Sorting std::list of pointers to objects with comparator

Question

I have the class which has list of pointers to objects which is needed to be sorted with comparator which is the member of this class:

class VoronoiDiagram
{
public:
    void BuildVoronoi();

private:
    list<shared_ptr<QPointF>> inputPoints
    QPointF currentVoronoiPoint;

    bool compairingPointsPredictate(shared_ptr<QPointF> a, shared_ptr<QPointF> b) {    float d1 = (a->x()-currentVoronoiPoint.x())*(a->x()-currentVoronoiPoint.x()) + (a->y()-currentVoronoiPoint.y())*(a->y()-currentVoronoiPoint.y());
                                                                                float d2 = (b->x()-currentVoronoiPoint.x())*(b->x()-currentVoronoiPoint.x()) + (b->y()-currentVoronoiPoint.y())*(b->y()-currentVoronoiPoint.y());
                                                                                return d1 < d2; };          
};

And I am calling in BuildVoronoi() this:

inputPoints.sort(compairingPointsPredictate);

But this gives to me two errors: C3867 with my comparator function and C2660 with std::list::sort (do not receive 1 argument).

Sorry for my English or for explanation. I really hope that someone here can help me out. Thanks.

The problem is that a member function has an implicit first parameter for this, so it doesn't match the signature required for std::list::sort. There are plenty of duplicates out there. — juanchopanza
– juanchopanza, Commented Mar 8, 2014 at 21:27

4pie0 · Accepted Answer · 2015-10-15 08:45:04Z

2

sort needs a two argument comparison function but the declaration as member function results in a call similar to:

a.compairingPointsPredicate( b, c)
//                     ^
//                   no 't', it is predicate not predictate ;p

There is no reason to make compairingPointsPredicate a member of a class. You can make it a free standing function (a friend of QPointF if access to private data is required).

class QPointF{

    friend bool compairingPointsPredicate(
                        shared_ptr<QPointF> a, shared_ptr<QPointF> b);          
};


bool compairingPointsPredicate(shared_ptr<QPointF> a, 
                                          shared_ptr<QPointF> b) {    
                                            float d1 = (a->x()-currentVoronoiPoint.x())
                                            *(a->x()-currentVoronoiPoint.x()) + 
                                            (a->y()-currentVoronoiPoint.y())
                                            *(a->y()-currentVoronoiPoint.y());
                                            float d2 = (b->x()-currentVoronoiPoint.x())
                                            *(b->x()-currentVoronoiPoint.x()) +
                                            (b->y()-currentVoronoiPoint.y())
                                            *(b->y()-currentVoronoiPoint.y());
                                            return d1 < d2;
}

usage:

inputPoints.sort(compairingPointsPredicate);

edited Oct 15, 2015 at 8:45

answered Mar 8, 2014 at 21:31

4pie0

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7 Comments

VIRUS Over a year ago

Can u please give me link to example of how to make friendly function with access to private data and how comparator must be implemented?

VIRUS Over a year ago

Hm... I tried this, but now compairingPointsPredictate "doesn't see" currentVoronoiPoint.

4pie0 Over a year ago

compairingPointsPredictate must have access to a->x() and its currentVoronoiPoint.x(). This is another issue not related to the: error std::list::sort (do not receive 1 argument)

VIRUS Over a year ago

What kind of issue? I don't understand: friendly function must have access to currentVoronoiPoint, but VC2010 just gives to me C2065 and C2228 errors with currentVoronoiPoint. But the main problem is fixed, i guess.

4pie0 Over a year ago

@VIRUS comparator must be a friend of QPointF

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