c++ - 用C++排序时的 map 会产生巨大的错误

Question

我是使用 map 的新手，当我尝试使用以下代码对其进行排序时

#include <iostream>
#include<map>
#include<algorithm>
using namespace std;

int main()
{
    map<int,int> mp;
    mp[2]++;
    mp[5]++;
    mp[1]++;
    sort(mp.begin(), mp.end());
}

引发错误:

In file included from /usr/include/c++/6/algorithm:62:0,
                 from main.cpp:11:
/usr/include/c++/6/bits/stl_algo.h: In instantiation of ‘void std::__sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = std::_Rb_tree_iterator<std::pair<const int, int> >; _Compare = __gnu_cxx::__ops::_Iter_less_iter]’:
/usr/include/c++/6/bits/stl_algo.h:4707:18:   required from ‘void std::sort(_RAIter, _RAIter) [with _RAIter = std::_Rb_tree_iterator<std::pair<const int, int> >]’
<span class="error_line" onclick="ide.gotoLine('main.cpp',21)">main.cpp:21:30</span>:   required from here
/usr/include/c++/6/bits/stl_algo.h:1966:22: error: no match for ‘operator-’ (operand types are ‘std::_Rb_tree_iterator >’ and ‘std::_Rb_tree_iterator >’)
     std::__lg(__last - __first) * 2,
               ~~~~~~~^~~~~~~~~
In file included from /usr/include/c++/6/bits/stl_algobase.h:67:0,
                 from /usr/include/c++/6/bits/char_traits.h:39,
                 from /usr/include/c++/6/ios:40,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from main.cpp:9:
/usr/include/c++/6/bits/stl_iterator.h:336:5: note: candidate: template decltype ((__x.base() - __y.base())) std::operator-(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_Iterator>&)
     operator-(const reverse_iterator<_Iterator>& __x,
     ^~~~~~~~
/usr/include/c++/6/bits/stl_iterator.h:336:5: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/6/algorithm:62:0,
                 from main.cpp:11:
/usr/include/c++/6/bits/stl_algo.h:1966:22: note:   ‘std::_Rb_tree_iterator >’ is not derived from ‘const std::reverse_iterator<_Iterator>’
     std::__lg(__last - __first) * 2,
               ~~~~~~~^~~~~~~~~

可能是什么问题？
我试图以不同的方式更改代码，但无济于事。我似乎没有发现我在代码中犯的小错误

Answer 1

What might be the problem?

您不需要对 std::map进行排序，它已经排序。
错误消息(肯定不是很有帮助)表明您将错误类型的迭代器传递给 std::sort。它需要随机访问迭代器(请参阅“类型要求” here)，但是 std::map不提供这些迭代器(它使用双向迭代器)。从错误消息:

error: no match for operator- (operand types are std::_Rb_tree_iterator > and std::_Rb_tree_iterator >)

这表明实例化 std::sort模板尝试在 operator -返回的迭代器上使用 mp.begin()。算术运算符仅存在于随机访问迭代器(和满足随机访问类别的指针)上。