add hash_value example

examples/hash_value.cc

@@ -0,0 +1,20 @@
+#include "hash_std.h"   // defines Person  with std::hash specialization
+
+#include <iostream>
+#include <parallel_hashmap/phmap.h>
+
+int main()
+{
+    // As we have defined a specialization of std::hash() for Person,
+    // we can now create sparse_hash_set or sparse_hash_map of Persons
+    // ----------------------------------------------------------------
+    phmap::flat_hash_set<Person> persons = 
+        { { "John", "Mitchell", 35 },
+          { "Jane", "Smith",    32 },
+          { "Jane", "Smith",    30 },
+        };
+
+    for (auto& p: persons)
+        std::cout << p._first << ' ' << p._last << " (" << p._age << ")" << '\n';
+
+}