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

C++ Deque::empty() Function

The C++ std::deque::empty() function is used to check whether the deque is empty or not. It returns a boolean value true if the deque is empty otherwise false. It is used to determine the state of the deque, ensuring efficient management of data insertion and retrieval operations.

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

bool empty() const noexcept;

It does not accepts any parameters.

It returns true if deque is empty otherwise false.

This function never throws exception.

The time complexity of this function is Constant i.e. O(1)

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

#include <iostream>
#include <deque>
int main()
{
    std::deque<int> a;
    if (a.empty()) {
        std::cout << "Deque Is Empty." << std::endl;
    } else {
        std::cout << "Deque Is Not Empty." << std::endl;
    }
    return 0;
}

Output of the above code is as follows −

Deque Is Empty.

Consider the following example, where we are going to add the elements to the deque and applying the empty() function.

#include <iostream>
#include <deque>
int main()
{
    std::deque<char> a;
    a.push_back('A');
    a.push_front('B');
    if (!a.empty()) {
        std::cout << "Deque Is Not Empty." << std::endl;
    } else {
        std::cout << "Deque Is Empty." << std::endl;
    }
    return 0;
}

Following is the output of the above code −

Deque Is Not Empty.

Let's look at the following example, where we are going to use the clear() function along with the empty() and observing the output.

#include <iostream>
#include <deque>
int main()
{
    std::deque<int> a {1,22,333};
    a.clear();
    if (a.empty()) {
        std::cout << "Deque Is Empty." << std::endl;
    } else {
        std::cout << "Deque Is Not Empty." << std::endl;
    }
    return 0;
}

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

Deque Is Empty.

Following is the example, where we are going to use resize(0) to make the deque size zero and apply the empty() function.

#include <iostream>
#include <deque>
int main() {
    std::deque<char> a {'A', 'B', 'C', 'D'};
    a.resize(0); 
    if (a.empty()) {
        std::cout << "Deque Is Empty." << std::endl;
    } else {
        std::cout << "Deque Is Not Empty." << std::endl;
    }
    return 0;
}

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

Deque Is Empty.

deque.htm

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