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C++ Queue::queue() Function

The C++ std::queue::queue() function constructor initializes a queue, a first-in, first-out (FIFO) data structure. The default constructor queue() creates an empty queue and we can initialize a queue with elements from another container using queue(const Container & cont).

copy constructors queue and move constructors queue allow us to create a new queue from an existing one, either by copying or moving its elements.

Following is the syntax for std::queue::queue() constructor.

explicit queue (const container_type& ctnr = container_type());
or
explicit queue (const container_type& ctnr);
or
queue( queue&& other);
or
queue( queue& other );

• ctnr − It indicates the container type which is second parameter of class template
• other − It indicates the another queue object of the same type.

The constructor never returns value.

Let's look at the following example, where we are going to use the default constructor.

#include <iostream>
#include <queue>
int main()
{
    std::queue<int> a;
    std::cout << "Size of the queue after default construction: " << a.size() << std::endl;
    return 0;
}

Following is the output of the above code −

Size of the queue after default construction: 0

Consider the another scenario, where we are going to use the copy constructor.

#include <iostream>
#include <queue>
int main()
{
    std::queue<int> a;
    a.push(11);
    a.push(2);
    std::queue<int> b(a);
    std::cout << "Size of the queue after copy construction: " << b.size() << std::endl;
    return 0;
}

Let us compile and run the above program, this will produce the following result −

Size of the queue after copy construction: 2

Following is the example, where we are going to use the move constructor.

#include <iostream>
#include <queue>
int main()
{
    std::queue<int> a;
    a.push(111);
    a.push(22);
    std::queue<int> b(std::move(a));
    std::cout << "Size of the queue after move construction: " << b.size() << std::endl;
    std::cout << "Size of the queue after move construction: " << a.size() << std::endl;
    return 0;
}

Output of the above code is as follows −

Size of the queue after move construction: 2
Size of the queue after move construction: 0

In the following example, we are going to construct a queue using the different container.

#include <iostream>
#include <queue>
#include <deque>
int main()
{
    std::deque<int> a = {11,22,3,4};
    std::queue<int, std::deque<int>> b(a);
    std::cout << "Queue size: " << b.size() << std::endl;
    return 0;
}

If we run the above code it will generate the following output −

Queue size: 4

queue.htm

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