Iterate through a queue in C++

This post will discuss how to iterate through a queue in C++.

1. Using `std::queue`

The std::queue container does not contain the std::begin function and there isn’t any overload of std::begin which accepts a std::queue. In other words, std::queue is not meant to be iterated over.

If you need to iterate over a std::queue, you can create a copy of it and remove items from the copy, one at a time, using the standard pop function after processing it. This way the original queue remains untouched, but its copy becomes empty.

#include <iostream>

#include <queue>

int main()

{

std::queue<int> q;

q.push(1);

q.push(2);

q.push(3);

q.push(4);

std::queue<int> q_copy = q;

while (!q_copy.empty())

{

int front = q_copy.front();

std::cout << front << std::endl;

q_copy.pop();

}

return 0;

}

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Output:

1
2
3
4

2. Using `std::deque`

The above solution creates a temporary copy of the queue just for iteration, which is not efficient. Consider using the std::deque instead, which supports all standard operations of a std::queue, and can be iterated over using the range-based for-loop. We can insert and remove elements at the end and the front of a deque using the push_back and pop_front operations.

#include <iostream>

#include <deque>

int main()

{

std::deque<int> q;

q.push_back(1);

q.push_back(2);

q.push_back(3);

q.push_back(4);

for(auto &i: q) {

std::cout << i << std::endl;

}

return 0;

}

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Output:

1
2
3
4

That’s all about iterating through a queue in C++.

Iterate through a queue in C++

1. Using std::queue

2. Using std::deque

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1. Using `std::queue`

2. Using `std::deque`