std::map 如何使用结构体作为自定义键值

在使用map时，有时候我们需要自定义键值，才能符合程序的需要。

比如我们需要使用自定义的结构体来作为map的键值：

struct  Test{int x;int y;};

这样直接使用的话，在编译时会出问题：

1>------ Build started: Project: MapRemove, Configuration: Debug Win32 ------1>  Source.cpp1>d:\program files (x86)\microsoft visual studio 12.0\vc\include\xstddef(193): error C2784: 'bool std::operator <(const std::tuple<_Types...> &,const std::tuple<_Types1...> &)' : could not deduce template argument for 'const std::tuple<_Types...> &' from 'const Test'1>          d:\program files (x86)\microsoft visual studio 12.0\vc\include\tuple(480) : see declaration of 'std::operator <'1>          d:\program files (x86)\microsoft visual studio 12.0\vc\include\xstddef(192) : while compiling class template member function 'bool std::less<_Kty>::operator ()(const _Ty &,const _Ty &) const'1>          with1>          [1>              _Kty=Test1>  ,            _Ty=Test1>          ]1>          d:\program files (x86)\microsoft visual studio 12.0\vc\include\map(228) : see reference to function template instantiation 'bool std::less<_Kty>::operator ()(const _Ty &,const _Ty &) const' being compiled1>          with1>          [1>              _Kty=Test1>  ,            _Ty=Test1>          ]1>          d:\program files (x86)\microsoft visual studio 12.0\vc\include\type_traits(572) : see reference to class template instantiation 'std::less<_Kty>' being compiled1>          with1>          [1>              _Kty=Test1>          ]1>          d:\program files (x86)\microsoft visual studio 12.0\vc\include\xtree(1023) : see reference to class template instantiation 'std::is_empty<std::less<_Kty>>' being compiled1>          with1>          [1>              _Kty=Test1>          ]1>          d:\program files (x86)\microsoft visual studio 12.0\vc\include\map(70) : see reference to class template instantiation 'std::_Tree<std::_Tmap_traits<_Kty,_Ty,_Pr,_Alloc,false>>' being compiled1>          with1>          [1>              _Kty=Test1>  ,            _Ty=std::string1>  ,            _Pr=std::less<Test>1>  ,            _Alloc=std::allocator<std::pair<const Test,std::string>>1>          ]1>          f:\xdd\xaudio2\mapremove\source.cpp(12) : see reference to class template instantiation 'std::map<Test,std::string,std::less<_Kty>,std::allocator<std::pair<const _Kty,_Ty>>>' being compiled1>          with1>          [1>              _Kty=Test1>  ,            _Ty=std::string1>          ]1>d:\program files (x86)\microsoft visual studio 12.0\vc\include\xstddef(193): error C2784: 'bool std::operator <(const std::_Tree<_Traits> &,const std::_Tree<_Traits> &)' : could not deduce template argument for 'const std::_Tree<_Traits> &' from 'const Test'1>          d:\program files (x86)\microsoft visual studio 12.0\vc\include\xtree(2259) : see declaration of 'std::operator <'1>d:\program files (x86)\microsoft visual studio 12.0\vc\include\xstddef(193): error C2784: 'bool std::operator <(const std::basic_string<_Elem,_Traits,_Alloc> &,const _Elem *)' : could not deduce template argument for 'const std::basic_string<_Elem,_Traits,_Alloc> &' from 'const Test'1>          d:\program files (x86)\microsoft visual studio 12.0\vc\include\xstring(2545) : see declaration of 'std::operator <'1>d:\program files (x86)\microsoft visual studio 12.0\vc\include\xstddef(193): error C2784: 'bool std::operator <(const _Elem *,const std::basic_string<_Elem,_Traits,_Alloc> &)' : could not deduce template argument for 'const _Elem *' from 'const Test'1>          d:\program files (x86)\microsoft visual studio 12.0\vc\include\xstring(2535) : see declaration of 'std::operator <'1>d:\program files (x86)\microsoft visual studio 12.0\vc\include\xstddef(193): error C2784: 'bool std::operator <(const std::basic_string<_Elem,_Traits,_Alloc> &,const std::basic_string<_Elem,_Traits,_Alloc> &)' : could not deduce template argument for 'const std::basic_string<_Elem,_Traits,_Alloc> &' from 'const Test'1>          d:\program files (x86)\microsoft visual studio 12.0\vc\include\xstring(2525) : see declaration of 'std::operator <'1>d:\program files (x86)\microsoft visual studio 12.0\vc\include\xstddef(193): error C2784: 'bool std::operator <(const std::move_iterator<_RanIt> &,const std::move_iterator<_RanIt2> &)' : could not deduce template argument for 'const std::move_iterator<_RanIt> &' from 'const Test'1>          d:\program files (x86)\microsoft visual studio 12.0\vc\include\xutility(1997) : see declaration of 'std::operator <'1>d:\program files (x86)\microsoft visual studio 12.0\vc\include\xstddef(193): error C2784: 'bool std::operator <(const std::reverse_iterator<_RanIt> &,const std::reverse_iterator<_RanIt2> &)' : could not deduce template argument for 'const std::reverse_iterator<_RanIt> &' from 'const Test'1>          d:\program files (x86)\microsoft visual studio 12.0\vc\include\xutility(1155) : see declaration of 'std::operator <'1>d:\program files (x86)\microsoft visual studio 12.0\vc\include\xstddef(193): error C2784: 'bool std::operator <(const std::_Revranit<_RanIt,_Base> &,const std::_Revranit<_RanIt2,_Base2> &)' : could not deduce template argument for 'const std::_Revranit<_RanIt,_Base> &' from 'const Test'1>          d:\program files (x86)\microsoft visual studio 12.0\vc\include\xutility(971) : see declaration of 'std::operator <'1>d:\program files (x86)\microsoft visual studio 12.0\vc\include\xstddef(193): error C2784: 'bool std::operator <(const std::pair<_Ty1,_Ty2> &,const std::pair<_Ty1,_Ty2> &)' : could not deduce template argument for 'const std::pair<_Ty1,_Ty2> &' from 'const Test'1>          d:\program files (x86)\microsoft visual studio 12.0\vc\include\utility(230) : see declaration of 'std::operator <'1>d:\program files (x86)\microsoft visual studio 12.0\vc\include\xstddef(193): error C2676: binary '<' : 'const Test' does not define this operator or a conversion to a type acceptable to the predefined operator========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

看错误是说，键值无法比较。因为map的键值是自动比较后进插入的，键值是递增的。

现在我们自定义的键值，编译器无法进行比较，找不到类似的模板，所以报错。

既然是没有‘<’，那我们自己重载小于操作符应该就可以了：

struct  Test{int x;int y;bool operator < (const Test &o) const{return x < o.x || y < o.y;}};

重载后，重新编译，顺利通过。测试代码如下：

#include <map>#include <iostream>struct  Test{int x;int y;bool operator < (const Test &o) const{return x < o.x || y < o.y;}};int main(){std::map<Test, std::string> mapTest;Test test = { 1, 2 };mapTest[test] = "Test1";for (auto it = mapTest.begin(); it != mapTest.end();it++){std::cout << it->first.x << " " << it->first.y << " " << it->second.c_str() << std::endl;}return 0;}

使用自定义结构体作为map的键值，答题思路就是这样。

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