How to properly hash the custom struct?

Question

In the C++ language there is the default hash-function template std::hash<T> for the most simple types, like std::string, int, etc. I suppose, that these functions have a good entropy and the corresponding random variable distribution is statistically uniform. If it's not, then let's pretend, that it is.

Then, I have a structure:

struct CustomType {
  int field1;
  short field2;
  string field3;
  // ...
};

I want to hash it, using separate hashes of some of it's fields, say, std::hash(field1) and std::hash(field2). Both hashes are in a set of possible values of the type size_t.

What is a good hash-function, that can combine both those results and map them back to size_t?

typ1232 · Accepted Answer · 2025-04-10 08:51:19Z

27

boost::hash_combine is really good one to hash different fields.

If you don't have boost library you can use this :

template <class T>
inline void hash_combine(std::size_t & s, const T & v)
{
  std::hash<T> h;
  s^= h(v) + 0x9e3779b9 + (s<< 6) + (s>> 2);
}

struct S {
  int field1;
  short field2;
  std::string field3;
  // ...
};

template <class T>
class MyHash;
 
template<>
struct MyHash<S>
{
    std::size_t operator()(S const& s) const 
    {
        std::size_t res = 0;
        hash_combine(res,s.field1);
        hash_combine(res,s.field2);
        hash_combine(res,s.field3);
        return res;
    }
};

And then probably std::unordered_set<S, MyHash<S>> s; , etc

edited Apr 10 at 8:51

typ1232

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answered Oct 5, 2013 at 7:52

P0W

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1 Comment

P0W Over a year ago

Read Magic number in boost::hash_combine

Yuushi · Accepted Answer · 2013-10-05 07:30:12Z

13

boost::hash_combine is something that could help you out here:

namespace std
{
template <>
struct hash<CustomType>
{
    std::size_t operator()(const CustomType& c) const
    {
        std::size_t result = 0;
        boost::hash_combine(result, field1);
        boost::hash_combine(result, field2);
        return result;
    }
};
}

See the boost doc here.

answered Oct 5, 2013 at 7:30

Yuushi

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Comments

Dave E · Accepted Answer · 2023-07-19 14:33:03Z

5

A simpler approach may be to add a toString() method and hash that.

struct CustomType {
    int field1;
    short field2;
    std::string field3;
    // ...

    std::string toString() const {
        return std::to_string(field1) + std::to_string(field2) + field3; // + ...
    }

    size_t hash() const {
        return std::hash<std::string>()(toString());
    }

};

Having the toString() method is also handy for logging and debugging.

answered Jul 19, 2023 at 14:33

Dave E

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