std::map 如何使用结构体作为自定义键值
2016-03-15 11:53
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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:
这样直接使用的话,在编译时会出问题:
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:
使用自定义结构体作为map的键值,答题思路就是这样。
交流QQ:1245178753
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.cpp 1>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> with 1> [ 1> _Kty=Test 1> , _Ty=Test 1> ] 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 compiled 1> with 1> [ 1> _Kty=Test 1> , _Ty=Test 1> ] 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 compiled 1> with 1> [ 1> _Kty=Test 1> ] 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 compiled 1> with 1> [ 1> _Kty=Test 1> ] 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 compiled 1> with 1> [ 1> _Kty=Test 1> , _Ty=std::string 1> , _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 compiled 1> with 1> [ 1> _Kty=Test 1> , _Ty=std::string 1> ] 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。
交流QQ:1245178753
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