使用lambda的多图谓词 [英] multimap predicate using lambda

问题描述

  std：我尝试使用多图表对一些关系数据进行排序， ：multimap < 
 std :: vector< Message>，std :: string，
 bool（*）（const std :: vector< Message>，const std :: vector< Message> 
 sortmap（[&]（const std :: vector< Message>& lhs，
 const std :: vector< Message>& rhs）{
 return lhs.size < rhs.size（）; 
}）; 
  

这些类型似乎是正确的，但视觉工作室另有说明。

解决方案

我将介绍一些提到的选项：

选项1 @Joachim Pilborg

  auto comparator = []（const std :: vector< Message>& lhs，
 const std :: vector< Message>& rhs）{
 return lhs.size（） rhs.size（）; 
}; 
 
 std :: multimap< std :: vector< Message>，std :: string，decltype（comparator）> sortmap（比较器）; 
 
消息msg; 
 std :: vector< Message>消息; 
 messages.push_back（msg）; 
 
 std :: string str; 
 auto test = std :: make_pair（messages，str）; 
 
 sortmap.emplace（test）; 
  

会导致

 code> 1> c：\program files（x86）\microsoft visual studio 12.0 \vc\include\xtree（1007）：错误C3497：您不能构造lambda的实例
 1。 c：\program files（x86）\microsoft visual studio 12.0 \vc\include\xtree（1006）：在编译类模板成员函数'main ::< lambda_2efe1f793bd1dfe8b84381ab9a3a87dc> std :: _ Tree_comp< false，_Traits> :: _ Getcomp（void）const'
 1> with 
 1> [
 1> _Traits = std :: _Tmap_traits< std :: vector< Message，std :: allocator< Message>>，std :: string，main ::< lambda_2efe1f793bd1dfe8b84381ab9a3a87dc> ;, std :: allocator< std :: pair& ：vector< Message，std :: allocator< Message>>，std :: string>>，true> 
 1> ] 
 1> c：\program files（x86）\microsoft visual studio 12.0 \vc\include\xtree（1789）：参见函数模板实例化'main ::< lambda_2efe1f793bd1dfe8b84381ab9a3a87dc> std :: _ Tree_comp  1> with 
 1> [
 1> _Traits = std :: _Tmap_traits< std :: vector< Message，std :: allocator< Message>>，std :: string，main ::< lambda_2efe1f793bd1dfe8b84381ab9a3a87dc> ;, std :: allocator< std :: pair& ：vector< Message，std :: allocator< Message>>，std :: string>>，true> 
 1> ] 
 1> c：\program files（x86）\microsoft visual studio 12.0 \vc\include\xtree（1024）：参考类模板实例化'std :: _ Tree_comp< false，_Traits& b 1> with 
 1> [
 1> _Traits = std :: _Tmap_traits< std :: vector< Message，std :: allocator< Message>>，std :: string，main ::< lambda_2efe1f793bd1dfe8b84381ab9a3a87dc> ;, std :: allocator< std :: pair& ：vector< Message，std :: allocator< Message>>，std :: string>>，true> 
 1> ] 
 1> c：\program files（x86）\microsoft visual studio 12.0 \vc\include\map（275）：参见类模板实例化的参考std :: _ Tree< std :: _ Tmap_traits< _Kty，_Ty，_Pr ，_Alloc，true>>'正在编译
 1> with 
 1> [
 1> _Kty = std :: vector< Message，std :: allocator< Message>> 
 1> ，_Ty = std :: string 
 1> ，_Pr = main ::< lambda_2efe1f793bd1dfe8b84381ab9a3a87dc> 
 1> ，_Alloc = std :: allocator< std :: pair< const std :: vector< Message，std :: allocator< Message>>，std :: string> 
 1> ] 
 1> c：\users\c\documents\visual studio 2013\projects\project1\project1\source.cpp（15）：参见类模板实例化'std :: multimap< std :: vector< ; Message，std :: allocator< _Ty>>，std :: string，main ::< lambda_2efe1f793bd1dfe8b84381ab9a3a87dc>，std :: allocator< std :: pair< const _Kty，std :: basic_string< char，std :: char_traits< ; char>，std :: allocator< char>>>>>'正在编译
 1> with 
 1> [
 1> _Ty = Message 
 1> ，_Kty = std :: vector< Message，std :: allocator< Message>> 
 1> ] 
  

选项2 @PlasmaHH

  std :: multimap< 
 std :: vector< Message>，std :: string，
 bool（*）（const std :: vector< Message>& const std :: vector< Message>& 
 sortmap（[&]（const std :: vector< Message>& lhs，
 const std :: vector< Message>& rhs）{
 return lhs.size < rhs.size（）; 
}）; 
消息msg; 
 std :: vector< Message>消息; 
 messages.push_back（msg）; 
 
 std :: string str; 
 auto test = std :: make_pair（messages，str）; 
 
 sortmap.emplace（test）; 
  

这个DOES编译，但奇怪的是在可视工作室错误列表中留下错误（并留下恼人的红色波浪）：

  1 IntelliSense：没有构造函数的实例std :: multimap< _Kty，_Ty，_Pr，_Alloc> :: multimap [with _Kty = std :: vector< Message，std :: allocator< Message>>>，_Ty = std :: string，_Pr = bool（*）（const std :: vector< Message，std :: allocator< Message> >&& const std :: vector< Message，std :: allocator< Message>&），_Alloc = std :: allocator< std :: pair< const std :: vector< Message，std :: allocator< ; message>>，std :: string>>]匹配参数列表
参数类型是：（lambda [] bool（const std :: vector< Message，std :: allocator< Message> & lhs，const std :: vector< Message，std :: allocator< Message>& rhs） - > bool）c：\Users\c\Documents\Visual Studio 2013\Projects\\ \\ Project1\Project1\Source.cpp 12 
  

}

我宁愿用

  auto comparator = []（const std :: vector< Message> & lhs，
 const std :: vector< Message> & rhs）{
 return lhs.size（）< rhs.size（）; 
}; 
 std :: multimap< std :: vector< Message>，std :: string，
 std :: function< bool（const std :: vector< Message>& 
 const std :: vector< Message>&）>> 
 sortmap（comparator）; 
消息msg; 
 std :: vector< Message>消息; 
 messages.push_back（msg）; 
 
 std :: string str; 
 auto test = std :: make_pair（messages，str）; 
 
 sortmap.emplace（test）; 
  

选项1似乎无效：

https://connect.microsoft.com/VisualStudio/feedback/details/727957/vc11-beta-compiler-fails-to-compile-lambda-key-compalle-for-maps-and-sets

显式定义类型工作，并且由于某种原因使用std :: function使得（烦人的）红色波动和错误列表条目消失。希望这有助于！

I am trying to sort some relational data using a multimap but am having issues getting the predicate defined correctly.

  std::multimap<
      std::vector<Message>, std::string,
      bool (*)(const std::vector<Message>, const std::vector<Message>)>
  sortmap([&](const std::vector<Message> &lhs,
              const std::vector<Message> &rhs) {
    return lhs.size() < rhs.size();
  });

The types seem to be correct but visual studio says otherwise.

解决方案

I'll go through some of the options mentioned:

Option 1 @Joachim Pilborg

  auto comparator = [](const std::vector<Message>& lhs,
    const std::vector<Message>& rhs) {
    return lhs.size() < rhs.size();
  };

  std::multimap<std::vector<Message>, std::string, decltype(comparator)> sortmap(comparator);

  Message msg;
  std::vector<Message> messages;
  messages.push_back(msg);

  std::string str;
  auto test = std::make_pair(messages, str);

  sortmap.emplace(test);

results in

1>c:\program files (x86)\microsoft visual studio 12.0\vc\include\xtree(1007): error C3497: you cannot construct an instance of a lambda
1>          c:\program files (x86)\microsoft visual studio 12.0\vc\include\xtree(1006) : while compiling class template member function 'main::<lambda_2efe1f793bd1dfe8b84381ab9a3a87dc> std::_Tree_comp<false,_Traits>::_Getcomp(void) const'
1>          with
1>          [
1>              _Traits=std::_Tmap_traits<std::vector<Message,std::allocator<Message>>,std::string,main::<lambda_2efe1f793bd1dfe8b84381ab9a3a87dc>,std::allocator<std::pair<const std::vector<Message,std::allocator<Message>>,std::string>>,true>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 12.0\vc\include\xtree(1789) : see reference to function template instantiation 'main::<lambda_2efe1f793bd1dfe8b84381ab9a3a87dc> std::_Tree_comp<false,_Traits>::_Getcomp(void) const' being compiled
1>          with
1>          [
1>              _Traits=std::_Tmap_traits<std::vector<Message,std::allocator<Message>>,std::string,main::<lambda_2efe1f793bd1dfe8b84381ab9a3a87dc>,std::allocator<std::pair<const std::vector<Message,std::allocator<Message>>,std::string>>,true>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 12.0\vc\include\xtree(1024) : see reference to class template instantiation 'std::_Tree_comp<false,_Traits>' being compiled
1>          with
1>          [
1>              _Traits=std::_Tmap_traits<std::vector<Message,std::allocator<Message>>,std::string,main::<lambda_2efe1f793bd1dfe8b84381ab9a3a87dc>,std::allocator<std::pair<const std::vector<Message,std::allocator<Message>>,std::string>>,true>
1>          ]
1>          c:\program files (x86)\microsoft visual studio 12.0\vc\include\map(275) : see reference to class template instantiation 'std::_Tree<std::_Tmap_traits<_Kty,_Ty,_Pr,_Alloc,true>>' being compiled
1>          with
1>          [
1>              _Kty=std::vector<Message,std::allocator<Message>>
1>  ,            _Ty=std::string
1>  ,            _Pr=main::<lambda_2efe1f793bd1dfe8b84381ab9a3a87dc>
1>  ,            _Alloc=std::allocator<std::pair<const std::vector<Message,std::allocator<Message>>,std::string>>
1>          ]
1>          c:\users\c\documents\visual studio 2013\projects\project1\project1\source.cpp(15) : see reference to class template instantiation 'std::multimap<std::vector<Message,std::allocator<_Ty>>,std::string,main::<lambda_2efe1f793bd1dfe8b84381ab9a3a87dc>,std::allocator<std::pair<const _Kty,std::basic_string<char,std::char_traits<char>,std::allocator<char>>>>>' being compiled
1>          with
1>          [
1>              _Ty=Message
1>  ,            _Kty=std::vector<Message,std::allocator<Message>>
1>          ]

Option 2 @PlasmaHH

  std::multimap<
      std::vector<Message>, std::string,
      bool (*)(const std::vector<Message> &, const std::vector<Message> &)>
  sortmap([&](const std::vector<Message> &lhs,
              const std::vector<Message> &rhs) {
    return lhs.size() < rhs.size();
  });
  Message msg;
  std::vector<Message> messages;
  messages.push_back(msg);

  std::string str;
  auto test = std::make_pair(messages, str);

  sortmap.emplace(test);

This DOES compile, but strangely leaves an error in visual studios error list (and leaves an annoying red squiggle):

1   IntelliSense: no instance of constructor "std::multimap<_Kty, _Ty, _Pr, _Alloc>::multimap [with _Kty=std::vector<Message, std::allocator<Message>>, _Ty=std::string, _Pr=bool (*)(const std::vector<Message, std::allocator<Message>> &, const std::vector<Message, std::allocator<Message>> &), _Alloc=std::allocator<std::pair<const std::vector<Message, std::allocator<Message>>, std::string>>]" matches the argument list
        argument types are: (lambda []bool (const std::vector<Message, std::allocator<Message>> &lhs, const std::vector<Message, std::allocator<Message>> &rhs)->bool)  c:\Users\c\Documents\Visual Studio 2013\Projects\Project1\Project1\Source.cpp   12

}

I instead settle with

  auto comparator = [](const std::vector<Message> &lhs,
    const std::vector<Message> &rhs) {
    return lhs.size() < rhs.size();
  };
  std::multimap<std::vector<Message>, std::string,
    std::function<bool(const std::vector<Message> &,
    const std::vector<Message> &) >>
    sortmap(comparator);
  Message msg;
  std::vector<Message> messages;
  messages.push_back(msg);

  std::string str;
  auto test = std::make_pair(messages, str);

  sortmap.emplace(test);

Option 1 appears not to work:

https://connect.microsoft.com/VisualStudio/feedback/details/727957/vc11-beta-compiler-fails-to-compile-lambda-key-comparer-for-maps-and-sets

explicitly defining the type works, and using std::function for some reason makes the (annoying!!) red squiggle and error list entry go away. Hope this helps!

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