Why use iter = lst.insert(iter, word) and not just lst.insert(iter, word)-CodePudding

Edit: I'm changing 1st to lst

I'm still learning about insert on C .

why use iter = here?

list<string> lst;
auto iter = lst.begin();
while (cin >> word)
    iter = lst.insert(iter, word);

why not like this?

list<string> lst;
auto iter = lst.begin();
while (cin >> word)
    lst.insert(iter, word);

I'm confused because there is also this case, which don't use slist.begin() =

list<string> slist;
slist.insert(slist.begin(), "Hello!");

CodePudding user response：

The difference becomes immediately apparent when you look at the output of the following for input:

1
2
3

#include <list>
#include <iostream>
using namespace std;

int main()
{
    std::list<std::string> st;
    auto iter = st.end();
    std::string word;
    while (cin >> word)
        iter = st.insert(iter, word);

    for (const auto& w : st) std::cout << w << " ";
}

output:

3 2 1

#include <list>
#include <iostream>
using namespace std;

int main()
{
    std::list<std::string> st;
    auto iter = st.end();
    std::string word;
    while (cin >> word)
        st.insert(iter, word);

    for (const auto& w : st) std::cout << w << " ";
}

output:

1 2 3

Note that I used end() instead of begin(), because begin() looks a little fishy when there is no element yet in the list. However, for an empty list begin() == end() so it does not change the result.

std::list::insert inserts the element before the one referenced by the iterator you pass and returns an iterator to the newly inserted element. Hence, A inserts the next element always before the last one that was inserted. On the other hand, B always inserts the next element at the end, because iter always points to the lists end.