Improve self resolvable.

include/cru/common/SelfResolvable.h

@@ -28,6 +28,11 @@ class ObjectResolver {
     return *this->shared_object_ptr_;
   }
 
+  /**
+   * @remarks So this class can be used as a functor.
+   */
+  T* operator()() const { return Resolve(); }
+
  private:
   void SetResolvedObject(T* o) {
     assert(IsValid());
@@ -38,6 +43,15 @@ class ObjectResolver {
   std::shared_ptr<T*> shared_object_ptr_;
 };
 
+/**
+ * @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: