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C++ Forward_list::cbefore_begin() Function
The C++ std::forward_list::cbefore_begin() function is used to retrieve a constant iterator to the element before the first element of the forward_list container.
The element of this forward_list acts as a placeholder, attempting to access it results in undefined behavior. The returned iterator can be used with the emplace_after(), erase_after(), insert_after(), and splice_after() functions to insert, erase, and delete an element at the returned iterator position.
The cbefore_begin() function is similar to the before_begin() function in C++ std:: forward_list.
Syntax
Following is the syntax of the C++ std::forward_list::cbefore_begin() function −
const_iterator cbefore_begin();
Parameters
- It does not accept any parameter.
Return value
This function returns a constant iterator to the element before the first element.
Example 1
If the forward_list is an int-type, the cbefore_begin() function returns a constant iterator to the element before the first element.
In the following program, we are using the C++ std::forward_list::cbefore_begin() function to retrieve a constant iterator to the element before the first element of this forward_list {1, 2, 3, 4, 5}.
#include<iostream> #include<forward_list> using namespace std; int main() { //create a forward_list forward_list<int> num_list = {1, 2, 3, 4, 5}; cout<<"The num_list contents are: "<<endl; for(int n : num_list){ cout<<n<<endl; } //using the cbefore_begin() function auto it = num_list.cbefore_begin(); cout<<"The constant iterator to the element before the first element is: "<<*it; }
Output
Let us compile and run the above program, this will produce the following result −
The num_list contents are: 1 2 3 4 5 The constant iterator to the element before the first element is: 0
Example 2
If the forward_list is char-type, this function returns a constant iterator to the element before the first char element.
Following is another example of the C++ std::forward_list::cbefore_begin() function. Here, we are creating a forward_list(type char) named char_list with contents {'B', 'C', 'E', 'F'}. Then, we use the cbefore_begin() function to retrieve a constant iterator to the element before the first element of this container, and using the emplace_after() function, insert a new element 'A' at the returned iterator position.
#include<iostream> #include<forward_list> using namespace std; int main() { //create a forward_list forward_list<char> char_list = {'B', 'C', 'D', 'E'}; cout<<"The char_list contents are: "<<endl; for(char c : char_list){ cout<<c<<endl; } //using the cbefore_begin() function auto it = char_list.cbefore_begin(); cout<<"The constant iterator is: "<<*it<<endl; //using emplace_after() function char_list.emplace_after(it, 'A'); cout<<"The char_list contents after inserting new element: "<<endl; for(char c: char_list){ cout<<c<<endl; } }
Output
This will generate the following output −
The char_list contents are: B C D E The constant iterator is: � The char_list contents after inserting new element: A B C D E
Example 3
Apart from the int-type and char-type forward_list, we can also retrieve a constant iterator of the string-type forward_list element.
In this example, we are creating a forward_list(type string) named fruits with contents {"Apple", "Banana", "Orange", "Grapes", "Guava"}. Then, using the cbefore_begin() function, we are trying to retrieve a constant iterator to the element before the first element of this container, and using the insert_after() function, we are trying to insert a new element "Pear" at the returned iterator position.
#include<iostream> #include<forward_list> using namespace std; int main() { //create a forward_list forward_list<string> fruits = {"Apple", "Banana", "Orange", "Grapes", "Guava"}; cout<<"The fruits forward_list contents are: "<<endl; for(string n : fruits){ cout<<n<<endl; } //using the cbefore_begin() function auto it = fruits.cbefore_begin(); cout<<"The constant iterator is: "<<*it<<endl; //using the insert_after()function fruits.insert_after(it, "Pear"); cout<<"The fruits forward_list contents after inserting new element: "<<endl; for(string s : fruits){ cout<<s<<endl; } }
Output
Following is the output of the above program −
The fruits forward_list contents are: Apple Banana Orange Grapes Guava The constant iterator is: The fruits forward_list contents after inserting new element: Pear Apple Banana Orange Grapes Guava