Re: If you can help... (map::find and map::insert)

From:
James Kanze <james.kanze@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Fri, 26 Jun 2009 01:46:46 -0700 (PDT)
Message-ID:
<4e7e94ba-aa22-4609-b728-87aeaeac8780@l28g2000vba.googlegroups.com>
On Jun 25, 10:40 pm, Paavo Helde <pa...@nospam.please.ee> wrote:

Ricardo <rbalbi...@gmail.com> kirjutas:

I have seen a lot of comments about this problem on the
lists and, even as I tried to implement the proper
corrections, I was really unable to do so... (or my problem
is somehow different...).

I have a map that uses a class as its key.. something like this:

struct SFlowId {
     uint32_t ingressInterface;
     uint16_t VLAN;
     uint32_t sourceIP;
     uint32_t destinationIP;
     uint16_t sourcePort;
     uint16_t destinationPort;
};

I defined a < function to compare those values....

// this is short cause the problem occurs both with 2, 3 or 4
parameters being compared (anything more than 1 causes my problem).
bool operator<(const SFlowId &left,const SFlowId &right) {
     return (left.destinationIP < right.destinationIP) ||
               (!(left.destinationIP < right.destinationIP) &&
               (left.sourceIP <
right.sourceIP));


Note that you have a redundancy here: !(left.destinationIP <
right.destinationIP) is always true if the first part of the expression
is not (left.destinationIP < right.destinationIP).

This already hints
that you have written something meaningless here. A proper weak ordering
predicate would be:

bool operator<(const SFlowId &left,const SFlowId &right) {
   return (left.destinationIP < right.destinationIP) ||
               ((left.destinationIP == right.destinationIP) &&
                (left.sourceIP < right.sourceIP));


or
    return (left.destinationIP < right.destinationIP
        || ( ! (right.destinationIP < left.destinationIP)
            && left.sourceIP < right.sourceIP ) ;

That way, you only need to use '<'.

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