std::queue iteration

Jack_48461 0
Viewed 189476

I need to iterate over std::queue. www.cplusplus.com says:

By default, if no container class is specified for a particular queue class, the standard container class template deque is used.

So can I somehow get to the queue's underlying deque and iterate over it?

c++-standard-library iteration queue c++
10 Answers

If you need to iterate over a queue then you need something more than a queue. The point of the standard container adapters is to provide a minimal interface. If you need to do iteration as well, why not just use a deque (or list) instead?

Charles_Bailey_195630

One indirect solution can be to use std::deque instead. It supports all operations of queue and you can iterate over it just by using for(auto& x:qu). It's much more efficient than using a temporary copy of queue for iteration.

user_109236670

while Alexey Kukanov's answer may be more efficient, you can also iterate through a queue in a very natural manner, by popping each element from the front of the queue, then pushing it to the back:

#include <iostream>
#include <queue>

using namespace std;

int main() {
    //populate queue
    queue<int> q;
    for (int i = 0; i < 10; ++i) q.push(i);

    // iterate through queue
    for (size_t i = 0; i < q.size(); ++i) {
        int elem = std::move(q.front());
        q.pop();
        elem *= elem;
        q.push(std::move(elem));
    }

    //print queue
    while (!q.empty()) {
        cout << q.front() << ' ';
        q.pop();
    }
}

output:

0 1 4 9 16 25 36 49 64 81
user_26409370

In short: No.

There is a hack, use vector as underlaid container, so queue::front will return valid reference, convert it to pointer an iterate until <= queue::back

user_1498180

I use something like this. Not very sophisticated but should work. 

    queue<int> tem; 

    while(!q1.empty()) // q1 is your initial queue. 
    {
        int u = q1.front(); 

        // do what you need to do with this value.  

        q1.pop(); 
        tem.push(u); 
    }


    while(!tem.empty())
    {
        int u = tem.front(); 
        tem.pop(); 
        q1.push(u); // putting it back in our original queue. 
    }

It will work because when you pop something from q1, and push it into tem, it becomes the first element of tem. So, in the end tem becomes a replica of q1.

user_62209550

If you need to iterate a queue ... queue isn't the container you need.
Why did you pick a queue?
Why don't you take a container that you can iterate over?

1.if you pick a queue then you say you want to wrap a container into a 'queue' interface: - front - back - push - pop - ...

if you also want to iterate, a queue has an incorrect interface. A queue is an adaptor that provides a restricted subset of the original container

2.The definition of a queue is a FIFO and by definition a FIFO is not iterable

user_1060640
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