Functor. Implementing comparator with additional parameter

[Maser]

Hi.
Im trying to implement a comparator to sort my QList<Custom>. Problem ist, the comparator for qSort obv have to be a static function, just taking two arguments. But i need a third argument, so i can do my comparison, or i need somehow to access member variables of my class. Is there a way to do this?
I came across something called functor, that somehow make a subclass of myclass and then use member variables and implementing "operator()" , but i dont get how to do it and if its even possible with the signal and slot stuff.

[SGaist]

Hi,

Can you show a sample of how you would like to do your comparison ?

[Maser]

Well, i have a QList<Village> containing villages with a name and two coordinates. I have a program that display i number of villages in a TableView. I want to sort the villages with respect to their distance to a specific targetvillage. Which means the sorting changes every time the targetvillage is changed. This means i need an additional argument for the comparison, namely a pointer, reference or whatever for the current targetvillage.

class Village
{
public:
...

private:
    QString villageName;
    int x;
    int y;
};

so far i had something like

 void TableDataModel::sort(int column, Qt::SortOrder order)
 {
       qSort(villageList->begin(), villageList->end(), compareTwoVillageLessThan);
 }

bool TableDataModel::compareTwoVillageLessThan(const Village &A, const Village &B )
{
    Village tempTargetVillage = *targetVillage;
    double distanceA = calculateDistance::getCalculateDistance(A, tempTargetVillage, c);
    double distanceB = calculateDistance::getCalculateDistance(B, tempTargetVillage, c);
    return distanceA < distanceB;
}

but then i realized the comparator have to be static and i cant access class member.

[SGaist]

Use a functor:

class CompareTwoVillages
{
private:
   const Village refVillage;

public:

   CompareTwoVillages(const Village &village) : refVillage(village)
   {
   }

   bool operator () (const Village& leftVillage, const Village& rightVillage)
   {
        bool result = false;
        // implement algorithm
        return result;
   }
};

And then call:

qSort(villageList.begin(), villageList.end(), CompareTwoVillages(targetVillage));

By the way, why do you allocate your QList on the heap ? That's usually not needed.

[kshegunov]

@Maser
I have two tiny remarks to the otherwise excellent guidance provided by @SGaist:

1. You could use a lambda function (which internally is a functor) instead of writing a whole class. It's just a more compact notation for what Samuel suggested.
2. You should use std::sort instead of qSort, as qSort is marked obsolete in Qt 5.

Kind regards.

[Maser]

Thank you very much, guys))
Your example helped me a lot, SGaist. Although i had to do a bit of reading on functors and function pointers to fully understand the concept.
I let it sink in a bit, and then im gonna read some more on lambdafunctions as kshegunov suggested.
Thubms up!