hashing function lambda failing to compile with unordered_map

Question

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?

When you get the error, is that the only message you get? Please edit your question to include a complete error log. — Some programmer dude
– Some programmer dude, Commented May 21, 2015 at 20:37
Lambdas are not default constructible and not assignable, so don't do either of those things with your map. — Kerrek SB
– Kerrek SB, Commented May 21, 2015 at 20:40

dev_nut · Accepted Answer · 2015-05-21 20:45:40Z

0

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
    };

answered May 21, 2015 at 20:45

dev_nut

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