Re: STL set with custom data type

From:

Bart van Ingen Schenau <Bart.van.Ingen.Schenau@ict.nl>

Newsgroups:

comp.lang.c++

Date:

Tue, 24 Feb 2009 04:56:26 -0800 (PST)

Message-ID:

<d5b2b7e6-f6d1-474f-9fe8-3e98072d945b@w35g2000yqm.googlegroups.com>

On Feb 24, 1:16 pm, cata.1...@hotmail.com wrote:

Hi,

I have structure representing a 2 dimesional point:

struct K2dCoords
{
    K2dCoords(void):m_x(0), m_y(0){};
    K2dCoords(double x, double y):m_x(x), m_y(y){};
    K2dCoords(const K2dCoords& rhs): m_x(rhs.m_x), m_y(rhs.m_y){};
    double getX() const {return m_x;}
    double getY() const {return m_y;}
    double m_x;
    double m_y;

};

I want to store pointers to instances of this in a STL set.
So I define the sorting criterion, such that two points are considered
equal
if the distance between them is negligeble (less than a tolerance):

<snip>

What am I doing wrong? Thanks.

You are using a sorting criterion that std::set<> can't handle.
std::set<> does not care about the equality of two points, it wants to
know if P1 comes before P2 or not. Only if neither P1 comes before P2
nor P2 comes before P1, then P1 and P2 are considered to be equal.

Another problem is your use of tolerances. std::set<> also requires
that the following equation holds:
if P1 == P2 && P2 == P3 then P1 == P3
When using tolerances, there is a big chance that this equation does
not hold for all points in the set, and that is bad.

Bart v Ingen Schenau