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81 lines
2.5 KiB
Plaintext
81 lines
2.5 KiB
Plaintext
[#combine]
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= Combining hash values
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:idprefix: combine_
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Say you have a point class, representing a two dimensional location:
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[source]
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----
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class point
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{
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int x;
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int y;
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public:
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point() : x(0), y(0) {}
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point(int x, int y) : x(x), y(y) {}
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bool operator==(point const& other) const
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{
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return x == other.x && y == other.y;
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}
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};
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----
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and you wish to use it as the key for an unordered_map. You need to customise the hash for this structure. To do this we need to combine the hash values for x and y. The function `boost::hash_combine` is supplied for this purpose:
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[source]
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----
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class point
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{
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...
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friend std::size_t hash_value(point const& p)
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{
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std::size_t seed = 0;
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boost::hash_combine(seed, p.x);
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boost::hash_combine(seed, p.y);
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return seed;
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}
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...
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};
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----
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Calls to `hash_combine` incrementally build the hash from the different members of `point`, it can be repeatedly called for any number of elements. It calls `hash_value` on the supplied element, and combines it with the seed.
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Full code for this example is at link:../../examples/point.cpp[/libs/container_hash/examples/point.cpp].
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[NOTE]
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====
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When using `boost::hash_combine` the order of the calls matters.
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[source]
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----
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std::size_t seed = 0;
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boost::hash_combine(seed, 1);
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boost::hash_combine(seed, 2);
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----
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results in a different seed to:
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[source]
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----
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std::size_t seed = 0;
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boost::hash_combine(seed, 2);
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boost::hash_combine(seed, 1);
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----
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If you are calculating a hash value for data where the order of the data doesn't matter in comparisons (e.g. a set) you will have to ensure that the data is always supplied in the same order.
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====
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To calculate the hash of an iterator range you can use `boost::hash_range`:
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[source]
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----
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std::vector<std::string> some_strings;
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std::size_t hash = boost::hash_range(some_strings.begin(), some_strings.end());
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----
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Note that when writing template classes, you might not want to include the main hash header as it's quite an expensive include that brings in a lot of other headers, so instead you can include the `<boost/container_hash/hash_fwd.hpp>` header which forward declares `boost::hash`, `boost::hash_range` and `boost::hash_combine`. You'll need to include the main header before instantiating `boost::hash`. When using a container that uses `boost::hash` it should do that for you, so your type will work fine with the boost hash containers. There's an example of this in link:../../examples/template.hpp[template.hpp] and link:../../examples/template.cpp[template.cpp].
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