以结构体为键值时的map报错

struct ch{ //选课 
	int st_id; // 学号 
	int sb_id; // 课程号 
};
map<ch, int> cho;

bool choose::check(ch f)
{
	int x = cho[f];
	return x != 0;
}
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也不知道为什么，总之好像只要直接用一个结构体的变量作为键值而不是手写的{a, b}这个样子的，就会报一堆奇怪的错误

52		D:\Dev-Cpp\MinGW64\lib\gcc\x86_64-w64-mingw32\4.9.2\include\c++\x86_64-w64-mingw32\bits\stdc++.h	                 from D:/Dev-Cpp/MinGW64/lib/gcc/x86_64-w64-mingw32/4.9.2/include/c++/x86_64-w64-mingw32/bits/stdc++.h
2		C:\Users\30504\Desktop\新建文件夹 (2)\main.cpp	                 from main.cpp
D:\Dev-Cpp\MinGW64\lib\gcc\x86_64-w64-mingw32\4.9.2\include\c++\bits\stl_function.h	In instantiation of 'bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = ch]':
498	31	D:\Dev-Cpp\MinGW64\lib\gcc\x86_64-w64-mingw32\4.9.2\include\c++\bits\stl_map.h	required from 'std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const key_type&) [with _Key = ch; _Tp = int; _Compare = std::less<ch>; _Alloc = std::allocator<std::pair<const ch, int> >; 
std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type = int; std::map<_Key, _Tp, _Compare, _Alloc>::key_type = ch]'
371	20	D:\Dev-Cpp\MinGW64\lib\gcc\x86_64-w64-mingw32\4.9.2\include\c++\bits\stl_function.h	[Error] no match for 'operator<' (operand types are 'const ch' and 'const ch')
371	20	D:\Dev-Cpp\MinGW64\lib\gcc\x86_64-w64-mingw32\4.9.2\include\c++\bits\stl_function.h	[Note] candidates are:
371	20	D:\Dev-Cpp\MinGW64\lib\gcc\x86_64-w64-mingw32\4.9.2\include\c++\bits\stl_function.h	[Note] candidates are:
58	0	D:\Dev-Cpp\MinGW64\lib\gcc\x86_64-w64-mingw32\4.9.2\include\c++\regex	In file included from D:/Dev-Cpp/MinGW64/lib/gcc/x86_64-w64-mingw32/4.9.2/include/c++/regex

108		D:\Dev-Cpp\MinGW64\lib\gcc\x86_64-w64-mingw32\4.9.2\include\c++\x86_64-w64-mingw32\bits\stdc++.h	                 from D:/Dev-Cpp/MinGW64/lib/gcc/x86_64-w64-mingw32/4.9.2/include/c++/x86_64-w64-mingw32/bits/stdc++.h
1428	5	D:\Dev-Cpp\MinGW64\lib\gcc\x86_64-w64-mingw32\4.9.2\include\c++\bits\regex.h	[Note] template<class _Bi_iter> bool std::operator<(const std::sub_match<_BiIter>&, const typename std::iterator_traits<_Iter>::value_type&)

1428	5	D:\Dev-Cpp\MinGW64\lib\gcc\x86_64-w64-mingw32\4.9.2\include\c++\bits\regex.h	[Note] template argument deduction/substitution failed:
48	0	D:\Dev-Cpp\MinGW64\lib\gcc\x86_64-w64-mingw32\4.9.2\include\c++\string	In file included from D:/Dev-Cpp/MinGW64/lib/gcc/x86_64-w64-mingw32/4.9.2/include/c++/string

......
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然后我查了一下原因，必须得加个排序才行

如下

struct ch{ //选课 
	int st_id; // 学号 
	int sb_id; // 课程号 
	bool operator < (const ch &a)const
    {
        if (st_id == a.st_id)   return sb_id < a.sb_id;
        else return st_id < a.st_id;
    }
};
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这样就好了

参考：https://www.cnblogs.com/ZhaoxiCheung/p/6790718.html