Find bucket in unordered_map from hash without a key

Question

I am using a std::unordered_map. I have a hash value and a way to determine if a given candidate key is the key that I am looking for, but I do not have an actual key. I want to

Answer 1

You cannot avoid constructing a key, but you can avoid constructing the entire key.

For example, let's say that you have a key class VectorKey that encapsulates an std::vector, and caches the computed hash code. Further suppose that you provide implementations of Hash and KeyEqual that access the cached hash code off your VectorKey, and compare encapsulated vectors for equality. You can define a constructor of VectorKey that always constructs an empty std::vector, and sets the cached hash code to a value passed to the constructor:

class VectorKey{
    int cached_hash;
    std::vector key;
public:
    VectorKey(const std::vector& _key)
    :    key(_key)
    ,    cached_hash(calc_hash(_key)) {
    }
    // *** This is the centerpiece of the solution: *** 
    // *** this constructor effectively lets you access *** 
    // *** a bucket with nothing more than a hash code. *** 
    VectorKey(int hash)
    :    cached_hash(hash) {
    }
    // More code goes here for getting cached_hash
    // and also for checking equality
private:
    int calc_hash(const std::vector& _key) {
         // calculate the hash code based on the vector
    }
};

With a key class like that, you can quickly find buckets by constructing a fake key:

size_type bucketIndex = myHashMap.bucket(VectorKey(precalculated_hash));