Accessing a base class member with .*& (accessing priority_queue container)

Question

I'm trying to understand the code below from

How to iterate over a priority_queue?

I gather that since HackedQueue is deriving privately from priority_queue, it can access its privates. So, I assume that *&HackedQueue::c returns the address of the base class object, and its called for q. Not fully clear, though, and even more how it's a valid syntax.

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Mentioning priority_queue, I was wondering if there's still no cleaner workaround. For example, the 4th c'tor on this link

priority_queue( const Compare& compare, Container&& cont );

https://en.cppreference.com/w/cpp/container/priority_queue/priority_queue

seems to provide a container to work with and not as read only input. I don't see it though in visual studio header file, and I'm not clear what is &&.

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Related, I don't understand opposing questions such as why I need access to the private container, it's not made for that. Well, how do you debug a priority queue, where the basic need is to print its elements?

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#include <queue>
#include <cstdlib>
#include <iostream>
using namespace std;

template <class T, class S, class C>
S& Container(priority_queue<T, S, C>& q) {
    struct HackedQueue : private priority_queue<T, S, C> {
        static S& Container(priority_queue<T, S, C>& q) {
            return q.*&HackedQueue::c;
        }
    };
    return HackedQueue::Container(q);
}

int main()
{
    priority_queue<int> pq;
    vector<int> &tasks = Container(pq);

    cout<<"Putting numbers into the queue"<<endl;
    for(int i=0;i<20;i++){
        int temp=rand();
        cout<<temp<<endl;
        pq.push(temp);
    }

    cout<<endl<<"Reading numbers in the queue"<<endl;
    for(vector<int>::iterator i=tasks.begin();i!=tasks.end();i++)
        cout<<*i<<endl;

    cout<<endl<<"Taking numbers out of the queue"<<endl;
    while(!pq.empty()){
        int temp=pq.top();
        pq.pop();
        cout<<temp<<endl;
    }

    return 0;
}

Answer 1

I'm trying to understand the code below from

Let's say in points:

• .* is pointer-to-member access operator. See https://en.cppreference.com/w/cpp/language/operator_member_access
• The code is using internal representation of the priority_queue, it's using the underlying container used by the queue to store data and accessing that data using protected member with the name c.

  • From glibc stl_queue.h:

    class priority_queue
    {
         ...
    protected:
         //  See queue::c for notes on these names.
         _Sequence  c;
         _Compare   comp;
    

  • See this note in queue::c about why it's just named c. And C++98 standard.

• HackedQueue inherits from priority_queue so it can access it's protected members,
• &HackedQueue::c is a pointer to member c in class HackedQueue (it is inherited from priority_queue),
• q.*(&HackedQueue::c); accesses that member c in object q using that pointer-to-member,
• and then the function returns a reference to that member in the passed object q.

what is &&.

Rvalue reference declarator. See https://en.cppreference.com/w/cpp/language/reference

how do you debug a priority queue, where the basic need is to print its elements?

With a debugger. Anyway, the question is also answered in the linked question How to iterate over a priority_queue? .