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

C++ Deque::size() Function

The C++ std::deque::size() function is used to return the number of elements currently stored in the deque. It retrieves the count of the elements, allowing dynamic size management. Unlike arrays, deques support fast insertion and deletion at both ends.

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

size_type size() const noexcept;

It does not accept any parameter.

It returns the number of elements present in the deque.

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 size() function.

#include <iostream>
#include <deque>
int main()
{
    std::deque<char> a = {'A', 'B', 'C', 'D'};
    std::cout << "Size of the deque is: " << a.size() << std::endl;
    return 0;
}

Output of the above code is as follows −

Size of the deque is: 4

Consider the following example, where we are going to get the size of the deque after removing few elements.

#include <iostream>
#include <deque>
int main()
{
    std::deque<int> a = {11,12,23,34,45};
    a.pop_front();
    a.pop_back();
    std::cout << "Size of deque after removing element :  " << a.size() << std::endl;
    return 0;
}

Following is the output of the above code −

Size of deque after removing element :  3

Let's look at the following example, where we are going to apply the size() function to the deque in its initial stage and after clearing the deque.

#include <iostream>
#include <deque>
int main()
{
    std::deque<char> x = {'A', 'B', 'C', 'D'};
    std::cout << "Initial size: " << x.size() << std::endl;
    x.clear();
    std::cout << "Size after clearing : " << x.size() << std::endl;
    return 0;
}

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

Initial size: 4
Size after clearing : 0

deque.htm

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