How to write a custom hash_function for std::unordered_map in cpp?

Question

I want to write my own Hash_function for an std::unordered_map instead of using the default one. I could find unordered_map::hash_function() on many websites. But using this i can only get the Hash value generated, using something like this :

/*Sample map of strings*/

unordered_map<string, string> sample;

// inserts key and elements
sample.insert({ "Tom", "MNNIT" });
sample.insert({ "Kate", "MNNIT" });

unordered_map<string, string>::hasher foo
    = sample.hash_function();

cout << foo("Tom") << endl;

But how can i have more control and create my own version of the hashing function ? So, that for example lets say for the key "Tom", i want hash value to be 100.

Use a good source code editor like [GNU emacs](https://www.gnu.org/software/emacs/) and read [this C++ reference](https://en.cppreference.com/w/cpp) and the documentation of your C++ compiler (e.g. [GCC](http://gcc.gnu.org/)) and debugger (e.g. [GDB](https://www.gnu.org/software/gdb/) — Basile Starynkevitch, May 03 '21 at 19:23
Read wikipedia about [hash table](https://en.wikipedia.org/wiki/Hash_table), the [*Introduction to algorithms*](https://en.wikipedia.org/wiki/Introduction_to_Algorithms), and explain in several paragraphs of written English why do you require that the hashcode of `"Tom"` should be 100. Provide some [mre] in your question. What will happen to you if the hashcode of `"Tom"` is *not* 100? Read also [documentation of `std::unordered_map`](https://en.cppreference.com/w/cpp/container/unordered_map) — Basile Starynkevitch, May 03 '21 at 19:25
This might be helpful https://stackoverflow.com/a/17017281/9676206 — Linux Geek, May 03 '21 at 19:28

Dean Johnson · Accepted Answer · 2021-05-03T19:41:29.337

7

std::unordered_map is templated off of a Hasher that defaults to std::hash<Key>. You can change your variable to std::unordered_map<string, string, CustomHasher>. unordered_map will then default construct a CustomHasher (it can also be passed in the constructor if you can't default construct your hashing object).

A custom hasher needs to provide a call operator such as the following:

struct CustomHasher
{
    // noexcept is recommended, but not required
    std::size_t operator()(const std::string& s) const /*noexcept*/
    {
        return /*hash computation here*/;
    }
};

Note: Writing code that depends on the hash values of something stored in an unordered_map is typically a bad design. There are valid use cases for wanting a custom hash function such as when you can exploit some information specific to your data to generate better hashes, but those cases are quite rare.

edited May 03 '21 at 19:41

answered May 03 '21 at 19:24

Dean Johnson

1,682
7
12

1

Just a note: it doesn't have to be `noexcept`. – Evg May 03 '21 at 19:33
@Evg good point, removed it from the example. – Dean Johnson May 03 '21 at 19:34
2

I didn't mean to ask to remove `noexcept`. Having a `noexcept` hasher is a good thing but the standard doesn't mandate non-throwing custom hashers. For example, libstdc++ does some internal optimization for `noexcept` hashers (it a hasher is not `noexcept` then a hash code has to be stored in a node, see https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/include/bits/hashtable.h#L44). In that particular implementation `noexcept` alone is not enough to trigger optimization (hasher should also be "fast"), but in some other implementation it might be important. – Evg May 03 '21 at 19:40

How to write a custom hash_function for std::unordered_map in cpp?

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