How to copy unordered_map with a const key?

Question

Simple code:

#include <unordered_map>

int main()
{
  std::unordered_map<const int, int> m;
  std::unordered_map<const int, int> m1 = m;
}

produces convoluted compile error message:

Error C2280 'std::hash<_Kty>::hash(void)': attempting to reference a deleted function

which basically says unordered_map in its internals doesn't expect the key to be constant

PS: I've read the answer for the analogous question:

The associative containers only expose the (key,value) pair as std::pair, so the additional const on the key type is superfluous.

But it doesn't explain why the hashmap with a const key is practically unusable and how to circumvent the issue

Answer 1

The type

std::unordered_map<const int, int> 

uses a defaulted third parameter std::hash<const int>. This hash type, unlike std::hash<int>, is not specialized by the standard library, and so is deleted (as the error message says).

A working hash is required when copying an unordered_set. To make a working hash:

1. You can specialize std::hash<const int> by yourself, so that it is no longer deleted:

   namespace std 
   { 
     // fixes it but is a bad idea - could break in future revisions of the standard
     template<>
     struct hash<const int> : hash<int>{};
   }
   

2. Or you can explicitly state your hash:

   std::unordered_map<const int, int, std::hash<int>> 
   

3. Or you can get rid of the const in the key (as it has no effect):

   std::unordered_map<int, int> 
   

Addendum:

Deleted means that the constructor of the non-specialized std::hash is deleted:

template <typename T>
struct hash
{
   hash() = delete;
   hash(const hash) = delete;
   // more deleted methods
};

By "deleted", it means that it does not exist (neither user-provided nor default).

You can see this at cppreference, where they use the terminology of enabled/disabled:

For every type Key for which neither the library nor the user provides an enabled specialization std::hash, that specialization exists and is disabled.

Since std::hash<const int> is not provided by the library, it is disabled unless it is provided by the user. Next, the text explains what disabled is:

Disabled specializations do not satisfy Hash, [...] std::is_default_constructible_v, std::is_copy_constructible_v [...] are all false. In other words, they exist, but cannot be used.

So, these constructors must be unavailable (and deleting them is the best way to do it).