在std：sort（）上使用类

Question

An example from here: http://www.cplusplus.com/reference/algorithm/sort/

Shows that

struct myclass {
  bool operator() (int i,int j) { return (i<j);}
} myobject;

int main () {
  int myints[] = {32,71,12,45,26,80,53,33};
  std::vector<int> myvector (myints, myints+8);               // 32 71 12 45 26 80 53 33

  // using object as comp
  std::sort (myvector.begin(), myvector.end(), myobject);     //(12 26 32 33 45 53 71 80)
}

This works fine, however I am trying to use class instead of structure. So what I am doing is:

CardComparer class:

bool CardComparer::operator() (Card* firstCard, Card* secondCard) {
    this->firstCard = firstCard;
    this->secondCard = secondCard;
    if (firstCard->GetRank() == secondCard->GetRank()) {
        return firstCard->GetSuit() > secondCard->GetSuit();
    }
    else {
        return firstCard->GetRank() > secondCard->GetRank();
    }
}

and this is main:

CardComparer* compare;
compare = new CardComparer();
sort(cards.begin(), cards.end(), compare->operator());

I am getting this long error:

hand.cpp: In member function 'void Hand::AddCard(Card*)':
hand.cpp:60:54: error: no matching function for call to 'sort(std::vector<Card*>::iterator, std::vector<Card*>::iterator, <unresolved overloaded function type>)'
hand.cpp:60:54: note: candidates are:
In file included from /usr/include/c++/4.7/algorithm:63:0,
                 from hand.cpp:4:
/usr/include/c++/4.7/bits/stl_algo.h:5463:5: note: template<class _RAIter> void std::sort(_RAIter, _RAIter)
/usr/include/c++/4.7/bits/stl_algo.h:5463:5: note:   template argument deduction/substitution failed:
hand.cpp:60:54: note:   candidate expects 2 arguments, 3 provided
In file included from /usr/include/c++/4.7/algorithm:63:0,
                 from hand.cpp:4:
/usr/include/c++/4.7/bits/stl_algo.h:5499:5: note: void std::sort(_RAIter, _RAIter, _Compare) [with _RAIter = __gnu_cxx::__normal_iterator<Card**, std::vector<Card*> >; _Compare = bool (CardComparer::*)(Card*, Card*)]
/usr/include/c++/4.7/bits/stl_algo.h:5499:5: note:   no known conversion for argument 3 from '<unresolved overloaded function type>' to 'bool (CardComparer::*)(Card*, Card*)'

I couldn't really find a solution because if i modify the sample and keep it as a struct it works fine, but does not work when i convert it into a class.

Answer 1

The third argument is called a functor, and is something that can be called. Either a pointer to a function, a C++11 lambda, or an object instance (not pointer) with an operator() member function.

In your case, don't dynamically allocate the functor object on the heap, it's enough to declare it as a temporary object in the std::sort call:

std::sort(cards.begin(), cards.end(), CardComparer());

In the above std::sort call, using CardComparer() creates an object on the stack, this object is temporary and only valid while std::sort is running. The std::sort function will call this object, which is the same as invoking the operator() function on the object.

Since this comparison functor is pretty simple, it doesn't need to store any data:

struct CardComparer
{
    bool operator() (const Card* firstCard, const Card* secondCard) const { ... }
};

So no need for the member data fields.