Question

I have struct that store info about ARP packet and I need to save the ARP request and reply packet together. I would like to use std::map for this, the key and mapped value is that ARP struct, and I would be using the find operator to math the reply with request.

My question is how can I say map::find to compare only some members from struct.

Example of what I’m trying to describe

struct arp_packet : public packet_info
{
    u_short type;   // 0x0001 request
                    // 0x0002 reply

    struct ipv4_address ip_sender;
    struct ipv4_address ip_target;
};

I would save all request in map like this

std::map<arp_packet*, arp_packet*>

First the mapped value is NULL but whet the reply packet comes I will use the find method to match it to request.

So how should I accomplist that the map would take as key the arp_packet *, and in find it would take the another arp_packet * and match it using ip_sender and ip_target?

Answer 1

A std::map accepts up to 4 template parameters (the last 2 have default values).

You would be using the 3rd parameter here: a functor meant to compare two keys.

struct ArpComparator {
  bool operator()(arp_packet* const left, arp_packet* const right) {
    // compare the fields you need here
  }
};

typedef std::map<arp_packet*, arp_packet*, ArpComparator> ArpMap;

Be careful about implementing a proper operator() semantics, it should match the mathematical properties of <, notably antisymmetry and transitivity.