Re: Key strategies for hash_map

Alberto Ganesh Barbati <>
Fri, 18 Jul 2008 13:00:28 CST
devdude ha scritto:


I have a hash_map indexed via a std::string. There are times when I
only have the value payload (e.g. key->value) but need to quickly find
it's key. Is it possible to quickly find the key in a hash_map given
it's paired value?

Assuming it's not, I'll presumably carry the key around in the value
payload. This obviously has a disadvantage of duplicating memory (for
key and payload). I intend to carry millions of records around in
this hash_map. My first thought is to use a shared_ptr<std::string>
as both the key and w/in the value payload, but it would seemingly be
awkward to force consumers to index via shared_ptr<std::string> and
likewise it would be awkward for me to translate incoming std::string
keys to shared_ptr<std::string> keys for millions of lookups/inserts.

Is there a better way?

Use a hash_set<>. This has the inconvenient that it makes the value part
immutable, so you have to litter the code with a const_cast, but if that
doesn't scare you, it works. Specifically:

   class Value { /* whatever */ };

   class Key { /* whatever */ };

   class KeyValuePair : public Key, public Value
      // constructors and adapters

      // be sure to avoid the "keying" methods like operator==
      // and hash() depending on the Value subobject

      Value& GetValue() const
         // ok, this is ugly, but we need it and it doesn't
         // violate the contract with hash_set
         return const_cast<KeyValuePair&>(*this);

   const Key& KeyFromValue(const Value& value)
   #ifdef _DEBUG
     // safe but slow version in debug builds
     return dynamic_cast<const KeyValue&>(value);
     // unsafe but quick version in release builds
     return static_cast<const KeyValue&>(value);

   hash_set<KeyValuePair /* blabla */> container;



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