Create serialization.md

presentations/rust-traits-c++-on-sea-2025/serialization.md

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+## Example of subtyping as subclassing: Typical Serialization in C++
+
+```c++
+#include <sstream>
+
+// Infrastructure:
+namespace user {
+
+struct TypeRegistry {
+    template<typename T>
+    std::uint64_t id() const;
+};
+
+extern TypeRegistry g_registry;
+
+struct ISerialize {
+    virtual std::ostream &serialize(std::ostream &) const;
+    virtual std::size_t length() const;
+};
+
+template<typename T>
+struct SerializeWrapper: ISerialize {
+    T value_;
+    virtual std::ostream &serialize(std::ostream &to) const override {
+        return
+            to << g_registry.id<T>() << ':' << value_;
+    }
+    virtual std::size_t length() const {
+        std::ostringstream temporary;
+        serialize(temporary);
+        return temporary.str().length();
+    }
+};
+
+}
+```