Add some hash examples.

[SVN r28139]

hash/examples/books.cpp

@@ -0,0 +1,55 @@
+
+//  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>
+// If we had std::unordered_set.
+//#include <unordered_set>
+#include <cassert>
+
+// This example illustrates how to use boost::hash with a custom hash function.
+// For full details, see the tutorial.
+
+namespace library
+{
+    struct book
+    {
+        int id;
+        std::string author;
+        std::string title;
+
+        book(int i, std::string const& a, std::string const& t)
+            : id(i), author(a), title(t) {}
+    };
+
+    bool operator==(book const& a, book const& b)
+    {
+        return a.id == b.id;
+    }
+
+    std::size_t hash_value(book const& b)
+    {
+        return boost::hash_value(b.id);
+    }
+}
+
+int main()
+{
+    library::book knife(3458, "Zane Grey", "The Hash Knife Outfit");
+    library::book dandelion(1354, "Paul J. Shanley", "Hash & Dandelion Greens");
+
+    boost::hash<library::book> book_hasher;
+    std::size_t knife_hash_value = book_hasher(knife);
+
+    // If we had std::unordered_set:
+    //
+    //std::unordered_set<library::book, boost::hash<library::book> > books;
+    //books.insert(knife);
+    //books.insert(library::book(2443, "Lindgren, Torgny", "Hash"));
+    //books.insert(library::book(1953, "Snyder, Bernadette M.",
+    //    "Heavenly Hash: A Tasty Mix of a Mother's Meditations"));
+
+    //assert(books.find(knife) != books.end());
+    //assert(books.find(dandelion) == books.end());
+}