Add some hash examples.

[SVN r28139]

hash/examples/portable.cpp

@@ -0,0 +1,56 @@
+
+//  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 customise boost::hash portably, so that
+// it'll work on both compilers that don't implement argument dependent lookup
+// and compilers that implement strict two-phase template instantiation.
+
+namespace foo
+{
+    struct custom_type
+    {
+        int value;
+
+        custom_type(int x) : value(x) {}
+
+#ifndef BOOST_NO_ARGUMENT_DEPENDENT_LOOKUP
+        friend inline std::size_t hash_value(custom_type x)
+        {
+            boost::hash<int> hasher;
+            return hasher(x.value);
+        }
+#else
+        std::size_t hash() const
+        {
+            boost::hash<int> hasher;
+            return hasher(value);
+        }
+#endif
+    };
+}
+
+#ifdef BOOST_NO_ARGUMENT_DEPENDENT_LOOKUP
+namespace boost
+{
+    std::size_t hash_value(foo::custom_type x)
+    {
+        return x.hash();
+    }
+}
+#endif
+
+int main()
+{
+    foo::custom_type x(1), y(2), z(1);
+
+    boost::hash<foo::custom_type> hasher;
+
+    assert(hasher(x) == hasher(x));
+    assert(hasher(x) != hasher(y));
+    assert(hasher(x) == hasher(z));
+}