错误：无法将“ minHeap”从“ std :: priority_queue，std :: greater>”转换为“ std :: priority_queue”

Question

I am a beginner in c++. I am trying to create a maxheap and a minheap using std::priority_queue. Creating just a maxheap works fine but not both. I can't seem to understand the error. I get the following error: could not convert 'minHeap' from 'std::priority_queue, compare>' to 'std::priority_queue' Tried searching the internet but to no avail. Below is the code.

void addNum(int num, priority_queue<int> maxHeap, priority_queue<int> minHeap) {
    if (minHeap.size() == 0 || num > minHeap.top()) 
        minHeap.push(num);
    else if (num < minHeap.top())
        maxHeap.push(num);
}

void createHeaps(vector<int> a) {
    priority_queue<int> maxHeap;
    priority_queue<int, vector<int>, greater<int> > minHeap;
    for (int i = 0; i < a.size(); ++i) {
        int num = a[i];
        addNum(num, maxHeap, minHeap);
    }
}

Answer 1

Your are trying to pass variable with priority_queue<int, vector<int>, greater<int> > type, but your function expects priority_queue<int> type.

Correct the prototype of function:

void addNum(int num, priority_queue<int>& maxHeap, priority_queue<int, vector<int>, greater<int> >& minHeap) {
    if (minHeap.size() == 0 || num > minHeap.top()) 
        minHeap.push(num);
    else if (num < minHeap.top())
        maxHeap.push(num);
}

You must use references. It is because you need to modify original collections.

Answer 2

You have to use correct type for your method too:

void addNum(int num,
            priority_queue<int, >& maxHeap,
            priority_queue<int, std::vector<int>, std::greater<int>>& minHeap) {
    if (minHeap.size() == 0 || num > minHeap.top()) 
        minHeap.push(num);
    else if (num < minHeap.top())
        maxHeap.push(num);
}

Or you might template your method to be more generic

template <typename ... Ts1, typename ... Ts2>
void addNum(int num,
            priority_queue<int, Ts1...>& maxHeap,
            priority_queue<int, Ts2...>& minHeap) {
    if (minHeap.size() == 0 || num > minHeap.top()) 
        minHeap.push(num);
    else if (num < minHeap.top())
        maxHeap.push(num);
}

or even, simply:

template <typename PriorityQueue1, typename PriorityQueue2>
void addNum(int num,
            PriorityQueue1& maxHeap,
            PriorityQueue2& minHeap) {
    if (minHeap.size() == 0 || num > minHeap.top()) 
        minHeap.push(num);
    else if (num < minHeap.top())
        maxHeap.push(num);
}