使用lambda的多图谓词

Question

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.

Answer 1

A lambda can be used in place of a function pointer, but a lambda is not a function pointer. Therefore the type of the third template argument is wrong.

Instead you could define the lambda first, and use eg decltype to get the lambda type. Something like the following code:

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);

Answer 2

Your comparator template parameter type is

bool (*)(const std::vector<Message>, const std::vector<Message>)

while the type the lambda can decay to is

bool (*)(const std::vector<Message>&, const std::vector<Message>&)

I suggest you make the multimap take references in its comparator parameter too.

Answer 3

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!