c++ - 哈希函数 lambda 无法使用 unordered_map 进行编译

Question

以下代码编译失败

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);

我注意到有时不包含 header 时会发生这种情况。这些是包含的 header :

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

注意:我在 VS 2013 中对优先级队列的比较器使用了相同的技术，并且它有效。我看到有一个使用 std::function 的替代方法，但我想让这个方法起作用。

编辑:完整的错误日志

 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>          ]

我在这里做错了什么？

Answer 1

在 Kerrek 的评论后想通了:

改变这个:

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
    };

至(注意引用):

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
    };