Erasing elements from unordered_map in a loop

To comply with C++11 you’re unfortunately a bit limited in how you can tackle this. Your options basically boil down to:

1. Iterate over the unordered_map and build a list of keys to delete like so:

   //std::unordered_map<...> mymap;
   std::vector<decltype(mymap)::key_type> vec;
   for (auto&& i : mymap)
       if (/*compare i*/)
           vec.emplace_back(i.first);
   for (auto&& key : vec)
       mymap.erase(key);
   

2. Iterate over the object and reset if we find something to remove – I’d really only recommend this for small datasets. those of you who feel goto is unconditionally bad, well, this option is arguably bad.

   //std::unordered_map<...> mymap;
   reset:
   for (auto&& i : mymap)
       if (/*compare i*/) {
           mymap.erase(i.first);
           goto reset;
       }
   

3. As a somewhat out there option, you could also just create a new unordered_map and move the elements that you want to keep. This is arguably a good option when you have more to delete than to keep.

   //std::unordered_map<...> mymap;
   decltype(mymap) newmap;
   for (auto&& i : mymap)
       if (/*i is an element we want*/)
           newmap.emplace(std::move(i));
   mymap.swap(newmap);