copy(product.begin(), product.end(), back_inserter(vals)); std::stable_sort(vals.begin(), vals.end(), [](const std::pair&lt;int,int&gt;&amp; left, const std::pair&lt;int,int&gt;&amp; right) { return left.second &lt; right.second; }); // dump to stdout for (auto val : vals) cout &lt;&lt; setw(3) &lt;&lt; val.first &lt;&lt; " ==&gt; " &lt;&lt; val.second &lt;&lt; endl; return EXIT_SUCCESS; } </code></pre> <p><strong>Sample Output</strong></p> <pre><code> 1 ==&gt; 48 2 ==&gt; 30 3 ==&gt; 25 4 ==&gt; 32 5 ==&gt; 34 6 ==&gt; 21 7 ==&gt; 26 8 ==&gt; 6 9 ==&gt; 50 10 ==&gt; 28 11 ==&gt; 50 12 ==&gt; 32 13 ==&gt; 35 14 ==&gt; 17 15 ==&gt; 33 16 ==&gt; 30 17 ==&gt; 13 18 ==&gt; 1 19 ==&gt; 50 18 ==&gt; 1 8 ==&gt; 6 17 ==&gt; 13 14 ==&gt; 17 6 ==&gt; 21 3 ==&gt; 25 7 ==&gt; 26 10 ==&gt; 28 2 ==&gt; 30 16 ==&gt; 30 4 ==&gt; 32 12 ==&gt; 32 15 ==&gt; 33 5 ==&gt; 34 13 ==&gt; 35 1 ==&gt; 48 9 ==&gt; 50 11 ==&gt; 50 19 ==&gt; 50 </code></pre>
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    1. POWhat data structure is better to use for storing and sorting <int, int> structure?
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