Using unordered_map on array of doubles

Question

My main data object is a array of doubles of a length that depends on a specific instantiation of my class. I would like to construct a very simple hash table to store/retr

Answer 1

One way would be to create struct to hold the pointer to the sequence of doubles:

struct DoubleRegion
{
    double* p;
    size_t size;
};

bool operator==(DoubleRegion a, DoubleRegion b)
{
    return a.size == b.size && memcmp(a.p, b.p, a.size) == 0;
}

size_t hash(DoubleRegion dr) 
{
    size_t h = 0;
    for (double* p = dr.p; p != dr.p + dr.size; ++p)
        h ^= hash(*p);
    return h;
}

And then use it:

unordered_map<DoubleRegion, DoubleRegion> cache;

Of course it is your problem to make sure the lifetime of the backing memory is a superset of the lifetime of the DoubleRegion.

Old Answer:

If you don't know until runtime how big the key and value is going to be, use a std::vector:

unordered_map<vector<double>, vector<double>> cache;

If you know at compile-time how big you can use an std::array:

unordered_map<array<double, N>, array<double, N>> cache;

In both cases the default hashing function will work by value as you want, and you do not need to define a custom one.