Added msgpack::type::variant and msgpack::type::variant_ref that is based on boost::variant.

You can convert to those types from any msgpack objects.

example/boost/msgpack_variant_mapbased.cpp

@@ -0,0 +1,95 @@
+// MessagePack for C++ example
+//
+// Copyright (C) 2015 KONDO Takatoshi
+//
+//    Licensed under the Apache License, Version 2.0 (the "License");
+//    you may not use this file except in compliance with the License.
+//    You may obtain a copy of the License at
+//
+//        http://www.apache.org/licenses/LICENSE-2.0
+//
+//    Unless required by applicable law or agreed to in writing, software
+//    distributed under the License is distributed on an "AS IS" BASIS,
+//    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//    See the License for the specific language governing permissions and
+//    limitations under the License.
+//
+
+#include <string>
+#include <sstream>
+#include <iostream>
+
+#include <msgpack.hpp>
+
+struct user {
+    std::string name;
+    int age;
+    std::string address;
+    MSGPACK_DEFINE_MAP(name, age, address);
+};
+
+struct proc:boost::static_visitor<void> {
+    // msgpack::type::MAP is converted to std::multimap, not std::map.
+    void operator()(std::multimap<msgpack::type::variant_ref, msgpack::type::variant_ref>& v) const {
+        std::cout << "match map" << std::endl;
+        std::multimap<msgpack::type::variant_ref, msgpack::type::variant_ref>::iterator it = v.begin();
+        std::multimap<msgpack::type::variant_ref, msgpack::type::variant_ref>::iterator end = v.end();
+        while(it != end) {
+            boost::string_ref const& key = boost::get<boost::string_ref>(it->first);
+            if (key == "name") {
+                boost::string_ref const& value = boost::get<boost::string_ref>(it->second);
+                if (value == "Takatoshi Kondo") {
+                    // You can add values to msgpack::type::variant_ref.
+                    v.insert(
+                        std::multimap<msgpack::type::variant_ref, msgpack::type::variant_ref>::value_type(
+                            std::string("role"),
+                            std::string("msgpack-c committer")
+                        )
+                    );
+                }
+                ++it;
+            }
+            else if (key == "age") {
+                // You can remove key-value pair from msgpack::type::variant_ref
+#if defined(MSGPACK_USE_CPP03)
+                v.erase(it++);
+#else
+                it = v.erase(it);
+#endif
+            }
+            else if (key == "address") {
+                // When you want to append string
+                // "Tokyo" -> "Tokyo, JAPAN"
+                // Use msgpack::type::variant instead of msgpack::type::varinat_ref
+                // or do as follows:
+                boost::string_ref const& value = boost::get<boost::string_ref>(it->second);
+                it->second = std::string(&value.front(), value.size()) + ", JAPAN";
+                ++it;
+            }
+        }
+    }
+    template <typename T>
+    void operator()(T const&) const {
+        std::cout << "  match others" << std::endl;
+    }
+};
+
+int main() {
+    std::stringstream ss;
+    user u;
+    u.name = "Takatoshi Kondo";
+    u.age = 42;
+    u.address = "Tokyo";
+    msgpack::pack(ss, u);
+
+    msgpack::unpacked unp = msgpack::unpack(ss.str().data(), ss.str().size());
+    msgpack::object const& obj = unp.get();
+    std::cout << "Unpacked msgpack object." << std::endl;
+    std::cout << obj << std::endl;
+    msgpack::type::variant_ref v = obj.as<msgpack::type::variant_ref>();
+    std::cout << "Applying proc..." << std::endl;
+    boost::apply_visitor(proc(), v);
+    msgpack::zone z;
+    std::cout << "Applied msgpack object." << std::endl;
+    std::cout << msgpack::object(v, z) << std::endl;
+}