C++ Deque::empty() Function

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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.

Syntax

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

bool empty() const noexcept;

Parameters

It does not accepts any parameters.

Return value

It returns true if deque is empty otherwise false.

Exceptions

This function never throws exception.

Time complexity

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

Example

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

Output of the above code is as follows −

Deque Is Empty.

Example

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;
}

Output

Following is the output of the above code −

Deque Is Not Empty.

Example

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;
}

Output

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

Deque Is Empty.

Example

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;
}

Output

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

Deque Is Empty.