Use vectors as key in unordered_map - c++

Question

So I want to use vectors as keys in my unordered_map. I have followed this answer and I have the following
#include <iostream>
#include <cstdlib>
#include <tuple>
#include <unordered_map>
#include <vector>
#include <boost/functional/hash.hpp> /* hash_combine */
template <typename T>
struct vectorHasher{
std::size_t operator()(std::vector<T> &in) const
{
using boost::hash_value;
using boost::hash_combine;
// Start with a hash value of 0
std::size_t seed = 0;
T value;
for (int i=0; i< in.size(); i++)
{
value = static_cast<T>(in[i]);
hash_combine(seed, hash_value(value));
}
return seed;
}
};
int main()
{
typedef std::unordered_map< std::vector<std::size_t>, int, vectorHasher<std::vector<std::size_t> > > map_type;
map_type mymap;
std::vector<size_t> vec (3,100);
mymap[vec] = 1;
return 0;
}
, but I get the following error compiling the code.
In file included from mytest_vectorhasher.cpp:6:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/unordered_map:404:17: error:
no matching function for call to object of type 'const
vectorHasher<std::__1::vector<unsigned long, std::__1::allocator<unsigned long> >
>'
{return static_cast<const _Hash&>(*this)(__x);}
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/__hash_table:1976:21: note:
in instantiation of member function
'std::__1::__unordered_map_hasher<std::__1::vector<unsigned long,
std::__1::allocator<unsigned long> >,
std::__1::__hash_value_type<std::__1::vector<unsigned long,
std::__1::allocator<unsigned long> >, int>, vectorHasher<std::__1::vector<unsigned
long, std::__1::allocator<unsigned long> > >, true>::operator()' requested here
size_t __hash = hash_function()(__k);
^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/unordered_map:1443:21: note:
in instantiation of function template specialization
'std::__1::__hash_table<std::__1::__hash_value_type<std::__1::vector<unsigned long,
std::__1::allocator<unsigned long> >, int>,
std::__1::__unordered_map_hasher<std::__1::vector<unsigned long,
std::__1::allocator<unsigned long> >,
std::__1::__hash_value_type<std::__1::vector<unsigned long,
std::__1::allocator<unsigned long> >, int>, vectorHasher<std::__1::vector<unsigned
long, std::__1::allocator<unsigned long> > >, true>,
std::__1::__unordered_map_equal<std::__1::vector<unsigned long,
std::__1::allocator<unsigned long> >,
std::__1::__hash_value_type<std::__1::vector<unsigned long,
std::__1::allocator<unsigned long> >, int>,
std::__1::equal_to<std::__1::vector<unsigned long, std::__1::allocator<unsigned
long> > >, true>,
std::__1::allocator<std::__1::__hash_value_type<std::__1::vector<unsigned long,
std::__1::allocator<unsigned long> >, int> >
>::__emplace_unique_key_args<std::__1::vector<unsigned long,
std::__1::allocator<unsigned long> >, const std::__1::piecewise_construct_t &,
std::__1::tuple<const std::__1::vector<unsigned long, std::__1::allocator<unsigned
long> > &>, std::__1::tuple<> >' requested here
return __table_.__emplace_unique_key_args(__k,
^
mytest_vectorhasher.cpp:41:10: note: in instantiation of member function
'std::__1::unordered_map<std::__1::vector<unsigned long,
std::__1::allocator<unsigned long> >, int, vectorHasher<std::__1::vector<unsigned
long, std::__1::allocator<unsigned long> > >,
std::__1::equal_to<std::__1::vector<unsigned long, std::__1::allocator<unsigned
long> > >, std::__1::allocator<std::__1::pair<const std::__1::vector<unsigned long,
std::__1::allocator<unsigned long> >, int> > >::operator[]' requested here
mymap[vec] = 1;
^
mytest_vectorhasher.cpp:13:17: note: candidate function not viable: no known conversion
from 'const std::__1::vector<unsigned long, std::__1::allocator<unsigned long> >'
to 'std::vector<vector<unsigned long, allocator<unsigned long> > > &' for 1st
argument
std::size_t operator()(std::vector<T> &in) const
^
1 error generated.
Is it something easy that I am messing up? What am I doing wrong?

Answer 1

There are two errors in this example. First, your hasher's operator() parameter should be const.
std::size_t operator()(const std::vector<T> &in) const
// Add const here ^^^^^
Second is you are defining your hashing type as vectorHasher<std::vector<std::size_t> > which would imply that T = std::vector<std::size_t>, meaning that you are trying to instantiate a hasher with std::size_t operator()(const std::vector<std::vector<std::size_t>> &in) const which is too many vectors. Remove the std::vector from your map type declaration like this :
typedef std::unordered_map< std::vector<std::size_t>, int, vectorHasher< std::size_t > > map_type;
// Remove std::vector from this template argument ^^^^^^^^^^^

Answer 2

The problem is you have the wrong parameter for operator(). When you construct the vectorHasher you use
vectorHasher<std::vector<std::size_t>>
Which means T is a
std::vector<std::size_t>
That means your operator() looks like
std::size_t operator()(std::vector<std::vector<std::size_t>> &in) const
Which is not what you want. What you can do is to use T directly like
std::size_t operator()(const T& in) const
Or change the vectorHasher to be
vectorHasher<std::size_t>

Answer 3

There are 2 errors in your code.
1) Since you defined your vectorHasher as
template <typename T>
struct vectorHasher{
std::size_t operator()(std::vector<T> &in) const
{
// ...
}
};
The following template instantiation vectorHasher<std::vector<std::size_t>> expands to:
struct vectorHasher{
std::size_t operator()(std::vector<std::vector<std::size_t>> &in) const
{
// ...
}
};
And, sure enough, std::vector<std::size_t> can't be passed to this function.
Change the instantiation of the template to vectorHasher<std::size_t>.
2) operator() of the struct used for hashing needs to take const value/reference, so you should change the signature of it to:
std::size_t operator()(std::vector<T> const &in) const
Live demo.