How to find Size of std::forward_list in C++ STL

Forward list in standard template library of C++. It comes under #include<forward_list> header file. It is implemented as a singly linked list. It was introduced in C++ 11 for the first time. Forward lists are sequence containers that allow constant time insert and erase operations from anywhere within the sequence. In the case of a forward list, fast random access is not supported.

Unlike other STL libraries, std::forward_list does not have any size() method. Hence, in this article, we will show you how to get the size of a std::forward_list in C++ STL.

There is a problem in retrieving the size of forward lists because std::forward_list doesn’t have any std::size() member function. To get the size of forward lists, one can use std::distance() function.

Approach:

Since std::distance() function takes two iterators as arguments and it returns an integer, the std::begin() and std::end() function can be passed which points to the address of the first item and the address just after the last item.

Syntax:

size = distance(forward_list.begin(), forward_list.end()); 

Below is the C++ code to implement the above approach:

C++14

// C++ program to implement
// the above approach

#include <forward_list>
#include <iostream>
using namespace std;

// Driver code
int main()
{

    forward_list<int> l1 = { 3, 5, 6, 9, 6 };

    // l.size() will throw an error, since
    // there is no size() method for
    // forward_list. So, to calculate the
    // size, we will use std::distance(iterator1,
    // iterator2), where iterator1 will be
    // l.begin() and iterator2 will be l.end()
    int size = distance(l1.begin(),
                        l1.end());

    cout << "Size of l1 is : " << size << endl;

    l1.remove(6);

    // It will erase all instances of 6
    // from the list
    size = distance(l1.begin(), l1.end());

    cout << "Size of l1, after removing all"
         << " instances of 6 is : " << size << endl;

    forward_list<int> l2 = { 6, 11, 0 };

    int size2 = distance(l2.begin(),
                         l2.end());

    cout << "Size of l2, before assigning"
         << " it to l1 : " << size2
         << endl;

    l1.splice_after(l1.begin(), l2);

    // It will assign l2 to l at the
    // provided iterator, making l1
    // as empty.
    size = distance(l1.begin(),
                    l1.end());
    size2 = distance(l2.begin(),
                     l2.end());

    cout << "Size of l1, after assigning"
         << " l2 to it : " << size << endl;

    cout << "Size of l2, after assigning"
         << " it to l1 : " << size2 << endl;
}

Output:

Size of l1 is : 5
Size of l1, after removing all instances of 6 is : 3
Size of l2, before assigning it to l1 : 3
Size of l1, after assigning l2 to it : 6
Size of l2, after assigning it to l1 : 0