CODEWITHSUNDEEP
c++templatesunordered-map
notaorb
notaorb

Reputation: 2190

Using default bucket count in unordered_map<> when specifying hash function

I'm using unordered_map<> and am curious, when specifying a hash function as the second argument (per code below) a size_type n bucket count must be specified as the first argument in the constructor. I have read the default bucket count should be used. Does anyone know how to use the default bucket count parameter when using your own hash function?

Interestingly, Stroustrup C++ 4th Ed Page 918 constructs an unordered_set<> without use of bucket size and does not agree with the documented constructor arguments.

explicit unordered_map ( size_type n = /* see below */,
                         const hasher& hf = hasher(),
                         const key_equal& eql = key_equal(),
                         const allocator_type& alloc = allocator_type() );

Example usage:

#include <unordered_map>
#include <functional>
#include <iostream>
using namespace std;

struct X {
    X(string n) : name{n} {}
    string name;
    bool operator==(const X& b0) const { return name == b0.name; }
};

namespace std {
    template<>
    struct hash<X> {
        size_t operator()(const X&) const;
    };
    size_t hash<X>::operator()(const X& a) const
    {
        cout << a.name << endl;
        return hash<string>{}(a.name);
    }
}

size_t hashX(const X& a)
{
    return hash<string>{}(a.name);
}

int main()
{
//    unordered_map<X,int,hash<X>> m(100, hash<X>{});
//    unordered_map<X,int,function<size_t(const X&)>> m(100, &hashX);
    unordered_map<X,int,size_t(*)(const X&)> m(100, &hashX);
    X x{"abc"};
    m[x] = 1;
    int i = m[x];
    cout << i << endl;
}

Upvotes: 7

Views: 1361

Answers (1)

andre
andre

Reputation: 7249

It seems like we can access the bucket_count value. I would just run the following code in your environment and check what values it gives you.

#include <iostream>
#include <unordered_map>

int main() {
    std::unordered_map<int, int> m;
    std::cout << m.bucket_count() << std::endl;
    return 0;
}

This outputs 1 in ideone

Upvotes: 4

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