add hash_value example

CMakeLists.txt

@@ -124,6 +124,7 @@ if (PHMAP_BUILD_EXAMPLES)
     add_executable(ex_bench examples/bench.cc)
     add_executable(ex_emplace examples/emplace.cc)
     add_executable(ex_hash_std examples/hash_std.cc)
+    add_executable(ex_hash_value examples/hash_value.cc)
     add_executable(ex_two_files examples/f1.cc examples/f2.cc)
     add_executable(ex_insert_bench examples/insert_bench.cc)
     add_executable(ex_knucleotide examples/knucleotide.cc)

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';
+
+}

examples/hash_value.h

@@ -0,0 +1,25 @@
+#ifndef phmap_example_hash_value_
+#define phmap_example_hash_value_
+
+#include <parallel_hashmap/phmap_utils.h> // minimal header providing phmap::HashState()
+#include <string>
+using std::string;
+
+struct Person
+{
+    bool operator==(const Person &o) const
+    { 
+        return _first == o._first && _last == o._last && _age == o._age; 
+    }
+
+    friend size_t hash_value(const Person &p) 
+    {
+            return phmap::HashState().combine(0, p._first, p._last, p._age);
+    }
+
+    string _first;
+    string _last;
+    int    _age;
+};
+
+#endif // phmap_example_hash_value_