Using set in c++ standard template library (STL) - c++

Question

Sets are the collection. There is a function in set isinsert().
It's return type : it returns the iterator pointing to the inserted element in the collection.
I write a code and its working :
#include<bits/stdc++.h>
using namespace std;
int main(){
set<int> s;
s.insert (1);
s.insert (4);
s.insert (2);
s.insert (5);
s.insert (3);
cout << "The elements in set are: ";
for (auto it = s.begin(); it!= s.end (); it++)
cout << *it;
return 0;
}
I write another code and it also working:
#include<bits/stdc++.h>
using namespace std;
int main(){
set<int> s;
auto itr=s.insert (s.begin(),5);
itr=s.insert (itr,4);
itr=s.insert (itr,2);
cout << "The elements in set are: ";
for (auto it = s.begin(); it!= s.end (); it++)
cout << *it;
return 0;
}
Now i merge the logic of both my previous code but now its not working why? :
#include<bits/stdc++.h>
using namespace std;
int main(){
set<int> s;
auto itr=s.insert (1);
s.insert (itr,4);
cout << "The elements in set are: ";
for (auto it = s.begin(); it!= s.end (); it++)
cout << *it;
return 0;
}
Output in above code :
/usr/include/c++/7/bits/stl_set.h:536:7: note: candidate: std::set<_Key, _Compare, _Alloc>::iterator std::set<_Key, _Compare, _Alloc>::insert(std::set<_Key, _Compare, _Alloc>::const_iterator, const value_type&) [with _Key = int; _Compare = std::less<int>; _Alloc = std::allocator<int>; std::set<_Key, _Compare, _Alloc>::iterator = std::_Rb_tree_const_iterator<int>; std::set<_Key, _Compare, _Alloc>::const_iterator = std::_Rb_tree_const_iterator<int>; std::set<_Key, _Compare, _Alloc>::value_type = int]
insert(const_iterator __position, const value_type& __x)
^~~~~~
/usr/include/c++/7/bits/stl_set.h:536:7: note: no known conversion for argument 1 from ‘std::pair<std::_Rb_tree_const_iterator<int>, bool>’ to ‘std::set<int>::const_iterator {aka std::_Rb_tree_const_iterator<int>}’
/usr/include/c++/7/bits/stl_set.h:541:7: note: candidate: std::set<_Key, _Compare, _Alloc>::iterator std::set<_Key, _Compare, _Alloc>::insert(std::set<_Key, _Compare, _Alloc>::const_iterator, std::set<_Key, _Compare, _Alloc>::value_type&&) [with _Key = int; _Compare = std::less<int>; _Alloc = std::allocator<int>; std::set<_Key, _Compare, _Alloc>::iterator = std::_Rb_tree_const_iterator<int>; std::set<_Key, _Compare, _Alloc>::const_iterator = std::_Rb_tree_const_iterator<int>; std::set<_Key, _Compare, _Alloc>::value_type = int]
insert(const_iterator __position, value_type&& __x)
^~~~~~
/usr/include/c++/7/bits/stl_set.h:541:7: note: no known conversion for argument 1 from ‘std::pair<std::_Rb_tree_const_iterator<int>, bool>’ to ‘std::set<int>::const_iterator {aka std::_Rb_tree_const_iterator<int>}’
/usr/include/c++/7/bits/stl_set.h:556:2: note: candidate: template<class _InputIterator> void std::set<_Key, _Compare, _Alloc>::insert(_InputIterator, _InputIterator) [with _InputIterator = _InputIterator; _Key = int; _Compare = std::less<int>; _Alloc = std::allocator<int>]
insert(_InputIterator __first, _InputIterator __last)
^~~~~~
/usr/include/c++/7/bits/stl_set.h:556:2: note: template argument deduction/substitution failed:
yo.cpp:14:16: note: deduced conflicting types for parameter ‘_InputIterator’ (‘std::pair<std::_Rb_tree_const_iterator<int>, bool>’ and ‘int’)
s.insert (itr,4)
^
In file included from /usr/include/c++/7/set:61:0,
from /usr/include/x86_64-linux-gnu/c++/7/bits/stdc++.h:87,
from yo.cpp:1:
/usr/include/c++/7/bits/stl_set.h:568:7: note: candidate: void std::set<_Key, _Compare, _Alloc>::insert(std::initializer_list<_Tp>) [with _Key = int; _Compare = std::less<int>; _Alloc = std::allocator<int>]
insert(initializer_list<value_type> __l)
^~~~~~
/usr/include/c++/7/bits/stl_set.h:568:7: note: candidate expects 1 argument, 2 provided

Answer 1

When you use the syntax:
auto itr = s.insert(5);
The return type of s.insert() with the parameter 5 gives a type of:
std::pair<std::set<int>::iterator, bool>
But there's no such known overloaded conversion from that type to std::set<int>::iterator in the syntax:
itr = s.insert(itr, 4); // error!
_______________^^^_____
To solve it, you need to declare itr correctly:
auto itr = s.insert(s.begin(), 5);
Rather than only:
auto itr = s.insert(5);
In the correct case, the return type of s.insert() is std::set<int>::iterator which is compatible for the next statements.