Re: hash_map insertion question

Oncaphillis <>
Mon, 14 Jul 2008 14:40:39 CST


Is it possible (or easily make it possible) to insert into a hash_map
if I've already calculated a hash value for my key? Because of the
way my application works, prior to calling the hash_map insert, I've
already ran my key through a hash function for something else. I
would like to save some cycles by using it for my hash_map insert. I
realize I can send in a hash function with the creation of my hash_map
but tracking the callback to the insert seems like more trouble then
it's worth. I am using Visual Studio's 2005 STL.


hash_map isn't a part of the STL. I only know the gnu implementation
which differs from the VS implementation. Nevertheless they both follow
the design that you may pass a functor class responsible for calculating
the hash value as a std::size_t. At least the gnu implementation
does not only call the functor during <key,value> insertion but it
also gets called during iteration over the container and value erasure.
So the hassle of trying to poke inside of the hash_map for
insertion does not really solve your problem.

If your hash function is very time-consuming it might be worth to store
the hash value within the key itself. So you might do something like.

  myKey k;

The the functor simply becomes:

   std::size_t operator()(const myKey &k) const {
     return k.getHash();

That's a question of time vs. memory traitoff, but if your key
data is very complex this approach also helps you in speeding up
other things like operator==() for example:

   bool operator==(const myKey & k) const {
     bool r=false;
     if(getHash()!=k.getHash()) {
       return r;

     // Otherwise do what ever is needed to
     // compare the key data

     return r;



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