C++ Boost

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Extending the class

User defined Axes

In case you need a histogram with an axes of an arbitrary type that is not identified by the C++ Standard Library, you need to provide a overload for the hashing function that is used in the histogram class.

GIL’s histogram class uses boost::hash_combine in a sub routine to generate a hash from the key.

So we need to provide an overload of boost::hash_combine for the purpose.

For example, let’s consider you need a histogram with an axis over class Test.

// File : ./test.hpp
#include <cstddef>
#include <functional>

struct Test
{
    int a{0};
    Test() = default;
    Test(int c) : a(c) {}
    bool operator==(Test const& other) const
    {
        return (a == other.a);
    }
};

namespace boost {
    std::size_t hash_value(Test const& t)
    {
        // Replace with your hashing code
        std::hash<int> hasher;
        return hasher(t.a);
    }
}

Now lets get to the usage example.

#include <test.hpp>
#include <boost/gil.hpp>
#include <iostream>
// Mind the order of include i.e. test.hpp before boost/gil.hpp

using namespace boost::gil;

int main()
{
    boost::gil::histogram<Test> h;
    Test t(1);
    h(t) = 1;
    std::cout<<h(t);
}
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