public member function
<forward_list>
std:: forward_list::before_begin
iterator before_begin() noexcept;
const_iterator before_begin() const noexcept;
Return iterator to before beginning
Returns an iterator pointing to the position before the first element in the container.
The iterator returned shall not be dereferenced: It is meant to be used as an argument for member functions emplace_after, insert_after, erase_after or splice_after, to specify the beginning of the sequence as the location where the action is performed.
Return Value
An iterator to the position before the beginning of the sequence.
If the forward_list object is const-qualified, the function returns a const_iterator. Otherwise, it returns an iterator.
Member types iterator and const_iterator are forward iterator types (pointing to an element and to a const element, respectively).
Example
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
|
// forward_list::before_begin
#include <iostream>
#include <forward_list>
int main ()
{
std::forward_list<int> mylist = {20, 30, 40, 50};
mylist.insert_after ( mylist.before_begin(), 11 );
std::cout << "mylist contains:";
for ( int& x: mylist ) std::cout << ' ' << x;
std::cout << '\n';
return 0;
}
| |
Output:
mylist contains: 11 20 30 40 50
|
Iterator validity
No changes.
Data races
The container is accessed (neither the const nor the non-const versions modify the container).
No contained elements are accessed by the call, but the iterator returned can be used to access or modify elements. Concurrently accessing or modifying different elements is safe.
Exception safety
No-throw guarantee: this member function never throws exceptions.
The copy construction or assignment of the returned iterator is also guaranteed to never throw.