Forward List in C++ | Set 1 (Introduction and Important Functions) - GeeksforGeeks

17 Jan, 2022
Forward list in STL implements singly linked list. Introduced from C++11, forward list are more useful than other containers in insertion, removal, and moving operations (like sort) and allow time constant insertion and removal of elements. 
It differs from the list by the fact that the forward list keeps track of the location of only the next element while the list keeps track of both the next and previous elements, thus increasing the storage space required to store each element. The drawback of a forward list is that it cannot be iterated backward and its individual elements cannot be accessed directly. 
Forward List is preferred over the list when only forward traversal is required (same as the singly linked list is preferred over doubly linked list) as we can save space. Some example cases are, chaining in hashing, adjacency list representation of the graph, etc.
Operations on Forward List: 1. assign(): This function is used to assign values to the forward list, its other variant is used to assign repeated elements.  
CPP
// C++ code to demonstrate forward list// and assign()#include <forward_list>#include <iostream>using namespace std; // Driver Codeint main(){    // Declaring forward list    forward_list<int> flist1;     // Declaring another forward list    forward_list<int> flist2;     // Assigning values using assign()    flist1.assign({ 1, 2, 3 });     // Assigning repeating values using assign()    // 5 elements with value 10    flist2.assign(5, 10);     // Displaying forward lists    cout << "The elements of first forward list are : ";    for (int& a : flist1)        cout << a << " ";    cout << endl;     cout << "The elements of second forward list are : ";    for (int& b : flist2)        cout << b << " ";    cout << endl;     return 0;}
The elements of first forward list are : 1 2 3 
The elements of second forward list are : 10 10 10 10 10 
2. push_front(): This function is used to insert the element at the first position on forward list. The value from this function is copied to the space before first element in the container. The size of forward list increases by 1.
3. emplace_front(): This function is similar to the previous function but in this no copying operation occurs, the element is created directly at the memory before the first element of the forward list.
4. pop_front(): This function is used to delete the first element of the list.  
CPP
// C++ code to demonstrate working of// push_front(), emplace_front() and pop_front()#include <forward_list>#include <iostream>using namespace std; // Driver Codeint main(){    // Initializing forward list    forward_list<int> flist = { 10, 20, 30, 40, 50 };     // Inserting value using push_front()    // Inserts 60 at front    flist.push_front(60);     // Displaying the forward list    cout        << "The forward list after push_front operation : ";    for (int& c : flist)        cout << c << " ";    cout << endl;     // Inserting value using emplace_front()    // Inserts 70 at front    flist.emplace_front(70);     // Displaying the forward list    cout << "The forward list after emplace_front "            "operation : ";    for (int& c : flist)        cout << c << " ";    cout << endl;     // Deleting first value using pop_front()    // Pops 70    flist.pop_front();     // Displaying the forward list    cout << "The forward list after pop_front operation : ";    for (int& c : flist)        cout << c << " ";    cout << endl;     return 0;}
The forward list after push_front operation : 60 10 20 30 40 50 
The forward list after emplace_front operation : 70 60 10 20 30 40 50 
The forward list after pop_front operation : 60 10 20 30 40 50 
5. insert_after(): This function gives us a choice to insert elements at any position in forward list. The arguments in this function are copied at the desired position.
6. emplace_after(): This function also does the same operation as the above function but the elements are directly made without any copy operation.
7. erase_after(): This function is used to erase elements from a particular position in the forward list. 
CPP
// C++ code to demonstrate working of// insert_after(), emplace_after()// and erase_after()#include <forward_list>#include <iostream>using namespace std; // Driver Codeint main(){    // Initializing forward list    forward_list<int> flist = { 10, 20, 30 };     // Declaring a forward list iterator    forward_list<int>::iterator ptr;     // Inserting value using insert_after()    // starts insertion from second position    ptr = flist.insert_after(flist.begin(), { 1, 2, 3 });     // Displaying the forward list    cout << "The forward list after insert_after operation "            ": ";    for (int& c : flist)        cout << c << " ";    cout << endl;     // Inserting value using emplace_after()    // inserts 2 after ptr    ptr = flist.emplace_after(ptr, 2);     // Displaying the forward list    cout << "The forward list after emplace_after "            "operation : ";    for (int& c : flist)        cout << c << " ";    cout << endl;     // Deleting value using erase.after Deleted 2    // after ptr    ptr = flist.erase_after(ptr);     // Displaying the forward list    cout << "The forward list after erase_after operation "            ": ";    for (int& c : flist)        cout << c << " ";    cout << endl;     return 0;}
The forward list after insert_after operation : 10 1 2 3 20 30 
The forward list after emplace_after operation : 10 1 2 3 2 20 30 
The forward list after erase_after operation : 10 1 2 3 2 30 
8. remove(): This function removes the particular element from the forward list mentioned in its argument.
9. remove_if(): This function removes according to the condition in its argument. 
CPP
// C++ code to demonstrate// working of remove() and// remove_if()#include <forward_list>#include <iostream>using namespace std; // Driver Codeint main(){    // Initializing forward list    forward_list<int> flist = { 10, 20, 30, 25, 40, 40 };     // Removing element using remove()    // Removes all occurrences of 40    flist.remove(40);     // Displaying the forward list    cout << "The forward list after remove operation : ";    for (int& c : flist)        cout << c << " ";    cout << endl;     // Removing according to condition. Removes    // elements greater than 20. Removes 25 and 30    flist.remove_if([](int x) { return x > 20; });     // Displaying the forward list    cout << "The forward list after remove_if operation : ";    for (int& c : flist)        cout << c << " ";    cout << endl;     return 0;}
The forward list after remove operation : 10 20 30 25 
The forward list after remove_if operation : 10 20 
10. splice_after(): This function transfers elements from one forward list to other.  
CPP
// C++ code to demonstrate working of// splice_after()#include <forward_list> // for splice_after()#include <iostream>using namespace std; // Driver Codeint main(){    // Initializing forward list    forward_list<int> flist1 = { 10, 20, 30 };     // Initializing second list    forward_list<int> flist2 = { 40, 50, 60 };     // Shifting elements from first to second    // forward list after 1st position    flist2.splice_after(flist2.begin(), flist1);     // Displaying the forward list    cout << "The forward list after splice_after operation "            ": ";    for (int& c : flist2)        cout << c << " ";    cout << endl;     return 0;}
The forward list after splice_after operation : 40 10 20 30 50 60 
Method
Definition
For Second Set Refer to this Article – Forward List in C++ | Set 2 (Manipulating Functions)
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