diff --git a/src/engine/CMakeLists.txt b/src/engine/CMakeLists.txt
index 39c48b0..cfc5c6d 100644
--- a/src/engine/CMakeLists.txt
+++ b/src/engine/CMakeLists.txt
@@ -8,6 +8,7 @@ if(INCLUDEJS_BACKEND STREQUAL "JavaScriptCore")
   list(APPEND INCLUDEJS_ENGINE_SOURCES javascript_core/engine_context.cc)
   list(APPEND INCLUDEJS_ENGINE_SOURCES javascript_core/engine_promise.cc)
   list(APPEND INCLUDEJS_ENGINE_SOURCES javascript_core/engine_value.cc)
+  list(APPEND INCLUDEJS_ENGINE_SOURCES javascript_core/engine_private_data.cc)
 elseif(INCLUDEJS_BACKEND STREQUAL "V8")
   list(APPEND INCLUDEJS_ENGINE_SOURCES v8/engine.cc)
 else()
diff --git a/src/engine/include/includejs/engine_private_data.h b/src/engine/include/includejs/engine_private_data.h
index ad755d1..5539218 100644
--- a/src/engine/include/includejs/engine_private_data.h
+++ b/src/engine/include/includejs/engine_private_data.h
@@ -12,33 +12,20 @@ namespace includejs {
 // Inspired by https://github.com/sourcemeta/jsontoolkit
 /// @ingroup engine
 struct INCLUDEJS_ENGINE_EXPORT PrivateObjectData {
-  std::unique_ptr<std::map<void *, Value::Function>> functions;
+  std::unique_ptr<std::map<void *, Value::Function>> functions =
+      std::make_unique<std::map<void *, Value::Function>>();
 
-  PrivateObjectData() {
-    functions = std::make_unique<std::map<void *, Value::Function>>();
-  }
-
-  auto data() -> void * { return data_; }
+  ~PrivateObjectData();
 
+  auto data() -> void *;
   auto set_data(void *new_data, std::function<void(void *)> new_deleter)
-      -> void {
-    clear();
-    data_ = new_data;
-    deleter_ = new_deleter;
-  }
-
-  ~PrivateObjectData() { clear(); }
+      -> void;
 
 private:
   void *data_ = nullptr;
   std::function<void(void *)> deleter_;
 
-  auto clear() -> void {
-    if (data_ != nullptr && deleter_ != nullptr) {
-      deleter_(data_);
-      data_ = nullptr;
-    }
-  }
+  auto clear() -> void;
 };
 
 } // namespace includejs
diff --git a/src/engine/javascript_core/engine_private_data.cc b/src/engine/javascript_core/engine_private_data.cc
new file mode 100644
index 0000000..5959690
--- /dev/null
+++ b/src/engine/javascript_core/engine_private_data.cc
@@ -0,0 +1,24 @@
+#include <includejs/engine_private_data.h>
+
+namespace includejs {
+
+PrivateObjectData::~PrivateObjectData() { clear(); }
+
+auto PrivateObjectData::data() -> void * { return data_; }
+
+auto PrivateObjectData::set_data(void *new_data,
+                                 std::function<void(void *)> new_deleter)
+    -> void {
+  clear();
+  data_ = new_data;
+  deleter_ = new_deleter;
+}
+
+auto PrivateObjectData::clear() -> void {
+  if (data_ != nullptr && deleter_ != nullptr) {
+    deleter_(data_);
+    data_ = nullptr;
+  }
+}
+
+} // namespace includejs