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forward_list::before_begin() in C++ STL
In this article we will be discussing the working, syntax and examples of forward_list::before_begin() function in C++.
What is a Forward_list in STL?
Forward list are sequence containers that allow constant time insert and erase operations anywhere within the sequence. Forward list are implement as a singly-linked lists. The ordering is kept by the association to each element of a link to the next element in the sequence.
What is forward_list::before_begin()?
forward_list::before_begin() is an inbuilt function in C++ STL which is declared in <forward_list> header file. before_begin() returns the iterator which is pointing to the element before the first element in the forward_list container.
Syntax
forwardlist_container.before_begin();
This function accepts no parameter.
Return Value
This function returns a iterator to the position before the beginning of the sequence.
Example
/* In the below code we are creating a forward list and then using the before_begin() function we will point to the first element in a forward list and after that we will try to insert a new element in front of the forward list using insert_after() function. Now, we will notice the changes in the output. */
#include <bits/stdc++.h>
using namespace std;
int main() {
//creating and initializing forward list
forward_list<int> forwardList = { 3, 6, 1, 2, 4 };
//calling before_begin function
auto i = forwardList.before_begin();
//inserting element before forward list
forwardList.insert_after(i, 7);
cout<< "Element of the forward list are:" << endl;
for (auto j = forwardList.begin(); j != forwardList.end(); ++j)
cout << *j << " ";
return 0;
}Output
If we run the above code it will generate the following output
Element of the forward list are: 7 3 6 1 2 4
Example
#include <bits/stdc++.h>
using namespace std;
int main() {
forward_list<int> forwardList = {2, 23, 12, 11};
forwardList.insert_after(forwardList.before_begin(), 19 );
cout << "Elements in the forward lists are : ";
for (auto j = forwardList.begin(); j != forwardList.end(); ++j)
cout << *j << " ";
return 0;
}Output
If we run the above code it will generate the following output
Elements in the forward lists are : 19 2 23 12 11
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