comparison osx/include/kj/refcount.h @ 134:41e769c91eca

Add Capnp and KJ builds for OSX

+// Copyright (c) 2013-2014 Sandstorm Development Group, Inc. and contributors
+// Licensed under the MIT License:
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+
+#include "memory.h"
+
+#ifndef KJ_REFCOUNT_H_
+#define KJ_REFCOUNT_H_
+
+#if defined(__GNUC__) && !KJ_HEADER_WARNINGS
+#pragma GCC system_header
+#endif
+
+namespace kj {
+
+class Refcounted: private Disposer {
+  // Subclass this to create a class that contains a reference count. Then, use
+  // `kj::refcounted<T>()` to allocate a new refcounted pointer.
+  //
+  // Do NOT use this lightly.  Refcounting is a crutch.  Good designs should strive to make object
+  // ownership clear, so that refcounting is not necessary.  All that said, reference counting can
+  // sometimes simplify code that would otherwise become convoluted with explicit ownership, even
+  // when ownership relationships are clear at an abstract level.
+  //
+  // NOT THREADSAFE:  This refcounting implementation assumes that an object's references are
+  // manipulated only in one thread, because atomic (thread-safe) refcounting is surprisingly slow.
+  //
+  // In general, abstract classes should _not_ subclass this.  The concrete class at the bottom
+  // of the hierarchy should be the one to decide how it implements refcounting.  Interfaces should
+  // expose only an `addRef()` method that returns `Own<InterfaceType>`.  There are two reasons for
+  // this rule:
+  // 1. Interfaces would need to virtually inherit Refcounted, otherwise two refcounted interfaces
+  //    could not be inherited by the same subclass.  Virtual inheritance is awkward and
+  //    inefficient.
+  // 2. An implementation may decide that it would rather return a copy than a refcount, or use
+  //    some other strategy.
+  //
+  // TODO(cleanup):  Rethink above.  Virtual inheritance is not necessarily that bad.  OTOH, a
+  //   virtual function call for every refcount is sad in its own way.  A Ref<T> type to replace
+  //   Own<T> could also be nice.
+
+public:
+  virtual ~Refcounted() noexcept(false);
+
+  inline bool isShared() const { return refcount > 1; }
+  // Check if there are multiple references to this object. This is sometimes useful for deciding
+  // whether it's safe to modify the object vs. make a copy.
+
+private:
+  mutable uint refcount = 0;
+  // "mutable" because disposeImpl() is const.  Bleh.
+
+  void disposeImpl(void* pointer) const override;
+  template <typename T>
+  static Own<T> addRefInternal(T* object);
+
+  template <typename T>
+  friend Own<T> addRef(T& object);
+  template <typename T, typename... Params>
+  friend Own<T> refcounted(Params&&... params);
+};
+
+template <typename T, typename... Params>
+inline Own<T> refcounted(Params&&... params) {
+  // Allocate a new refcounted instance of T, passing `params` to its constructor.  Returns an
+  // initial reference to the object.  More references can be created with `kj::addRef()`.
+
+  return Refcounted::addRefInternal(new T(kj::fwd<Params>(params)...));
+}
+
+template <typename T>
+Own<T> addRef(T& object) {
+  // Return a new reference to `object`, which must subclass Refcounted and have been allocated
+  // using `kj::refcounted<>()`.  It is suggested that subclasses implement a non-static addRef()
+  // method which wraps this and returns the appropriate type.
+
+  KJ_IREQUIRE(object.Refcounted::refcount > 0, "Object not allocated with kj::refcounted().");
+  return Refcounted::addRefInternal(&object);
+}
+
+template <typename T>
+Own<T> Refcounted::addRefInternal(T* object) {
+  Refcounted* refcounted = object;
+  ++refcounted->refcount;
+  return Own<T>(object, *refcounted);
+}
+
+}  // namespace kj
+
+#endif  // KJ_REFCOUNT_H_