在c++标准模板库(STL)中使用set

Question

集就是集合。集合中有一个函数是insert().

它是 return 类型：它 return 是指向集合中插入元素的迭代器。

我写了一个代码及其工作原理：

#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;
}

我写了另一个代码，它也能工作：

#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;
}

现在我合并了我之前代码的逻辑，但现在它不起作用，为什么？ :

#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;
}

以上代码输出：


 
/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

当您使用语法时：

auto itr = s.insert(5);

带有参数5的s.insert()的return类型给出了一个类型：

std::pair<std::set<int>::iterator, bool>

但是语法中没有从该类型到 std::set<int>::iterator 的已知重载转换：

itr = s.insert(itr, 4); // error!
_______________^^^_____

要解决，需要正确声明itr：

auto itr = s.insert(s.begin(), 5);

而不只是：

auto itr = s.insert(5);

在正确的情况下，s.insert() 的 return 类型是 std::set<int>::iterator，它与下一个语句兼容。

在c++标准模板库(STL)中使用set

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

c++

stl

insert

set