Boost
C++ Libraries
...one of the most highly
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Coding Standards
boost::container::flat_multimap
// In header: <boost/container/flat_map.hpp> template<typename InputIterator> flat_multimap(InputIterator, InputIterator);
Deduction guide: allows a flat_multimap to be constructed from the iterator range [first, last), deducing the key and mapped types from the pair value type of InputIterator and using the default comparator and allocator.