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Say you have a point class, representing a two dimensional location:
class point { int x; int y; public: point() : x(0), y(0) {} point(int x, int y) : x(x), y(y) {} bool operator==(point const& other) const { return x == other.x && y == other.y; } };
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:
class point { ... friend std::size_t hash_value(point const& p) { std::size_t seed = 0;boost::hash_combine(seed, p.x);boost::hash_combine(seed, p.y); return seed; } ... };
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.
Full code for this example is at /libs/functional/hash/examples/point.cpp.
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Note |
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When using
std::size_t seed = 0;
boost::hash_combine(seed, 1);
boost::hash_combine(seed, 2);
results in a different seed to:
std::size_t seed = 0;
boost::hash_combine(seed, 2);
boost::hash_combine(seed, 1);
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. |
To calculate the hash of an iterator range you can use boost::hash_range:
std::vector<std::string> some_strings;
std::size_t hash = boost::hash_range(some_strings.begin(), some_strings.end());
| Copyright © 2005, 2006 Daniel James |