#include <bits/stdc++.h>
std::unordered_map<std::pair<int,int>, int> mp;
int main()
{
mp[make_pair(1, 2)]++;
}
When using the [] operator, I get this
error: no match for ‘operator[]’ (operand types are ‘std::unordered_map<std::pair<int, int>, int>’ and ‘std::pair<int, int>’)
However, when doing the same with std::map, no error occurs. why?
How can I make it works with std::unorderd_m?
when doing the same with
std::map, no error occurs. why? And how can I make it works withstd::unorderd_map?
Because they are simply different.
std::unorderd_map the elements are placed according to the hash of its keys.
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;
whereas the std::map needs simply a comparison function in order to sort the keys.
template<
class Key,
class T,
class Compare = std::less<Key>,
class Allocator = std::allocator<std::pair<const Key, T> >
> class map;
The reason why your std::map<std::pair<int,int>, int> got compiled is, the operator< is defined for std::pair and std::map uses it to sort its keys, whereas the hash function for std::pair has not been defined and hence std::unorderd_map need one to keep elemets in its buckets. This you need to define.
For instance, you can define a custom hash function as follows:
#include <unordered_map>
#include <cstddef>
#include <functional>
struct CustomHash
{
template <typename T, typename U>
std::size_t operator()(const std::pair<T, U> &x) const
{
return std::hash<T>()(x.first) ^ std::hash<U>()(x.second);
}
};
int main()
{
std::unordered_map<std::pair<int,int>, int, CustomHash> mp;
mp[std::make_pair(1, 2)]++;
return 0;
}
PS: #include <bits/stdc++.h>is a bad coding practice. Why? see this .
