Using a const key for unordered_map

Question

I've been switching my code over from std::map to std::unordered_map where appropriate. With std::map, I typically write the following just to make sure the key cannot be modified:

std::map<const std::string, int>

Frankly, I never checked if this const was of any value. This has always compiled and worked with g++.

Now, with std::unordered_map, the following fails to link with g++ 4.5.1.

std::unordered_map<const std::string, std::string> m;
m["foo"] = "bar";

with this link error:

Undefined symbols: "std::hash<std::basic_string<char, std::char_traits<char>, std::allocator<char> > const>::operator()(std::basic_string<char, std::char_traits<char>, std::allocator<char> >) const", referenced from:

The fix is simple, to remove const, but besides that, is there even a point in STL with any of the associative container classes to use a const key type? Are there no methods that let you get a reference to the key for any associative container?

MSN · Accepted Answer · 2010-10-22 16:42:40Z

28

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

answered Oct 22, 2010 at 16:42

MSN

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4 Comments

Jonathan Grynspan Over a year ago

Or in other words: std::unordered_map<std::string, std::string> will do what he wants. (Says Jonathan, restating the obvious-to-most.)

MSN Over a year ago

Heh, yes, I can see how my answer requires looking at the original question to understand it :)

Theko Lekena Over a year ago

This makes so much sense. The things I struggle with in the morning. Thanks @MSN

Mikhail T. Over a year ago

This answer tells the asker, he does not need to do, what he was doing. The question, why he cannot do it, remains...

user904963 · Accepted Answer · 2021-12-22 10:00:19Z

2

std::unordered_map<std::string const, std::string> uses std::hash<std::string const>, but no specialization exists for hash<std::string const>. However, there is one for std::hash<std::string>. You can provide one if you wish to use a std::string const as your key:

struct constant_string_hash {
    std::size_t operator () (std::string const &s) const {
        return hash<std::string>{}(s);
    }
};

You can then declare your std::unordered_map like this:

std::unordered_map<std::string const, std::string, constant_string_hash> m;
m["test key"] = "test value";

answered Dec 22, 2021 at 10:00

user904963

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