Own type as key in unordered_map [duplicate]

Question

I have a wrapper class around uint32_t:

class IPv4 
{ 
  uint32_t octets_;
public:
  operator uint32_t() {return octets;} 
};

Also I have a structure inherited from IPv4 class:

struct routerID : public IPv4 
{};

I want to create an unordered_map:

std::unordered_map<RouterID, std::string> routers;

But I’ve got an error: attempting to reference a deleted function.

Is there a way to make it working without explicitly defining hash_function and == for my IPv4 class?

@pptaszni My RouterID class, is, in fact, uint32_t (it can be implicitly casted to uint32_t). So, I don’t want to define hash function and comparison operator just for uint32_t. Or is it necessary for unordered map? — razenkovv
– razenkovv, Commented May 8, 2024 at 9:14
A hash<IPv4>, delegating to existing hash, would be a one-liner. — BoP
– BoP, Commented May 8, 2024 at 9:16
If you would do using RouterID = std::uint32_t; , then yes, you could say it is in fact uint32_t. Otherwise, it's a distinct type, no matter how simple. — pptaszni
– pptaszni, Commented May 8, 2024 at 9:18
Having an implicit conversion to uint32_t removes some of the advantages you gain by having a custom type in the first place, IMO. — HolyBlackCat
– HolyBlackCat, Commented May 8, 2024 at 9:24

Christian Stieber · Accepted Answer · 2024-05-08 09:30:14Z

1

Just add the stupid hash. There is no benefit in trying to minimize lines of code.

#include <unordered_map>
#include <cstdint>
#include <string>

class IPv4
{
  uint32_t octets_;
public:
  operator uint32_t() const {return octets_;}
};

struct RouterID : public IPv4
{};

template <> struct std::hash<RouterID>
{
    std::size_t operator()(const RouterID& routerId) const noexcept
    {
        return std::hash<uint32_t>{}(routerId);
    }
};

std::unordered_map<RouterID, std::string> routers;

You could, also, just do a

std::unordered_map<RouterID, std::string, std::hash<uint32_t>> routers;

Myself, I'm not the biggest fan of this -- having an actual std::hash for the RouterID does tell the reader that a hash is used somewhere, and makes it explicit how that hash is calculated.

edited May 8, 2024 at 9:30

answered May 8, 2024 at 9:22

Christian Stieber

12.6k30 silver badges27 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

razenkovv Over a year ago

Ok, thank you, I thought defining operator uint32_t() would be enough

Christian Stieber Over a year ago

You could, conceivably, also do something like this: std::unordered_map<RouterID, std::string, std::hash<uint32_t>> routers;

razenkovv Over a year ago

No, I checked and that does not compile

Caleth Over a year ago

@razenkovv works here

razenkovv Over a year ago

@Caleth Thank you, forget to add const to operator uint32_t()

Collectives™ on Stack Overflow

Own type as key in unordered_map [duplicate]

1 Answer 1

5 Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Linked

Related