1 files changed, 153 insertions, 0 deletions
diff --git a/include/cru/base/SelfResolvable.h b/include/cru/base/SelfResolvable.h
new file mode 100644
index 00000000..84fa54f6
--- /dev/null
+++ b/include/cru/base/SelfResolvable.h
@@ -0,0 +1,153 @@
+#pragma once
+
+#include <cassert>
+#include <functional>
+#include <memory>
+#include <type_traits>
+
+namespace cru {
+template <typename T>
+class SelfResolvable;
+
+template <typename T>
+class ObjectResolver {
+  friend SelfResolvable<T>;
+  template <typename U>
+  friend class ObjectResolver;
+
+ private:
+  template <typename U>
+  using Accessor_ = std::function<U*(const std::shared_ptr<void*>&)>;
+  using ThisAccessor_ = Accessor_<T>;
+
+  explicit ObjectResolver(T* o)
+      : shared_object_ptr_(new void*(o)),
+        accessor_([](const std::shared_ptr<void*>& ptr) {
+          return static_cast<T*>(*ptr);
+        }) {}
+  explicit ObjectResolver(std::shared_ptr<void*> ptr, ThisAccessor_ accessor)
+      : shared_object_ptr_(std::move(ptr)), accessor_(std::move(accessor)) {}
+
+  template <typename U>
+  static ThisAccessor_ CreateAccessor(Accessor_<U> parent_accessor) {
+    return [parent_accessor =
+                std::move(parent_accessor)](const std::shared_ptr<void*>& ptr) {
+      return static_cast<T*>(parent_accessor(ptr));
+    };
+  }
+
+ public:
+  template <typename U,
+            typename = std::enable_if_t<std::is_convertible_v<U*, T*>>>
+  ObjectResolver(const ObjectResolver<U>& other)
+      : shared_object_ptr_(other.shared_object_ptr_),
+        accessor_(CreateAccessor(other.accessor_)) {}
+
+  template <typename U,
+            typename = std::enable_if_t<std::is_convertible_v<U*, T*>>>
+  ObjectResolver(ObjectResolver<U>&& other)
+      : shared_object_ptr_(std::move(other.shared_object_ptr_)),
+        accessor_(CreateAccessor(std::move(other.accessor_))) {}
+
+  ObjectResolver(const ObjectResolver&) = default;
+  ObjectResolver& operator=(const ObjectResolver&) = default;
+  ObjectResolver(ObjectResolver&&) = default;
+  ObjectResolver& operator=(ObjectResolver&&) = default;
+  ~ObjectResolver() = default;
+
+  template <typename U,
+            typename = std::enable_if_t<std::is_convertible_v<U*, T*>>>
+  ObjectResolver& operator=(const ObjectResolver<U>& other) {
+    if (this != &other) {
+      this->shared_object_ptr_ = other.shared_object_ptr_;
+      this->accessor_ = CreateAccessor(other.accessor_);
+    }
+    return *this;
+  }
+
+  template <typename U,
+            typename = std::enable_if_t<std::is_convertible_v<U*, T*>>>
+  ObjectResolver& operator=(ObjectResolver<U>&& other) {
+    if (this != &other) {
+      this->shared_object_ptr_ = std::move(other.shared_object_ptr_);
+      this->accessor_ = CreateAccessor(std::move(other.shared_object_ptr_));
+    }
+    return *this;
+  }
+
+  bool IsValid() const { return this->shared_object_ptr_ != nullptr; }
+
+  T* Resolve() const {
+    assert(IsValid());
+    return this->accessor_(this->shared_object_ptr_);
+  }
+
+  /**
+   * @remarks So this class can be used as a functor.
+   */
+  T* operator()() const { return Resolve(); }
+
+  template <typename U,
+            typename = std::enable_if_t<std::is_convertible_v<T*, U*>>>
+  operator ObjectResolver<U>() const {
+    return ObjectResolver<U>(*this);
+  }
+
+ private:
+  void SetResolvedObject(T* o) {
+    assert(IsValid());
+    *this->shared_object_ptr_ = o;
+  }
+
+ private:
+  std::shared_ptr<void*> shared_object_ptr_;
+  std::function<T*(const std::shared_ptr<void*>&)> accessor_;
+};
+
+/**
+ * @remarks
+ * This class is not copyable and movable since subclass is polymorphic and
+ * copying is then nonsense. However, you can even delete move capability in
+ * subclass because it may also be nonsense for subclass. The move capability is
+ * optional.
+ *
+ * Whether this class needs to be thread-safe still has to be considered.
+ */
+template <typename T>
+class SelfResolvable {
+ public:
+  SelfResolvable() : resolver_(CastToSubClass()) {}
+  SelfResolvable(const SelfResolvable&) = delete;
+  SelfResolvable& operator=(const SelfResolvable&) = delete;
+
+  // Resolvers to old object will resolve to new object.
+  SelfResolvable(SelfResolvable&& other)
+      : resolver_(std::move(other.resolver_)) {
+    this->resolver_.SetResolvedObject(CastToSubClass());
+  }
+
+  // Old resolvers for this object will resolve to nullptr.
+  // Other's resolvers will now resolve to this.
+  SelfResolvable& operator=(SelfResolvable&& other) {
+    if (this != &other) {
+      this->resolver_ = std::move(other.resolver_);
+      this->resolver_.SetResolvedObject(CastToSubClass());
+    }
+    return *this;
+  }
+
+  virtual ~SelfResolvable() {
+    if (this->resolver_.IsValid()) {
+      this->resolver_.SetResolvedObject(nullptr);
+    }
+  }
+
+  ObjectResolver<T> CreateResolver() { return resolver_; }
+
+ private:
+  T* CastToSubClass() { return static_cast<T*>(this); }
+
+ private:
+  ObjectResolver<T> resolver_;
+};
+}  // namespace cru