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copied!<p>Even though correct answers have already been posted, I thought I'd add a demo of how you can do this cleanly:</p> <pre><code>template<typename A, typename B> std::pair<B,A> flip_pair(const std::pair<A,B> &p) { return std::pair<B,A>(p.second, p.first); } template<typename A, typename B> std::multimap<B,A> flip_map(const std::map<A,B> &src) { std::multimap<B,A> dst; std::transform(src.begin(), src.end(), std::inserter(dst, dst.begin()), flip_pair<A,B>); return dst; } int main(void) { std::map<int, double> src; ... std::multimap<double, int> dst = flip_map(src); // dst is now sorted by what used to be the value in src! } </code></pre> <hr> <p><strong>Generic Associative Source (requires C++11)</strong></p> <p>If you're using an alternate to <code>std::map</code> for the source associative container (such as <code>std::unordered_map</code>), you could code a separate overload, but in the end the action is still the same, so a generalized associative container using variadic templates can be used for <em>either</em> mapping construct:</p> <pre><code>// flips an associative container of A,B pairs to B,A pairs template<typename A, typename B, template<class,class,class...> class M, class... Args> std::multimap<B,A> flip_map(const M<A,B,Args...> &src) { std::multimap<B,A> dst; std::transform(src.begin(), src.end(), std::inserter(dst, dst.begin()), flip_pair<A,B>); return dst; } </code></pre> <p>This will work for both <code>std::map</code> and <code>std::unordered_map</code> as the source of the flip.</p>

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