std::映射服装比较函数和函数/lambda错误

Question

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
2. 输入CCD_ 2已被下一个输入Map1["Aam"] = 8;重写

方法1：我想用一个函子来管理它。我有两个错误：

||=== Build: Debug in MyTestProgram (compiler: GNU GCC Compiler) ===|
d:mingwincludec++7.3.0bitsstl_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:ProgrammingC++CPP ProgramsMyTestProgramMyTestProgram.cpp|22|required from here|
d:mingwincludec++7.3.0bitsstl_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:ProgrammingC++CPP ProgramsMyTestProgramMyTestProgram.cpp|12|note: candidate: bool compare_functor::operator()(const T&, const T&) const|
D:ProgrammingC++CPP ProgramsMyTestProgramMyTestProgram.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:mingwincludec++7.3.0bitsstl_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:mingwincludec++7.3.0bitsstl_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:mingwincludec++7.3.0bitsstl_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:mingwincludec++7.3.0bitsstl_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:mingwincludec++7.3.0bitsstl_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：我用lambda实现的逻辑和我得到的错误几乎相同：

||=== Build: Debug in MyTestProgram (compiler: GNU GCC Compiler) ===|
d:mingwincludec++7.3.0bitsstl_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:mingwincludec++7.3.0bitsstl_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:mingwincludec++7.3.0bitsstl_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:mingwincludec++7.3.0bitsstl_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:mingwincludec++7.3.0bitsstl_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:mingwincludec++7.3.0bitsstl_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:mingwincludec++7.3.0bitsstl_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)) ===|

我在网上提到了很多答案。但他们中的大多数人在使用functor/lambda进行比较时都考虑了Key类型或Value类型，而我的情况并非如此。插入映射时，我想比较比较的Value和Key。

有人能向我解释一下为什么这些实现失败吗？

其次，在插入时，我应该做些什么来获得上述结果(PS:我仍然希望将string作为我的映射密钥，如果可能的话，我不希望任何临时映射产生此结果。(？

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;
}

Answer 1

std::map<int, std::string>就是您想要的。

为了获得相反的语法，您可以使用包装器，比如：

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

演示