Re: [Boost-users] Is MultiIndex right for me?

On Mon, 22 Jul 2013, at 04:42 PM, Sensei wrote:

I was playing with std::unordered_multimap trying to achieve the following.

I need to store values in no particular order. I need to find, however, all values (let's assume they are integers) that satisfy some given criteria that is NOT the simple equality.

For example, I'd like to store integers, and find all integers that are equal modulo p, for instance given two integers u, v, I may write the equality predicate as return (u % p) == (v % p).

This requirement might change (at compile time, of course), for example, in another map I'd like to find all integers that have a particular bit set to 1.

I tried, as I said, with std::unordered_multimap, but I was told (correctly, AFAIK) that the ISO requires that items matching values MUST match also with hash values.

Of course, in my case matching integers do not possess matching hashes.

You can use a custom hash function. Here's an example. I made the modulo dynamic, but if it's a compile time constant, then the container will be a tad bit smaller, and you won't need to pass the hash function and equality predicate as parameters. #include #include <cassert> struct modulo_hash { int p; explicit modulo_hash(int p) : p(p) {} std::size_t operator()(int value) const { std::hash<int> hf; return hf(value % p); } }; struct modulo_equals { int p; explicit modulo_equals(int p) : p(p) {} std::size_t operator()(int x, int y) const { return x % p == y % p; } }; int main() { std::unordered_multimap x( 0, modulo_hash(53), modulo_equals(53)); x.insert(std::make_pair(54, 10)); assert(x.find(1) == x.find(54)); assert(x.find(1)->second == 10); x.insert(std::make_pair(57, 15)); x.insert(std::make_pair(4, 13)); assert(x.count(4) == 2); }