std::priority_queue parameterised Comparison struct

Question

I am working with std::priority_queue for the first time for a University assignment. The assignment is a simulation of process scheduling. I would like to pass a parameter to my Comparison struct constructor to initialise and I thought I saw it on another forum, but I'm unable to find the source again. I looked on SO before posting but I didn't see anything similar.

Here is my priority_queue:

/* schedules.hpp / .cpp */
#include "process.hpp"

namespace my = procschedassignment;

int tick = 0;
std::priority_queue<my::Process, _
        std::vector<my::Process>,
        PrioritiseHighestResponseRatioNext(tick) > rq;
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
line 100 - compiler errors are here
// ...

Here is my Comparison struct:

/* prioritise_process.hpp / .cpp */
#include "process.hpp"

namespace my = procschedassignment;

struct PrioritiseHighestResponseRatioNext {
    public:
        explicit PrioritiseHighestResponseRatioNext(int const& cpu_time)
                : cpu_time_(cpu_time) {};
        bool PrioritiseHighestResponseRatioNext::operator()(my::Process const& lhs,
                my::Process const& rhs) {
            bool ret;

            if (lhs.wait_time() > cpu_time_) {
                ret = (lhs.ResponseRatio() > rhs.ResponseRatio());
            } else if (rhs.wait_time() > cpu_time_) {
                ret = (lhs.ResponseRatio() < rhs.ResponseRatio());
            }

            return ret;
        };

    private:
        const int cpu_time_;
};

The compiler errors I get with this code are:

../src/schedules.cpp:100: error: ‘time’ cannot appear in a constant-expression
../src/schedules.cpp:100: error: template argument 3 is invalid
../src/schedules.cpp:100: error: invalid type in declaration before ‘;’ token

Is it possible to have a parameterised Comparison struct with std::priority_queue? I am new to STL so I apologise that I don't have a better understanding of what's happening here.

Answer 1

You are trying to pass an object as template-parameter. This will not work. You should provide your comparator as argument to the constructor, and the type of the comparator as template-argument.

// declare type
typedef std::priority_queue<my::Process,
                            std::vector<my::Process>,
                            PrioritiseHighestResponseRatioNext > process_queue;
                            // ^^^ just a type, no object ^^^
// create object
process_queue rq(PrioritiseHighestResponseRatioNext(tick));