C++ STL: forward_list (Complete Guide)

Forward_list in STL is a sequential container. It is very similar to list in STL but the list stores both the location of the previous and the next elements of the list while forward_list only stores the location of the next element, thus it saves memory. Insertion and deletion are efficient in forward_list and take only constant time. It is implemented as a singly LinkedList in memory. Forward_lists are included in #include <forward_list> header file.

Forward_list Declaration

Syntax:

forward_list<data_type> name;

data_type: data type of the elements to be stored in the list.

Example:

forward_list<int> l; //initializes a forward list of size 0 which stores integer values 

Forward_list Initialization

forward_list<data_type> name={initial_values};

initial_values: optional parameter which initializes the list with the given values.

Example:

forward_list<int> l = {1,2,3,4,5}; //initializes a forward list of size 5 with values 1,2,3,4,5 

Functions on forward_list

• push_front(element): It adds a new element to the front of the list. 
  Parameters: the element to be inserted
  Return value: void
   
• pop_front(): It deletes the first element of the list. 
  Parameters: None
  Return value: void
   
• insert_after(iterator,{values}): It inserts the values after the given position. 
  Parameters: iterator pointing to the position after which the value needs to be inserted, values that need to be inserted Return value: iterator, pointing to the last inserted element
   
• erase_after(iterator): It deletes the values after the given position. 
  Parameters: iterator, pointing to the position after which the value needs to be deleted
  Return value: void
   
• remove(element): It removes all the occurrences of the given element. 
  Parameters: element that needs to be deleted
  Return value: void
   
• front(): It returns the first element of the list. 
  Parameters: None 
  Return value: void
   
• begin(): It returns an iterator pointing to the first element of the list. 
  Parameters: None 
  Return value: iterator, pointing to the first element
   
• end(): It returns an iterator pointing to the last element of the list. 
  Parameters: None 
  Return value: iterator, pointing to the last element
   
• max_size(): It returns the maximum number of elements that can be stored by the forward_list. 
  Parameters: None 
  Return value: integer, the maximum size
   
• empty(): It tells us whether the list is empty or not
  Parameters: None
  Return value: boolean, true if the list is empty otherwise false.
   
• reverse(): It reverses the list. 
  Parameters: None
  Return value: void

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

void print(forward_list <int> l)
{
	forward_list<int>::iterator itr;
	for(itr=l.begin();itr!=l.end();++itr)
		cout<<*itr<<' ';
	cout<<'\n';
}

int main()
{
	forward_list<int> l={1,2,3};
	for(int i=0;i<5;++i)
	{
		l.push_front(i);
	}
	print(l);
	cout<<"First element " <<l.front()<<'\n';
	l.pop_front();
	l.insert_after(l.begin(),{5,6,7});
	print(l);
	l.erase_after(l.begin());
	print(l);
	l.remove(2);
	cout<<"List after removing all occurrences of 2: ";
	print(l);
	if(!l.empty()){
	cout<<"Max size of list "<<l.max_size()<<'\n';
	}
	return 0;
}

Output

4 3 2 1 0 1 2 3 
First element 4
3 5 6 7 2 1 0 1 2 3 
3 6 7 2 1 0 1 2 3 
List after removing all occurrences of 2: 3 6 7 1 0 1 3 
Max size of list 576460752303423487