Table of Contents
Generic Hashing for Tuples in Unordered Collections
The Need for a Generic Solution
A Standard-Conforming Approach
A Non-Standard Approach
Conclusion
Home Backend Development C++ How can you implement a generic hash function for tuples in unordered collections in C ?

How can you implement a generic hash function for tuples in unordered collections in C ?

Nov 06, 2024 pm 09:20 PM

How can you implement a generic hash function for tuples in unordered collections in C  ?

Generic Hashing for Tuples in Unordered Collections

In the realm of C standard libraries, the concept of tuples and their usage as keys in unordered collections like std::unordered_map and std::unordered_set can pose a challenge. By default, tuples do not have a generic hash function defined, leaving developers with the tedious task of manually defining one.

The Need for a Generic Solution

Defining a custom hash function for tuples can be cumbersome and prone to error. To address this issue, developers often seek a more generic solution that automates the process.

A Standard-Conforming Approach

While the standard does not explicitly provide a generic hash function for tuples, a standards-compliant approach is available. By moving the code into a custom namespace, it is possible to avoid undefined behavior associated with specializing in the std namespace.

In this approach, a custom namespace, hash_tuple, is created with its own implementation of the hash function. This implementation dispatches non-tuple types to the std::hash function.

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namespace hash_tuple{

template <typename TT>

struct hash

{

    size_t

    operator()(TT const&amp; tt) const

    {                                             

        return std::hash<TT>()(tt);                                

    }                                             

};

}

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The recursive template code is modified to utilize hash_tuple::hash instead of std::hash:

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namespace hash_tuple{

    namespace

    {

        template <class T>

        inline void hash_combine(std::size_t&amp; seed, T const&amp; v)

        {

            seed ^= hash_tuple::hash<T>()(v) + 0x9e3779b9 + (seed<<6) + (seed>>2);

        }

    }

}

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Finally, the std template specialization is placed within the hash_tuple namespace:

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namespace hash_tuple{

    template <typename ... TT>

    struct hash<std::tuple<TT...>>

    {

        size_t

        operator()(std::tuple<TT...> const&amp; tt) const

        {                                             

            size_t seed = 0;                            

            HashValueImpl<std::tuple<TT...> >::apply(seed, tt);   

            return seed;                                

        }                                             

    };

}

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To use this approach, users must specify the hash_tuple namespace in their unordered collection declarations:

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unordered_set<tuple<double, int>, hash_tuple::hash<tuple<double, int>>> test2;

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While this solution is standards-compliant, it requires specifying the namespace for each unordered collection declaration.

A Non-Standard Approach

An alternative approach, which is not compliant with the C standard, is to place the generic hash function code in the std namespace. This allows argument-dependent lookup (ADL) to automatically find the correct hash implementation.

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namespace std{

    namespace

    {

        // Code from boost

        // Reciprocal of the golden ratio helps spread entropy

        //     and handles duplicates.

        // See Mike Seymour in magic-numbers-in-boosthash-combine:

        //     http://stackoverflow.com/questions/4948780

 

        template <class T>

        inline void hash_combine(std::size_t&amp; seed, T const&amp; v)

        {

            seed ^= std::hash<T>()(v) + 0x9e3779b9 + (seed<<6) + (seed>>2);

        }

 

        // Recursive template code derived from Matthieu M.

        template <class Tuple, size_t Index = std::tuple_size<Tuple>::value - 1>

        struct HashValueImpl

        {

          static void apply(size_t&amp; seed, Tuple const&amp; tuple)

          {

            HashValueImpl<Tuple, Index-1>::apply(seed, tuple);

            hash_combine(seed, std::get<Index>(tuple));

          }

        };

 

        template <class Tuple>

        struct HashValueImpl<Tuple,0>

        {

          static void apply(size_t&amp; seed, Tuple const&amp; tuple)

          {

            hash_combine(seed, std::get<0>(tuple));

          }

        };

    }

 

    template <typename ... TT>

    struct hash<std::tuple<TT...>>

    {

        size_t

        operator()(std::tuple<TT...> const&amp; tt) const

        {                                             

            size_t seed = 0;                            

            HashValueImpl<std::tuple<TT...> >::apply(seed, tt);   

            return seed;                                

        }                                             

 

    };

}

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With this approach, unordered collection syntax remains simpler:

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unordered_set<tuple<double, int> > test_set;

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However, this technique carries the risk of undefined behavior due to the specialization in the std namespace.

Conclusion

The generic hashing of tuples in unordered collections is a non-trivial problem that can require a custom implementation. Both the standards-compliant and non-standard approaches outlined in this article provide viable solutions. Ultimately, the choice between these approaches depends on the developer's requirements and tolerance for potential undefined behavior.

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