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

From:

Paavo Helde <paavo@nospam.please.ee>

Newsgroups:

comp.lang.c++

Date:

Thu, 25 Jun 2009 15:40:06 -0500

Message-ID:

<Xns9C35F0C6E1A83nobodyebiee@216.196.109.131>

Ricardo <rbalbinot@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));

If you change your comparison operator to this style, will the problems
go away?

hth
Paavo