Dijkstras algorithm with priority_queue - c++

Question

I am trying to implement Dijkstra's algorithm. I am using this priority_queue
priority_queue<pair<PathInfo,string>,vector<pair<PathInfo,string> >,QueueComp> p;
where
class QueueComp{
PathComp* pc;
public:
QueueComp(PathComp*);
bool operator ()(const pair<PathInfo,string>&,const pair<PathInfo,string>& );
};
is my "Compare" function. The error is that QueueComp does not have a default constructor and I am not permitted to create one. What can I do to make my code compile? Btw this is the error
error: no matching function for call to 'QueueComp::QueueComp()'
This is the pathcomp.h
class PathComp{
public:
virtual bool betterThan(const PathInfo& path1,const PathInfo& path2)=0;
};
This is the pathcomppl.h
#include "pathcomp.h"
class PathCompPL:public PathComp{
public:
virtual bool betterThan(const PathInfo& path1,const PathInfo& path2);
};
This is the pathcomppl.cpp
#include "pathcomppl.h"
bool PathCompPL::betterThan(const PathInfo& path1,const PathInfo& path2){
if (path1.getTotalPrice()>path2.getTotalPrice())
return true;
if (path1.getTotalPrice()==path2.getTotalPrice() && path1.getTotalLength()>path2.getTotalLength())
return true;
return false;
}
Expanded error message
main.cpp: In constructor ‘std::priority_queue<_Tp, _Sequence, _Compare>::priority_queue(const _Compare&, const _Sequence&) [with _Tp = std::pair<PathInfo, std::basic_string<char> >; _Sequence = std::vector<std::pair<PathInfo, std::basic_string<char> > >; _Compare = QueueComp]’:
main.cpp:11:87: error: no matching function for call to ‘QueueComp::QueueComp()’
main.cpp:11:87: note: candidates are:
In file included from main.cpp:5:0:
queuecomp.h:14:5: note: QueueComp::QueueComp(PathComp*)
queuecomp.h:14:5: note: candidate expects 1 argument, 0 provided
queuecomp.h:10:7: note: QueueComp::QueueComp(const QueueComp&)
queuecomp.h:10:7: note: candidate expects 1 argument, 0 provided

Answer 1

You need to initialize your priority queue with additional parameter since you have non-default constructor.
priority_queue<pair<PathInfo,string>,vector<pair<PathInfo,string> >,QueueComp> p(QueueComp(ptrToPathCompObject));
The additional parameter (QueueComp(ptrToPathCompObject)) should fix your problem.
I am assuming that you have already implemented the operator() in QueueComp class.

Answer 2

You do not have a default constructor, because you are supposed to initialize the variable called pc. You have this constructor:
QueueComp(PathComp*);
You have to implement it so that pc is associated to the parameter.
As about your second question: The first element is your next priority, the second element is the set of lower priorities and the third is the queue comparison. I hope this helps you.

Answer 3

It looks like your issue is with implementing a proper comparator. One alternative you might consider is to create the comparator like the following
struct CompareEdgeWeights : public binary_function<PathInfo*, PathInfo*, bool>
{
bool operator()(const PathInfo* left, const PathInfo* right) const
{
return left->getEdgeWeight() > right->getEdgeWeight();
}
}; // end struct
// Priority queue of node edges
priority_queue<PathInfo*,vector<PathInfo*>,CompareEdgeWeights > * edgePriorityQueue;
Have this struct inherit from binary_function and overload the operator (). You can then use this as your comparator for keeping the edges sorted from lowest to highest weight value. Note: you may have to tweak this a bit to conform to your implementation. It is difficult to give a 100% correct suggestion without seeing more of your implementation.