forward_list emplace_after() and emplace_front() in C++ STL

1. The forward_list::emplace_after() is a builtin function in C++ STL which is used to insert a new element after the element at position specified in the argument. This insertion of the new element increases the size of the container by one.

   Syntax:

   forward_list_name.emplace_after(iterator position, elements)

   Parameters: The function accepts two mandatory parameter which are described below:

   • position: it specifies the iterator which points to the position in the container after which new element is to be inserted.
   • element: specifies the new element to be inserted after the position.

   Return Value :This function returns an iterator that points to the newly inserted element.

   Below program illustrates the above mentioned function:

   // C++ program to illustrate the
   // forward_list::emplace_after() function
   #include <forward_list>
   #include <iostream>
   
   using namespace std;
   
   int main()
   {
   
       forward_list<int> fwlist = { 1, 2, 3, 4, 5 };
   
       auto it_new = fwlist.before_begin();
   
       // use of emplace_after function
       // inserts elements at positions
       it_new = fwlist.emplace_after(it_new, 8);
       it_new = fwlist.emplace_after(it_new, 10);
   
       // cout << "The elements are: "
       for (auto it = fwlist.cbegin(); it != fwlist.cend(); it++) {
           cout << *it << " ";
       }
   
       return 0;
   }

   Output:

   The elements are: 8 10 1 2 3 4 5 

2. The forward_list::emplace_front() is a built-in function in C++ which is used to insert a new element at the beginning of the forward_list just before the first element. This increases the size of container by one.

   Syntax:

   forward_list_name.emplace_front(elements)

   Parameters: The function accepts one mandatory parameter element which is to be inserted at the start of the container.

   Return Value: It returns nothing.

   Below program illustrates the above function.

   // C++ program to illustrate the
   // forward_list::emplace_front() function
   #include <forward_list>
   #include <iostream>
   
   using namespace std;
   
   int main()
   {
   
       forward_list<int> fwlist = { 1, 2, 3, 4, 5 };
   
       // use of emplace_front function
       // inserts elements at front
       fwlist.emplace_front(8);
       fwlist.emplace_front(10);
   
       cout << "Elements are: ";
       // Auto iterator
       for (auto it = fwlist.cbegin(); it != fwlist.cend(); it++) {
           cout << *it << " ";
       }
   
       return 0;
   }

   Output:

   Elements are: 10 8 1 2 3 4 5