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forward_list insert_after() function in C++ STL

Last Updated : 16 Jun, 2022

The forward_list::insert_after() is a builtin function in C++ STL which gives us a choice to insert elements at the position just after the element pointed by a given iterator in the forward list. The arguments in this function are copied at the desired position. Syntax: 

forward_list_name.insert_after(iterator position, element)

or,

forward_list_name.insert_after(iterator position, n, element)

or,

forward_list_name.insert_after(iterator position, itr1, itr2)

or,

forward_list_name.insert_after(iterator position, list)

Parameters: The function accepts different parameters based on the above different syntaxes. Let's see every parameter in details and working of the above syntax.

• position, element: The parameter position is of type iterator and it points to the location after which the value of parameter element is to be inserted.
• position, n, element: The parameter position is of type iterator and it points to the location after which the value of parameter element is to be inserted. The parameter n specifies the number of times the value element is to inserted.
• position, itr1, itr2: The parameter position is of type iterator and it points to the location after which the values to be inserted. The iterators itr1 and itr2 denotes a range [itr1, itr2) and the elements in this range including itr1 and excluding itr2 will be inserted to the forward list after the iterator pointing to the given position.
• position, list: The parameter position is of type iterator and it points to the location after which the values are to be inserted. The second parameter list defines the list of elements to be inserted in the forward_list.

Return Value :This function returns an iterator that points to the last inserted element. Below program illustrates the above mentioned function: 

// C++ program to illustrate the 
// forward_list::insert_after() function 
#include <forward_list> 
#include <iostream> 
#include <list> 

using namespace std; 

int main() 
{ 

    forward_list<int> fwlist = { 1, 2, 3, 4, 5 }; 
    list<int> sampleList = { 8, 9, 10 }; 

    // This iterator points to the first element 
    auto it_new = fwlist.begin(); 

    // New element to be inserted 
    int element = 20; 

    /******************************/
    /** IMPLEMENTING SYNTAX 1 *****/
    /******************************/
    it_new = fwlist.insert_after(it_new, element); 

    cout << "After Syntax 1: "; 
    for (auto it = fwlist.cbegin(); it != fwlist.cend(); it++) { 
        cout << *it << " "; 
    } 

    // it_new points to new element inserted which is 20 
    // Make it to point to next element 
    it_new++; 

    /******************************/
    /** IMPLEMENTING SYNTAX 2 *****/
    /******************************/
    it_new = fwlist.insert_after(it_new, 3, element); 

    cout << "\n\nAfter Syntax 2: "; 
    for (auto it = fwlist.cbegin(); it != fwlist.cend(); it++) { 
        cout << *it << " "; 
    } 

    /******************************/
    /** IMPLEMENTING SYNTAX 3 *****/
    /******************************/
    it_new = fwlist.insert_after(it_new, sampleList.begin(), 
                                sampleList.end()); 

    cout << "\n\nAfter Syntax 3: "; 
    for (auto it = fwlist.cbegin(); it != fwlist.cend(); it++) { 
        cout << *it << " "; 
    } 

    /******************************/
    /** IMPLEMENTING SYNTAX 4 *****/
    /******************************/
    it_new = fwlist.insert_after(it_new, { 50, 60 }); 

    cout << "\n\nAfter Syntax 4: "; 
    for (auto it = fwlist.cbegin(); it != fwlist.cend(); it++) { 
        cout << *it << " "; 
    } 

    return 0; 
} 

After Syntax 1: 1 20 2 3 4 5 

After Syntax 2: 1 20 2 20 20 20 3 4 5 

After Syntax 3: 1 20 2 20 20 20 8 9 10 3 4 5 

After Syntax 4: 1 20 2 20 20 20 8 9 10 50 60 3 4 5

Time Complexity: O(n)

Auxiliary Space: O(1)

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