Can I override std::hash?

Question

I can replace the actual implementation of std::hash with my own definition of std::hash in C++ 11 ?

I mean from my codebase, without touching the standard library.

I can't see any use for virtual function/polymorphism in this case, so I suppose that I can't alter the definition of std::hash anyway ?

@Xeo yes, I know that, but I was interested in a global change. — user2485710, Aug 06 '13 at 11:35
Possible duplicate of [How to specialize std::hash::operator() for user-defined type in unordered containers?](https://stackoverflow.com/questions/8157937/how-to-specialize-stdhashkeyoperator-for-user-defined-type-in-unordered) — M.J. Rayburn, Sep 01 '17 at 20:28

score 9 · Answer 1 · edited May 23 '17 at 12:00

9

You can specialise hash for specific types. See here and here e.g. like this

namespace std {
  template <> struct hash<Foo>
  {
    size_t operator()(const Foo & x) const
    {
      /* your code here, e.g. "return hash<int>()(x.value);" */
    }
  };
}

If you think you can do better than the library implementors for existing versions you are either 1. wrong or 2. clever

edited May 23 '17 at 12:00

Community

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answered Aug 06 '13 at 11:33

doctorlove

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_3. lazy_ , so I can get a new hash function without writing X new types. – user2485710 Aug 06 '13 at 11:35
"X new types"? Just write a myHash function and use that instead, but that's probably not lazy - you will have to read Knuth for hours first to get it right. – doctorlove Aug 06 '13 at 11:37
the lazy part was about "writing" indeed :D but thanks for the reference to the creator of Tex and other bozillion of algorithms and books. – user2485710 Aug 06 '13 at 11:38

score 7 · Accepted Answer · answered Aug 06 '13 at 11:32

7

Yes it's okay, and you don't have to modify the standard library in any way, just use template specialization:

namespace std
{
    template<>
    struct hash<YourSpecialType>
    {
        // ...
    };
}

answered Aug 06 '13 at 11:32

Some programmer dude

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@user2485710 Ah, then no. Then you have fork the standard library. – Some programmer dude Aug 06 '13 at 11:34
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@user2485710: Global replacement of the hash function doesn't make much if any sense. What would it accomplish? – Xeo Aug 06 '13 at 11:42
@Xeo changing the hash function used by any structure in just one move. – user2485710 Aug 06 '13 at 11:51
@user2485710 How do you implement `std::hash` for any arbitrary type in C++? (hint: it doesn't work) – R. Martinho Fernandes Aug 06 '13 at 12:00
You can implement type with a different name, like struct HashInt{}, and specify it where use it, e.g unordered_map – where23 Oct 29 '19 at 02:44

Can I override std::hash?

2 Answers2

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