comparison with a third variable in priority_queue, c++ - c++

Question

On this page: http://comsci.liu.edu/~jrodriguez/cs631sp08/c++priorityqueue.html
author gives a nice example of using priority queue in c++. In this the author shows how to order various times serially. I have a similar problem in which I would like to order the times on basis of proximity to a given point in time. My question is how to add a third input to comparer, so that the extra parameter can be considered. I.e. how can we make t3 inside comparer a variable that is passed from outside.
#include <iostream>
#include <queue>
#include <iomanip>
#include <cstdlib>
using namespace std;
struct Time {
int h; // >= 0
int m; // 0-59
int s; // 0-59
};
class CompareTime {
public:
bool operator()(Time& t1, Time& t2)
{
Time t3 = {{2,9,0}};
if (abs(t3.h - t2.h) > (t3.h - t1.h))
return true;
return false;
}
};
int main()
{
priority_queue<Time, vector<Time>, CompareTime> pq;
// Array of 4 time objects:
Time t[4] = { {3, 2, 40}, {3, 2, 26}, {5, 16, 13}, {5, 14, 20}};
for (int i = 0; i < 4; ++i)
pq.push(t[i]);
while (! pq.empty()) {
Time t2 = pq.top();
cout << setw(3) << t2.h << " " << setw(3) << t2.m << " " <<
setw(3) << t2.s << endl;
pq.pop();
}
return 0;
}
thanks.

Answer 1

Your comparator itself can be parameterized at instantiation:
class CompareTime
{
Time t;
public:
CompareTime(const Time& arg) : t{arg} {}
bool operator()(const Time& t1, const Time& t2) const
{
return (abs(t.h - t2.h) > abs(t.h - t1.h));
}
};
Declared like this:
Time myTime; // modify to your leisure...
// ...then create your queue with myTime as the fixed param
priority_queue<Time, vector<Time>, CompareTime> pq{CompareTime{myTime};
Apologies in advance if the syntax isn't spot-on. I'm without a compiler at this moment, but I hope the idea is clear enough.

Answer 2

Finally, with helpful pointers from WhozCraig and user783920, following solution seems to work.
#include <iostream>
#include <queue>
#include <iomanip>
#include <cstdlib>
using namespace std;
struct Time {
int h; // >= 0
int m; // 0-59
int s; // 0-59
};
class CompareTime
{
Time t;
public:
CompareTime(const Time& arg) {
std::cout << "struct constructor \n";
t=arg;
}
// CompareTime(){}
bool operator()(const Time& t1, const Time& t2) const
{
return (abs(t.h - t2.h) > abs(t.h - t1.h));
}
};
int main()
{
Time mytime ={0};
mytime.h=6;
priority_queue<Time, vector<Time>, CompareTime> pq{CompareTime(mytime)};
// Array of 4 time objects:
Time t[4] = { {3, 2, 40}, {2, 2, 26}, {5, 16, 13}, {1, 14, 20}};
for (int i = 0; i < 4; ++i)
pq.push(t[i]);
while (! pq.empty()) {
Time t2 = pq.top();
cout << setw(3) << t2.h << " " << setw(3) << t2.m << " " <<
setw(3) << t2.s << endl;
pq.pop();
}
return 0;
}