Re: STL set with custom data type
cata.1972@hotmail.com wrote:
I have structure representing a 2 dimesional point and want to store
pointers to these in STL map. I define the sorting criterion, such
that two points are considered equal if the distance between them is
negligeble (less than a tolerance):The problem is that after I've
inserted them I cannot find them anymore:
Others already pointed out some errors, but there is one more in your
thinking:
Let's for the sake of simplicity assume that you have just scalars that you
compare (you can base your vector comparison on the scalar comparison of
each element). You then say that A is considered equal to B if the distance
between A and B is less than some delta. Now, consider the values A,
B=A+delta/2 and C=A+delta. The distance between A and B or B and C is
delta/2, so they are considered equal. However, the distance between A and
C is exactly delta, so they are considered unequal.
Note that one more problem is that floating point numbers have
representations for NaN and for positive and negative infinity, which would
completely mess up your sorting.
Now, what can you do? One thing is to map your values to integers, i.e to
multiples of your granularity. In order to do that properly, there are a
few preparations necessary:
1. You need to take care of Nan and infinity. Blocking such values could be
done in a wrapper class that throws std::out_of_range.
2. You have further bounds which occur e.g. when your number is so large
that the granularity can not be represented any more. In that case, you
have the effect that A+delta will actually result in A, because the least
significant digits are cut off. Those borders need to be determined and
also checked in mentioned wrapper class.
Uli
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