I just made C++ God and it's scary

src/dawn/state/StateOwner.hpp

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+// Copyright (c) 2023 Dominic Masters
+// 
+// This software is released under the MIT License.
+// https://opensource.org/licenses/MIT
+
+#pragma once
+#include "assert/assert.hpp"
+
+namespace Dawn {
+  template<class V>
+  class StateProperty;
+
+  class StateOwner {
+    private:
+      std::map<void*, std::vector<std::function<void()>>> effects;
+
+    public:
+      /**
+       * Invoked event listener for the subclass that listens for all changes
+       * to any part of this state updated.
+       */
+      virtual void onStateUpdate() = 0;
+
+      /**
+       * Creates a new state property and listens for its change.
+       * 
+       * @tparam V The type of the state that is held.
+       * @param initial The initial value of this state.
+       * @return The state that can then be listened for.
+       */
+      template<class V>
+      StateProperty<V> useState(V initial) {
+        auto property = StateProperty<V>();
+        property.value = initial;
+        property.owner = this;
+        return property;
+      }
+
+      /**
+       * Listen for changees to a state property and involke the provided func
+       * when the value is changed.
+       * 
+       * @tparam V The type of the state property.
+       * @param property Property to listen for affect changees to.
+       * @param fn The callback to be invoked when the state value changes.
+       */
+      template<class V>
+      void useEffect(StateProperty<V> &property, const std::function<void()> &fn) {
+        this->effects[(void*)&property].push_back(fn);
+      }
+
+      /**
+       * Internal method (that has to be exposed) to listen for changes for when
+       * a state property that belongs to this state owner is updated.
+       * 
+       * @tparam V The value type of the state property.
+       * @param prop The property that has its value changed in question.
+       * @param n The new, current value of the property.
+       * @param o The old, previous value of the property.
+       */
+      template<class V>
+      void onStatePropertyUpdated(StateProperty<V> *prop, V n, V o) {
+        this->onStateUpdate();
+
+        auto eff = &this->effects[prop];
+        auto itEff = eff->begin();
+        while(itEff != eff->end()) {
+          (*itEff)();
+          ++itEff;
+        }
+      }
+  };
+}