Priority Queue Comparison

Question

I\'m trying to declare a priority queue in c++ using a custom comparison function...

So , I declare the queue as follows:

std::priority_queue

        

Answer 1

std::priority_queue<int, std::vector<int>, bool (*)compare(int, int)> pq(compare);

Is another way not mentioned.

Answer 2

You can use C++11 lambda function. You need to create lambda object, pass it to the template using decltype and also pass it to the constructor. It looks like this:

auto comp = [] (int &a, int &b) -> bool { return a < b; };
std::priority_queue<int,std::vector<int>, decltype(comp) > pq (comp);

Answer 3

You can use a typedef. This compiles very well:

typedef bool (*comp)(int,int);
bool compare(int a, int b)
{
   return (a<b);
}
int main()
{
    std::priority_queue<int,std::vector<int>, comp> pq(compare);
    return 0;
}

Answer 4

This worked perfectly for me.

struct compare{
    bool operator() (const int& p1,const int& p2 ){
         return p1<p2;
    }
};

int main(){
    priority_queue<int,vector<int>, compare > qu;
return 0;
}

Answer 5

you have to specify function type and instantiate the function in priority_queue constructor.

#include <functional>

bool compare(int a, int b)
{
   return (a<b);
}

std::priority_queue<int, std::vector<int>,
                              std::function<bool(int, int)>> pq(compare);

Answer 6

The template parameter should be the type of the comparison function. The function is then either default-constructed or you pass a function in the constructor of priority_queue. So try either

std::priority_queue<int, std::vector<int>, decltype(&compare)> pq(&compare);

or don't use function pointers but instead a functor from the standard library which then can be default-constructed, eliminating the need of passing an instance in the constructor:

std::priority_queue<int, std::vector<int>, std::less<int> > pq;

http://ideone.com/KDOkJf

If your comparison function can't be expressed using standard library functors (in case you use custom classes in the priority queue), I recommend writing a custom functor class, or use a lambda.