Something along the lines of
std::vector<std::pair<int, int>>
find_nonmatching_values(const std::unordered_multimap<int, int> & thing,
int key, int value) {
std::vector<std::pair<int, int>> ret;
auto range = thing.equal_range(key);
std::copy_if(range.first, range.second, std::back_inserter(ret),
[value](const std::pair<const int, int> &p)
{ return p.second != value; });
return ret;
}
Demo. Templatizing this code is left as an exercise for the reader.
solved Search variable/item/attribute using STL in c++?