Add example demonstrating saving nested hashmaps to binary file.

examples/dump_nested.cc

@@ -0,0 +1,88 @@
+/*
+ *
+ * Example of dumping a map, containing values which are phmap maps or sets
+ * building this requires c++17 support
+ *
+ */
+
+#include <iostream>
+#include <parallel_hashmap/phmap_dump.h>
+
+template <class K, class V>
+class MyMap : public phmap::flat_hash_map<K, phmap::flat_hash_set<V>>
+{
+public:
+    using Set = phmap::flat_hash_set<V>;
+
+    void dump(const std::string &filename) 
+    {
+        phmap::BinaryOutputArchive ar_out (filename.c_str());
+
+        ar_out.dump(this->size());
+        for (auto& [k, v] : *this) 
+        {
+            ar_out.dump(k);
+            v.dump(ar_out);
+        }
+    }
+
+    void load(const std::string & filename) 
+    {
+        phmap::BinaryInputArchive ar_in(filename.c_str());
+
+        size_t size;
+        ar_in.load(&size);
+        this->reserve(size);
+
+        while (size--)
+        {
+            K k;
+            Set v;
+
+            ar_in.load(&k);
+            v.load(ar_in);
+
+            this->insert_or_assign(std::move(k), std::move(v));
+        }
+    }
+
+    void insert(K k, V v) 
+    {
+        Set &set = (*this)[k];
+        set.insert(v);
+    }
+
+    friend std::ostream& operator<<(std::ostream& os, const MyMap& map)
+    {
+        for (const auto& [k, m] : map)
+        {
+            os << k << ": [";
+            for (const auto& x : m)
+                os << x << ", ";
+            os << "]\n";
+        }
+        return os;
+    }
+};
+
+int main()
+{
+    MyMap<size_t, size_t> m;
+    m.insert(1, 5);
+    m.insert(1, 8);
+    m.insert(2, 3);
+    m.insert(1, 15);
+    m.insert(1, 27);
+    m.insert(2, 10);
+    m.insert(2, 13);
+    
+    std::cout << m << "\n";
+    
+    m.dump("test_archive");
+    m.clear();
+    m.load("test_archive");
+    
+    std::cout << m << "\n";
+
+    return 0;
+}