How can I return unordered_map with a hash function to something expecting just `unordered_map<string, string>`

Question

I want to implement an unordered_map<string, string> that ignores case in the keys. My code looks like:

    std::unordered_map<std::string, std::string> noCaseMap()
    {
        struct hasher {
            std::size_t operator()(const std::string& key) const {
                return std::hash<std::string>{}(toLower(key));
            }
        };
        std::unordered_map<std::string, std::string, hasher> ret;
        return ret;
    }

but XCode flags the return statement with this error:

foo.cpp:181:20 No viable conversion from returned value of type 'unordered_map<[2 * ...], hasher>' to function return type 'unordered_map<[2 * ...], (default) std::hash<std::string>>'

I tried casting ret to <std::unordered_map<std::string, std::string>>, but XCode wasn't having it.

I tried making my hasher a subclass of std::hash<std::string> but that made no difference.

Edit: this is a slight oversimplification of the problem; I know I also have to implement a case-insensitive equal_to() functor as well.

You can't. There's a reason it's part of the type: efficiency. What you can do is e.g. store everything lowercase. — lorro
– lorro, Commented Nov 4, 2022 at 21:24
Yes, I had an earlier implementation that did store everything lower case, but I want to return this struct to client code that might not convert the key to lower case before reading from the map. — Edward Falk
– Edward Falk, Commented Nov 4, 2022 at 21:27
But if you cant, you can't. 😕 Make your comment an answer, and I'll accept it. — Edward Falk
– Edward Falk, Commented Nov 4, 2022 at 21:30
Why not declare hasher at namespace scope and adjust the return type? Also note that std::unordered_map<std::string, std::string, hasher> probably doesn't do what you want: this just uses a worse hash function, but it doesn't change anything about key equality. Inserting values for "a" and "A" into your map will still result in 2 entries, but those 2 entries will always result in a hash collision. — fabian
– fabian, Commented Nov 4, 2022 at 21:40
you can return a std::unordered_map<Foo, std::string> where Foo can be constructed from a string and stores a lower case string. When the user then looks up the_map["AsDf"] then their string will be converted to lower case — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Nov 4, 2022 at 22:23

lorro · Accepted Answer · 2022-11-04 21:48:37Z

2

You can't. There's a reason it's part of the type: efficiency. What you can do is e.g. store everything lowercase. If you need both lowercase and case-preserving, you might need two maps; but, at this point, I'd consider requesting an interface change.

answered Nov 4, 2022 at 21:48

lorro

10.9k25 silver badges37 bronze badges

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

Collectives™ on Stack Overflow

How can I return unordered_map with a hash function to something expecting just `unordered_map<string, string>`

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related