Condensed for loop.. (C++) [duplicate] - c++

Question

This question already has answers here:
std::forward_list and std::forward_list::push_back
(5 answers)
Closed 5 years ago.
I have stumbled across this piece of code and trying to understand it, certain section are clear but not for loop:
forward_list<Class> obj;
auto b_end = obj.before_begin();
for (auto& _ : obj)
++b_end ;
obj.insert_after(b_end, Class newObj);
Not exactly sure what for loop is doing. Could some break the for loop down and help me understand it?
Edit:
This question is marked duplicate but other threads do not provide sufficient information on it and/or are too old.

Answer 1

std::forward_list is a sequential container with forward_iterators. It is a one-sided singly-linked list. To insert a value into the list you can use only the method insert_after specifying an iterator in the range [begin(), end()) or the iterator returned by the method before_begin().
From the C++ Standard
5 Requires: position is before_begin() or is a dereferenceable
iterator in the range [begin(), end()).
So if you are going to append a new value to the end of a list you have to move an iterator in the position that corresponds to the last element in the list.
So this loop named range-based for loop
for (auto& _ : obj)
++b_end ;
moves step by step the iterator b_end in the position occupied by the last element in the list. So now using the iterator you can append a new value to the list using the method insert_after.
Consider a simple demonstrative program.
#include <iostream>
#include <forward_list>
int main()
{
std::forward_list<int> lst = { 1, 2 };
auto b_end = lst.before_begin();
for (const auto &_ : lst)
{
++b_end;
std::cout << *b_end << '\n';
}
std::cout << std::endl;
lst.insert_after(b_end, 3);
for (const auto &_ : lst) std::cout << _ << ' ';
std::cout << std::endl;
return 0;
}
The program output is
1
2
1 2 3
At first the iterator b_end points to before the first element in the list.
auto b_end = lst.before_begin();
^^^^^^^^^^^^^^
After that in the loop that will have two iterations because the list contains only two elements the iterator at first will be moved in the position that corresponds to the first element and then in the position that corresponds to the second element.
Now using the obtained iterator we can add the value 3 after the value 2.

Answer 2

This:
forward_list<Class> obj;
declares a std::forward_list of Class-es named obj. The std:: part is omitted due to use of using namespace std; somewhere in the code.
This:
auto b_end = obj.before_begin();
defines an iterator pointing at the std::forward_list::before_begin location in your obj list.
This:
for (auto& _ : obj)
++b_end;
is a range based for loop that uses an oddly named _ variable passed by reference. In this loop the b_end gets incremented in each iteration.
Finally:
obj.insert_after(b_end, Class newObj);
tries to insert a new newObj of type Class after the b_end position and fails to compile because it should probably be:
Class newObj;
obj.insert_after(b_end, newObj);
That being said, the _ is just a variable name. It is a (somewhat unusual yet) valid variable identifier. In here the _ variable itself is not used in any way in the for loop so the whole thing could have been rewritten as:
for (auto el : obj){
++b_end;
}
The auto keyword stands for an auto specifier.