Add some hash examples.

[SVN r28139]

hash/examples/point.cpp

@@ -0,0 +1,51 @@
+
+//  Copyright Daniel James 2005. Use, modification, and distribution are
+//  subject to the Boost Software License, Version 1.0. (See accompanying
+//  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#include <boost/functional/hash.hpp>
+#include <cassert>
+
+// This example illustrates how to use boost::hash_combine to generate a hash
+// value from the different members of a class. For full details see the hash
+// tutorial.
+
+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;
+    }
+
+    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;
+    }
+};
+
+int main()
+{
+    boost::hash<point> point_hasher;
+
+    point p1(0, 0);
+    point p2(1, 2);
+    point p3(4, 1);
+    point p4 = p1;
+
+    assert(point_hasher(p1) == point_hasher(p4));
+
+    // These tests could legally fail, but if they did it'd be a pretty bad
+    // hash function.
+    assert(point_hasher(p1) != point_hasher(p2));
+    assert(point_hasher(p1) != point_hasher(p3));
+}