Pass template to function as an argument in c++

Question

I want to create a queue of structure (Category) sorted based on a parameter (action). For that, I'm using std::priority_queue (pQueue) template. When I build the code, I get the following errors:

1. error: redefinition of ‘pQueue::pQueue()’ -> Used #pragma once to avoid multiple header file inclusions, still getting this error.
2. error: use of deleted function ‘pQueue::pQueue(constpQueue&)’ note: ‘pQueue::pQueue(const pQueue&)’ is implicitly deleted because the default definition would be ill-formed: -> How do I define custom constructor which can incorporate the information as shown in the code below?
3. error: ‘q’ was not declared in this scope (scope: queueWork in file.cpp) Where do I declare 'q'?

How do I resolve these errors?

The main objective is to queue the data when function() is executed multiple times and later on use that data. The below code snippets are a part of a bigger project. Hence, might miss out on a few details.

file.cpp

#include "File.h"
using namespace std;

pQueue::pQueue()
{
  std::priority_queue<std::pair<int, Category*>> q;
}

pQueue::~pQueue()
{
}

void queueWork(std::priority_queue<std::pair<int, Category*>> q, int action, Category* data)
{
  q.emplace(make_pair(action, data));
}

void doWork(std::priority_queue<std::pair<int, Category*>> q, bool flag)
{
  while (true)
  {
    std::pair<int, Category*> queueElement;
    queueElement = q.top();
    Category* data = queueElement.second;
    // Perform some operation, get return value
    q.pop();
  }
}

file.h

#pragma once
#include <queue>
#include <thread>

class pQueue
{
public:
  pQueue() {
  }

  ~pQueue() {
  }

 void queueWork(std::priority_queue<std::pair<int, Category*>> q, int action, Category* data);

private:
  void doWork(std::priority_queue<std::pair<int, Category*>> q, bool flag);
};

main.cpp

#include "File.h"
bool function(Category* data)
{
    bool result;
    if (data)
    {
        // Will set action values later
        auto action = 1;
        pQueue::queueWork(Queue, action, data);

        // TODO: Implement getResult function to get the return value
        // result = pQueue.getResult()
    }
    return true;
}

Answer 1

In your file.h you do not only declare the constructor, but also define it:

class pQueue
{
public:
    pQueue() {
        flag = false;
}

Next, in your file.c file you redeclare the constructor:

pQueue::pQueue()
{
    std::priority_queue<std::pair<int, Category*>> q;
}

That's not possible. C++ has the One Definition Rule. There must only be one definition here.