散列函数lambda无法使用unordered_map进行编译 [英] hashing function lambda failing to compile with unordered_map

问题描述

以下代码无法使用

编译：

错误C3497：无法构建lambda的实例：

  auto hasher = [&]（const cv :: Vec3b& color） - > size_t {
 std :: hash< int> int_hasher; 
 return int_hasher（color [0]）+ int_hasher（color [1]）+ int_hasher（color [2]）; 
 //上面的代码可能是一个错误的构造一个散列的方法，
 //但我想让它先编译
}; 
 
 std :: unordered_map< cv :: Vec3b，int，decltype（hasher）> color_counts（10，hasher）; 
  

我注意到，有时这种情况发生时，不包括标题。这些是包含的标题：

  #include< unordered_map> 
 #include< functional> 
 #includeopencv2 / core / core.hpp
  

注意：技术用于VS 2013中的优先级队列的比较器，并且它工作。我看到有一个选择使用std :: function，但我想让这个方法工作。

编辑：完成错误日志

 错误C3497：您不能构造lambda的实例
 2> E：\Program Files（x86）\Microsoft Visual Studio 12.0 \VC\include\xhash（164）：在编译类模板成员函数'OSLSegmentation :: read_images ::< lambda_52090ebe4a9b9afa82eb49e6ee9eb824& std :: _ Hash_oper1< false，_Hasher> :: _ Gethash（void）const'
 2> with 
 2> [
 2> _Hasher = OSLSegmentation :: read_images ::< lambda_52090ebe4a9b9afa82eb49e6ee9eb824> 
 2> ] 
 2> E：\Program Files（x86）\Microsoft Visual Studio 12.0 \VC\include\xhash（242）：请参阅对函数模板实例化的引用OSLSegmentation :: read_images ::< lambda_52090ebe4a9b9afa82eb49e6ee9eb824& std :: _ Hash_oper1< false，_Hasher> :: _ Gethash（void）const'正在编译
 2> with 
 2> [
 2> _Hasher = OSLSegmentation :: read_images ::< lambda_52090ebe4a9b9afa82eb49e6ee9eb824> 
 2> ] 
 2> E：\Program Files（x86）\Microsoft Visual Studio 12.0 \VC\include\xhash（198）：参见类模板实例化'std :: _ Hash_oper1< false，_Hasher& b 2> with 
 2> [
 2> _Hasher = OSLSegmentation :: read_images ::< lambda_52090ebe4a9b9afa82eb49e6ee9eb824> 
 2> ] 
 2> E：\Program Files（x86）\Microsoft Visual Studio 12.0 \VC\include\xhash（220）：参见类模板实例化'std :: _ Hash_oper2< false，_Hasher，_Keyeq& b $ b 2> with 
 2> [
 2> _Hasher = OSLSegmentation :: read_images ::< lambda_52090ebe4a9b9afa82eb49e6ee9eb824> 
 2> ，_Keyeq = std :: equal_to< cv :: Vec3b>。 
 2> ] 
 2> E：\Program Files（x86）\Microsoft Visual Studio 12.0 \VC\include\unordered_map（23）：参见类模板实例化的参考'std :: _ Uhash_compare< _Kty，_Hasher，_Keyeq& b $ b 2> with 
 2> [
 2> _Kty = cv :: Vec3b 
 2> ，_Hasher = OSLSegmentation :: read_images ::< lambda_52090ebe4a9b9afa82eb49e6ee9eb824> 
 2> ，_Keyeq = std :: equal_to< cv :: Vec3b> 
 2> ] 
 2> E：\Program Files（x86）\Microsoft Visual Studio 12.0 \VC\include\xhash（255）：参见类模板实例化的引用'std :: _ Umap_traits <_Kty，_Ty，std :: _Uhash_compare< _Kty ，_Hasher，_Keyeq>，_ Alloc，false>'编译
 2> with 
 2> [
 2> _Kty = cv :: Vec3b 
 2> ，_Ty = int 
 2> ，_Hasher = OSLSegmentation :: read_images ::< lambda_52090ebe4a9b9afa82eb49e6ee9eb824> 
 2> ，_Keyeq = std :: equal_to< cv :: Vec3b> 
 2> ，_Alloc = std :: allocator< std :: pair< const cv :: Vec3b，int>> 
 2> ] 
 2> E：\Program Files（x86）\Microsoft Visual Studio 12.0 \VC\include\unordered_map（81）：参见类模板实例化的参考std :: _ Hash< std :: _ Umap_traits< _Kty，_Ty，std :: _ Uhash_compare< _Kty，_Hasher，_Keyeq>，_ Alloc，false>>'正在编译
 2> with 
 2> [
 2> _Kty = cv :: Vec3b 
 2> ，_Ty = int 
 2> ，_Hasher = OSLSegmentation :: read_images ::< lambda_52090ebe4a9b9afa82eb49e6ee9eb824> 
 2> ，_Keyeq = std :: equal_to< cv :: Vec3b> 
 2> ，_Alloc = std :: allocator< std :: pair< const cv :: Vec3b，int>> 
 2> ] 
 2> oslsegmentation.cpp（42）：参见类模板实例化的引用'std :: unordered_map< cv :: Vec3b，int，OSLSegmentation :: read_images ::< lambda_52090ebe4a9b9afa82eb49e6ee9eb824>，std :: equal_to< _Kty>，std :: allocator& std :: pair< const _Kty，_Ty>>>'正在编译
 2> with 
 2> [
 2> _Kty = cv :: Vec3b 
 2> ，_Ty = int 
 2> ] 
  

我在这里做错了什么？

改变这个：

pre> auto hasher = [&]（const cv :: Vec3b& color） - > size_t {
std :: hash< int> int_hasher;
return int_hasher（color [0]）+ int_hasher（color [1]）+ int_hasher（color [2]）;
//上面的代码可能是一个错误的构造一个散列的方法，
//但我想让它先编译
};


（注意参考）：

  auto& hasher = [&]（const cv :: Vec3b& color） - > size_t {
 std :: hash< int> int_hasher; 
 return int_hasher（color [0]）+ int_hasher（color [1]）+ int_hasher（color [2]）; 
 //上面的代码可能是一个错误的构造一个散列的方法，
 //但我想让它先编译
}; 
  

The following code is failing to compile with

error C3497: you cannot construct an instance of a lambda`:

    auto hasher = [&](const cv::Vec3b& color) -> size_t {
        std::hash<int> int_hasher;
        return int_hasher(color[0]) + int_hasher(color[1]) + int_hasher(color[2]); 
// the above code is probably a wrong way of constructing a hash,
// but I'd like to get it to compile first
    };

std::unordered_map<cv::Vec3b, int, decltype(hasher)> color_counts(10, hasher);

I noticed sometimes this happens when headers are not included. These are the included headers:

#include <unordered_map>
#include <functional>
#include "opencv2/core/core.hpp"

Note: I use the same technique for a comparator for priority queues in VS 2013, and it works. I see there's an alternative using std::function but I'd like to make this method work.

EDIT: Complete error log

 error C3497: you cannot construct an instance of a lambda
2>          E:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\xhash(164) : while compiling class template member function 'OSLSegmentation::read_images::<lambda_52090ebe4a9b9afa82eb49e6ee9eb824> std::_Hash_oper1<false,_Hasher>::_Gethash(void) const'
2>          with
2>          [
2>              _Hasher=OSLSegmentation::read_images::<lambda_52090ebe4a9b9afa82eb49e6ee9eb824>
2>          ]
2>          E:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\xhash(242) : see reference to function template instantiation 'OSLSegmentation::read_images::<lambda_52090ebe4a9b9afa82eb49e6ee9eb824> std::_Hash_oper1<false,_Hasher>::_Gethash(void) const' being compiled
2>          with
2>          [
2>              _Hasher=OSLSegmentation::read_images::<lambda_52090ebe4a9b9afa82eb49e6ee9eb824>
2>          ]
2>          E:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\xhash(198) : see reference to class template instantiation 'std::_Hash_oper1<false,_Hasher>' being compiled
2>          with
2>          [
2>              _Hasher=OSLSegmentation::read_images::<lambda_52090ebe4a9b9afa82eb49e6ee9eb824>
2>          ]
2>          E:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\xhash(220) : see reference to class template instantiation 'std::_Hash_oper2<false,_Hasher,_Keyeq>' being compiled
2>          with
2>          [
2>              _Hasher=OSLSegmentation::read_images::<lambda_52090ebe4a9b9afa82eb49e6ee9eb824>
2>  ,            _Keyeq=std::equal_to<cv::Vec3b>
2>          ]
2>          E:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\unordered_map(23) : see reference to class template instantiation 'std::_Uhash_compare<_Kty,_Hasher,_Keyeq>' being compiled
2>          with
2>          [
2>              _Kty=cv::Vec3b
2>  ,            _Hasher=OSLSegmentation::read_images::<lambda_52090ebe4a9b9afa82eb49e6ee9eb824>
2>  ,            _Keyeq=std::equal_to<cv::Vec3b>
2>          ]
2>          E:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\xhash(255) : see reference to class template instantiation 'std::_Umap_traits<_Kty,_Ty,std::_Uhash_compare<_Kty,_Hasher,_Keyeq>,_Alloc,false>' being compiled
2>          with
2>          [
2>              _Kty=cv::Vec3b
2>  ,            _Ty=int
2>  ,            _Hasher=OSLSegmentation::read_images::<lambda_52090ebe4a9b9afa82eb49e6ee9eb824>
2>  ,            _Keyeq=std::equal_to<cv::Vec3b>
2>  ,            _Alloc=std::allocator<std::pair<const cv::Vec3b,int>>
2>          ]
2>          E:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\unordered_map(81) : see reference to class template instantiation 'std::_Hash<std::_Umap_traits<_Kty,_Ty,std::_Uhash_compare<_Kty,_Hasher,_Keyeq>,_Alloc,false>>' being compiled
2>          with
2>          [
2>              _Kty=cv::Vec3b
2>  ,            _Ty=int
2>  ,            _Hasher=OSLSegmentation::read_images::<lambda_52090ebe4a9b9afa82eb49e6ee9eb824>
2>  ,            _Keyeq=std::equal_to<cv::Vec3b>
2>  ,            _Alloc=std::allocator<std::pair<const cv::Vec3b,int>>
2>          ]
2>          oslsegmentation.cpp(42) : see reference to class template instantiation 'std::unordered_map<cv::Vec3b,int,OSLSegmentation::read_images::<lambda_52090ebe4a9b9afa82eb49e6ee9eb824>,std::equal_to<_Kty>,std::allocator<std::pair<const _Kty,_Ty>>>' being compiled
2>          with
2>          [
2>              _Kty=cv::Vec3b
2>  ,            _Ty=int
2>          ]

What am I doing wrong here?

解决方案

Figured it out after Kerrek's comment:

change this:

auto hasher = [&](const cv::Vec3b& color) -> size_t {
        std::hash<int> int_hasher;
        return int_hasher(color[0]) + int_hasher(color[1]) + int_hasher(color[2]); 
// the above code is probably a wrong way of constructing a hash,
// but I'd like to get it to compile first
    };

to (note the reference):

auto& hasher = [&](const cv::Vec3b& color) -> size_t {
        std::hash<int> int_hasher;
        return int_hasher(color[0]) + int_hasher(color[1]) + int_hasher(color[2]); 
// the above code is probably a wrong way of constructing a hash,
// but I'd like to get it to compile first
    };

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