std::unordered_map and key built of multiple elements

Question

I'd like to store in map objects wrapping network connections, where key should be IP address + port number.

My question is how should I handle such key with two elements?

I can define std::unordered_map<std::pair<std::string, uint16_t>, Connection>, but I'm not sure how should I implement hash object for it. Only naive implementation comes to my mind:

std::size_t operator() (const pair<std::string, uint16_t>& key) const
{
    std::hash<std::string> ip_hash;
    std::hash<uint16_t>    port_hash;

    return ip_hash (key.first) + port_hash (port.second);
}

I assume that simple addition of two hash values is rather bad idea. Are there any general rules that i should obey to when implementing hash functions?

(I know, that i can build a string from IP address and port number, but I'm just curious).

It would be easier to offer suggestions if you explained what you have in mind for ip_hash and port_hash. Especially the latter -- a port number can serve excellently as its own hash (i.e. hash == identity). If you don't expect many connections from the same IP address you can even take the port out of the equation entirely. — Jon
– Jon, Commented Nov 26, 2013 at 23:32
@Jon Well, I will have multiple connections from the same IP, because this software is going to be used in LAN with couple NATs. — Goofy
– Goofy, Commented Nov 26, 2013 at 23:58
The solution looks ok, but I would combine the values using xor instead of addition since addition will mess up the distribution. — Johan
– Johan, Commented Nov 30, 2017 at 11:15

Praetorian · Accepted Answer · 2013-11-26 23:38:16Z

1

If using boost is an option, boost::hash_combine makes this really easy (otherwise the implementation is available on the linked page).

std::size_t operator()(const pair<std::string, uint16_t>& key) const
{
    std::size_t seed = 0;
    boost::hash_combine(seed, key.first);
    boost::hash_combine(seed, key.second);
    return seed;
}

answered Nov 26, 2013 at 23:38

Praetorian

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yasen · Accepted Answer · 2013-11-26 23:46:00Z

1

A trivial solution will be to append the uint16_t port number to the string, representing the IP address. Then you can use std:unordered_map<string, Connection>.

answered Nov 26, 2013 at 23:46

yasen

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egur · Accepted Answer · 2013-11-27 14:28:06Z

0

Your hashing is mostly fine for non security practices. You can make it more robust this way:

struct ip_port_hash
{
    size_t operator() (const std::pair< std::string, uint16_t >& key) const
    {
        std::hash<const char*> ip_hash;
        std::hash<size_t>      size_t_hash;

        return size_t_hash( ip_hash (key.first.c_str()) + key.second);
    }
};

answered Nov 27, 2013 at 14:28

egur

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