std ::既映射服装比较函子和函数/ lambda错误

时间：2018-07-29 16:10:06

标签： c++ c++11 compare stdmap

我有以下内容：

inputs:    
    Map1["Ram"] = 8;
    Map1["Aam"] = 8;
    Map1["Some"] = 2;
    Map1["He"] = 5;
    Map1["He"] = 6;

插入std::map<std::string, int>后的预期输出应为：

Output:
    "Some" 2  
    "He"   5
    "He"   6
    "Aam"  8

请注意：

  1. 排序是根据值而不是键进行的。
  2. 输入Map1["Ram"] = 8;已被下一个输入Map1["Aam"] = 8;覆盖

方法1 ：我正在考虑使用函子进行管理。我有这两个错误：

||=== Build: Debug in MyTestProgram (compiler: GNU GCC Compiler) ===|
d:\mingw\include\c++\7.3.0\bits\stl_map.h||In instantiation of 'std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](std::map<_Key, _Tp, _Compare, _Alloc>::key_type&&) [with _Key = std::__cxx11::basic_string<char>; _Tp = int; _Compare = compare_functor; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, int> >; std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type = int; std::map<_Key, _Tp, _Compare, _Alloc>::key_type = std::__cxx11::basic_string<char>]':|
D:\Programming\C++\CPP Programs\MyTestProgram\MyTestProgram.cpp|22|required from here|
d:\mingw\include\c++\7.3.0\bits\stl_map.h|511|error: no match for call to '(std::map<std::__cxx11::basic_string<char>, int, compare_functor>::key_compare {aka compare_functor}) (std::map<std::__cxx11::basic_string<char>, int, compare_functor>::key_type&, const std::__cxx11::basic_string<char>&)'|
D:\Programming\C++\CPP Programs\MyTestProgram\MyTestProgram.cpp|12|note: candidate: bool compare_functor::operator()(const T&, const T&) const|
D:\Programming\C++\CPP Programs\MyTestProgram\MyTestProgram.cpp|12|note:   no known conversion for argument 1 from 'std::map<std::__cxx11::basic_string<char>, int, compare_functor>::key_type {aka std::__cxx11::basic_string<char>}' to 'const T& {aka const std::pair<std::__cxx11::basic_string<char>, int>&}'|
d:\mingw\include\c++\7.3.0\bits\stl_tree.h||In instantiation of 'std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_lower_bound(std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Link_type, std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Base_ptr, const _Key&) [with _Key = std::__cxx11::basic_string<char>; _Val = std::pair<const std::__cxx11::basic_string<char>, int>; _KeyOfValue = std::_Select1st<std::pair<const std::__cxx11::basic_string<char>, int> >; |
d:\mingw\include\c++\7.3.0\bits\stl_tree.h|1187|required from 'std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::lower_bound(const key_type&) [with _Key = std::__cxx11::basic_string<char>; _Val = std::pair<const std::__cxx11::basic_string<char>, int>; _KeyOfValue = std::_Select1st<std::pair<const std::__cxx11::basic_string<char>, int> >; _Compare = compare_functor; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, int> >; std::_Rb_tree<_Key, _Val, _KeyOfVa|
d:\mingw\include\c++\7.3.0\bits\stl_map.h|1234|required from 'std::map<_Key, _Tp, _Compare, _Alloc>::iterator std::map<_Key, _Tp, _Compare, _Alloc>::lower_bound(const key_type&) [with _Key = std::__cxx11::basic_string<char>; _Tp = int; _Compare = compare_functor; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, int> >; std::map<_Key, _Tp, _Compare, _Alloc>::iterator = std::_Rb_tree_iterator<std::pair<const std::__cxx11::basic_string<char>, int> >; std::map<_Key, _Tp, _Compare, _Alloc>::key_type = std::__cxx11::basic_string<char|
d:\mingw\include\c++\7.3.0\bits\stl_map.h|509|required from 'std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](std::map<_Key, _Tp, _Compare, _Alloc>::key_type&&) [with _Key = std::__cxx11::basic_string<char>; _Tp = int; _Compare = compare_functor; _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, int> >; std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type = int; std::map<_Key, _Tp, _Compare, _Alloc>::key_type = std::__cxx11::basic_string<char>]'|
Program.cpp|22|required from here|
d:\mingw\include\c++\7.3.0\bits\stl_tree.h|1872|error: no match for call to '(compare_functor) (const std::__cxx11::basic_string<char>&, const std::__cxx11::basic_string<char>&)'|
Program.cpp|12|note: candidate: bool compare_functor::operator()(const T&, const T&) const|
Program.cpp|12|note:   no known conversion for argument 1 from 'const std::__cxx11::basic_string<char>' to 'const T& {aka const std::pair<std::__cxx11::basic_string<char>, int>&}'|
||=== Build failed: 2 error(s), 7 warning(s) (0 minute(s), 0 second(s)) ===|

方法2 ：我使用lambda实施的逻辑相同，但也出现了几乎相同的错误：

||=== Build: Debug in MyTestProgram (compiler: GNU GCC Compiler) ===|
d:\mingw\include\c++\7.3.0\bits\stl_map.h||In instantiation of 'std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](std::map<_Key, _Tp, _Compare, _Alloc>::key_type&&) [with _Key = std::__cxx11::basic_string<char>; _Tp = int; _Compare = bool (*)(const std::pair<std::__cxx11::basic_string<char>, int>&, const std::pair<std::__cxx11::basic_string<char>, int>&); _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, int> >; std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type = int; std:|
Program.cpp|37|required from here|
d:\mingw\include\c++\7.3.0\bits\stl_map.h|511|error: invalid initialization of reference of type 'const std::pair<std::__cxx11::basic_string<char>, int>&' from expression of type 'std::map<std::__cxx11::basic_string<char>, int, bool (*)(const std::pair<std::__cxx11::basic_string<char>, int>&, const std::pair<std::__cxx11::basic_string<char>, int>&)>::key_type {aka std::__cxx11::basic_string<char>}'|
d:\mingw\include\c++\7.3.0\bits\stl_tree.h||In instantiation of 'std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_lower_bound(std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Link_type, std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Base_ptr, const _Key&) [with _Key = std::__cxx11::basic_string<char>; _Val = std::pair<const std::__cxx11::basic_string<char>, int>; _KeyOfValue = std::_Select1st<std::pair<const std::__cxx11::basic_string<char>, int> >; |
d:\mingw\include\c++\7.3.0\bits\stl_tree.h|1187|required from 'std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::lower_bound(const key_type&) [with _Key = std::__cxx11::basic_string<char>; _Val = std::pair<const std::__cxx11::basic_string<char>, int>; _KeyOfValue = std::_Select1st<std::pair<const std::__cxx11::basic_string<char>, int> >; _Compare = bool (*)(const std::pair<std::__cxx11::basic_string<char>, int>&, const std::pair<std::__cxx11::basic_string<char>, int>&); _Alloc = |
d:\mingw\include\c++\7.3.0\bits\stl_map.h|1234|required from 'std::map<_Key, _Tp, _Compare, _Alloc>::iterator std::map<_Key, _Tp, _Compare, _Alloc>::lower_bound(const key_type&) [with _Key = std::__cxx11::basic_string<char>; _Tp = int; _Compare = bool (*)(const std::pair<std::__cxx11::basic_string<char>, int>&, const std::pair<std::__cxx11::basic_string<char>, int>&); _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, int> >; std::map<_Key, _Tp, _Compare, _Alloc>::iterator = std::_Rb_tree_iterator<std::pair<const std::__cxx11::ba|
d:\mingw\include\c++\7.3.0\bits\stl_map.h|509|required from 'std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](std::map<_Key, _Tp, _Compare, _Alloc>::key_type&&) [with _Key = std::__cxx11::basic_string<char>; _Tp = int; _Compare = bool (*)(const std::pair<std::__cxx11::basic_string<char>, int>&, const std::pair<std::__cxx11::basic_string<char>, int>&); _Alloc = std::allocator<std::pair<const std::__cxx11::basic_string<char>, int> >; std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type = int; std::map<_|
Program.cpp|37|required from here|
d:\mingw\include\c++\7.3.0\bits\stl_tree.h|1872|error: invalid initialization of reference of type 'const std::pair<std::__cxx11::basic_string<char>, int>&' from expression of type 'const std::__cxx11::basic_string<char>'|
||=== Build failed: 2 error(s), 7 warning(s) (0 minute(s), 0 second(s)) ===|

我在网上提到了许多答案。但是他们中的大多数人在使用函子/ lambda进行比较时都考虑了键类型或值类型，这不是我的情况。我想在插入地图时比较“值”和“键比较” 。

有人可以向我解释为什么这些实现失败吗？

其次，在插入（ PS：）时，我应该做些什么才能获得以上结果：我仍然想保留string作为地图的关键，如果可能的话，我不希望任何临时地图执行此结果。）

这是我的代码：

#include <iostream>
#include <map>
#include <string>
#include <tuple>

typedef std::pair<std::string, int> T;
typedef bool(*compare_functional_type)(const T&, const T&);

// Way - 1: using functor
struct compare_functor
{
   bool operator()(const T& A ,const T& B)const
   {
      return std::tie(A.second, A.first) < std::tie(B.second, B.first);
      // OR in other-words:
      //return (A.second == B.second) ? A.first < B.first : A.second < B.second;
   };
};
int main()
{
   std::map<std::string, int, compare_functor> Map1;
   Map1["Ram"] = 8;
   Map1["Aam"] = 8;
   Map1["Some"] = 2;
   Map1["He"] = 5;
   Map1["He"] = 6;

   // Way - 2: using lambdas and function pointers
   compare_functional_type Lambda = [](const T& A, const T& B)
   {
      return std::tie(A.second, A.first) < std::tie(B.second, B.first);
      // OR in other-words:
      //return (A.second == B.second) ? A.first < B.first : A.second < B.second;
   };

   std::map<std::string, int, compare_functional_type> Map2(Lambda);
   Map2["Ram"] = 8;
   Map2["Aam"] = 8;
   Map2["Some"] = 2;
   Map2["He"] = 5;
   Map2["He"] = 6;

   return 0;
}

1 个答案:

答案 0 :(得分：2)

std::map<int, std::string>是您想要的。

要使用反向语法，可以使用包装器，例如：

class Wrapper
{
public:
    Wrapper(std::map<int, std::string>& m, const std::string& s) : m(m), s(s) {}

    void operator =(int k) { m[k] = s; }
private:
    std::map<int, std::string>& m;
    std::string s;
};

class InvertedMap
{
public:
    Wrapper operator[](const std::string& s) { return {m, s}; }

    auto begin() const { return m.begin(); }
    auto end() const { return m.end(); }
private:
    std::map<int, std::string> m;  
};

然后：

InvertedMap Map1;
Map1["Ram"] = 8;
Map1["Aam"] = 8;
Map1["Some"] = 2;
Map1["He"] = 5;
Map1["He"] = 6;

for (const auto& p : Map1) {
    std::cout << p.second << " " << p.first << std::endl;
}

// Output:
// Some 2
// He 5
// He 6
// Aam 8

Demo

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