compilation error with std::priority_queue derived class using std::shared_ptr

Question

I derived from std::priority_queue to implement a few specialized methods. One of these methods somekind of a fixed queue when I add an element and the queue is full, the smallest element is dropped from the queue.

template<typename T,
     typename Sequence = std::vector<T>,
     typename Compare = std::less<typename Sequence::value_type> >
class fixed_priority_queue : public std::priority_queue<T, Sequence, Compare> {

    friend class BCQueue_; // to access maxSize_

public:

fixed_priority_queue(unsigned int maxSize) 
: maxSize_(maxSize) {}

void insertWithOverflow(const T& x) {
    if (this->size() == maxSize_) {
        auto beg = this->c.begin();
        auto end = this->c.end();
        auto min = std::min_element(beg, end);
        if(x > *min) {
            *min = x;
            std::make_heap(beg, end);
        }
    }
    else {
       this->push(x);
    }
}

// ...

private:
    fixed_priority_queue() {} 
    const unsigned int maxSize_;
};

This is my comparator I use:

class ScoreLessThan : public std::binary_function<
    std::shared_ptr<CCandidate>, std::shared_ptr<CCandidate>, bool> {
public:

    bool operator()(
        const std::shared_ptr<CCandidate>& a, const std::shared_ptr<CCandidate>& b) const {
        return a->score > b->score;
    }
};

My derived fixed_priority_queue class I wrap to keep functionality a little separated, so finally I have this:

class BCQueue_ : public fixed_priority_queue<
    std::shared_ptr<CCandidate>, std::vector<std::shared_ptr<CCandidate> >,      ScoreLessThan> {
public:
    BCQueue_(size_t maxSize)
    : fixed_priority_queue(maxSize) {}

    bool willInsert(float score) {
        return size() < maxSize_ || top()->score < score;
    }
};

I can use this like:

BCQueue_ queue(30);

The CCandidate is just a holder for some data. One property is the score field, that I use in the comparator above.

When I use above class with CCandidate as raw pointer all compiles with problem and works fine, now I want to replace the raw pointer with std::shared_ptr (as I did above) and I get a compilation error:

    ... 199:5: error: no match for ‘operator>’ in ‘x >min.__gnu_cxx::__normal_iterator::operator* [with _Iterator =  std::shared_ptr*, _Container = std::vector >, __gnu_cxx::__normal_iterator::reference = std::shared_ptr&]()’
...
... :199:5: note: candidates are:
... :199:5: note: operator>(int, int) 
... :199:5: note:   no known conversion for argument 2 from ‘std::shared_ptr’ to ‘int’

Maybe this is a simple issue. I am not shure if I defined the comparator properly or if I need to change the comparison in insertWithOverflow() x > *min, actually I do not know what I should change there.

I should mention, that I have found the implementation of `insertWithOverflow' here on stackoverflow which just fits what I need. See here: How to make STL's priority_queue fixed-size

Like said, with raw pointers all this is no problem. Could somebody please help me out on this. Thanks in advance!

Answer 1

You need to use the specified comparison function everywhere in your function:

using std::priority_queue<T, Sequence, Compare>::comp;

void insertWithOverflow(const T& x) {
    if (this->size() == maxSize_) {
        auto beg = this->c.begin();
        auto end = this->c.end();
        auto min = std::min_element(beg, end, comp);
        if(comp(*min, x)) {
            *min = x;
            std::make_heap(beg, end, comp);
        }
    }
    else {
       this->push(x);
    }
}