Using tuple in unordered_map
c++, hashmap, stdtuple, unordered-map
Solution
The template arguments for an unordered_map looks like this:
template<
class Key,
class T,
class Hash = std::hash<Key>,
class KeyEqual = std::equal_to<Key>,
class Allocator = std::allocator< std::pair<const Key, T> >
> class unordered_map;
`std::hash` is not specialized for tuples (scroll down to Standard specializations for library types). Therefore you need to provide your own, something like this:
typedef std::tuple<int, char, char> key_t;
struct key_hash : public std::unary_function<key_t, std::size_t>
{
std::size_t operator()(const key_t& k) const
{
return std::get<0>(k) ^ std::get<1>(k) ^ std::get<2>(k);
}
};
// ..snip..
typedef std::unordered_map<const key_t,data,key_hash,key_equal> map_t;
// ^ this is our custom hash
And finally, as Benjamin Lindley answer already addresses, you need to use `std::make_tuple`:
// d is data
m[std::make_tuple(1, 'a', 'b')] = d;
auto itr = m.find(std::make_tuple(1, 'a', 'b'));
The code was grabbed from Using a std::tuple as key for std::unordered_map and here is the Live Example.
Problem
I want to use tuple consisting of `int`,`char`,`char` in my `unordered_map`. I am doing like this: ``` #include <string> #include <unordered_map> #include <cstring> #include <iostream> #include <tuple> using namespace std; tuple <int,char,char> kk; unordered_map<kk,int> map; int main() { map[1,"c","b"]=23; return 0; } ``` but this gives me following errors: ``` map.cpp:9:21: error: type/value mismatch at argument 1 in template parameter list for ‘template<class _Key, class _Tp, class _Hash, class _Pred, class _Alloc> class std::unordered_map’ map.cpp:9:21: error: expected a type, got ‘kk’ map.cpp:9:21: error: template argument 3 is invalid map.cpp:9:21: error: template argument 4 is invalid map.cpp:9:21: error: template argument 5 is invalid map.cpp:9:26: error: invalid type in declaration before ‘;’ token map.cpp: In function ‘int main()’: map.cpp:14:16: error: assignment of read-only location ‘"b"[map]’ ``` What I am doing wrong in this?