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Showing content from https://www.tutorialspoint.com/cpp_standard_library/cpp_queue_empty.htm below:

C++ Queue Empty Function

The C++ std::queue::empty() function is used to check whether the queue is empty or not and returns a boolean value, true if the queue is empty and does not contains any elements otherwise false. This function can also be used in the conjunction with loops or conditionals. The time complexity of this function is constant O(1).

Following is the syntax for std::queue::empty() function.

bool empty() const;

It does not accepts any parameters.

This function returns true if queue is empty otherwise false.

In the following example, we are going to demonstrate the basic usage of empty() function.

#include <iostream>
#include <queue>
int main()
{
    std::queue<int> a;
    if (a.empty()) {
        std::cout << "TRUE" << std::endl;
    }
    a.push(1);
    if (!a.empty()) {
        std::cout << "FALSE" << std::endl;
    }
    return 0;
}

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

TRUE
FALSE

Consider the following example, where we are going to use the empty() in a loop to process and remove elements until the queue is empty.

#include <iostream>
#include <queue>
int main() {
    std::queue<int> x;
    x.push(1);
    x.push(2);
    while (!x.empty()) {
        std::cout << " " << x.front() << std::endl;
        x.pop();
    }
    if (x.empty()) {
        std::cout << "The queue was empty now." << std::endl;
    }
    return 0;
}

Following is the output of the above code −

 1
 2
The queue was empty now.

Let's look at the following example, where we are going to check the state of the queue in different scenarios.

#include <iostream>
#include <queue>
int main()
{
    std::queue<int> a;
    std::cout << "Initial stage: " << (a.empty() ? "empty" : "not empty") << std::endl;
    a.push(1);
    std::cout << "After pushing: " << (a.empty() ? "empty" : "not empty") << std::endl;
    a.pop();
    std::cout << "After popping: " << (a.empty() ? "empty" : "not empty") << std::endl;
    return 0;
}

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

Initial stage: empty
After pushing: not empty
After popping: empty

queue.htm

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