I want to be able to initialize a class that holds a boost::container::flat_map<std::string, std::unique_ptr<AbstractBaseClass>> in the constructor. Example:

class MapHolder {
public:
   MapHolder(boost::container::flat_map<std::string, std::unique_ptr<AbstractBaseClass>> &&map):
   _map(std::move(map)) {}

private:
   boost::container::flat_map<std::string, std::unique_ptr<AbstractBaseClass>> _map;
};

MapHolder m({
   {"test", std::make_unique<ConcreteDerivedClass>("val")}
});

I keep getting the error that it is trying to copy the unique_ptrs instead of moving them. Is there any way I can fix this?