Chris@50: // Copyright (c) 2013-2014 Sandstorm Development Group, Inc. and contributors
Chris@50: // Licensed under the MIT License:
Chris@50: //
Chris@50: // Permission is hereby granted, free of charge, to any person obtaining a copy
Chris@50: // of this software and associated documentation files (the "Software"), to deal
Chris@50: // in the Software without restriction, including without limitation the rights
Chris@50: // to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
Chris@50: // copies of the Software, and to permit persons to whom the Software is
Chris@50: // furnished to do so, subject to the following conditions:
Chris@50: //
Chris@50: // The above copyright notice and this permission notice shall be included in
Chris@50: // all copies or substantial portions of the Software.
Chris@50: //
Chris@50: // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
Chris@50: // IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
Chris@50: // FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
Chris@50: // AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
Chris@50: // LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
Chris@50: // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
Chris@50: // THE SOFTWARE.
Chris@50: 
Chris@50: #include "memory.h"
Chris@50: 
Chris@50: #ifndef KJ_REFCOUNT_H_
Chris@50: #define KJ_REFCOUNT_H_
Chris@50: 
Chris@50: #if defined(__GNUC__) && !KJ_HEADER_WARNINGS
Chris@50: #pragma GCC system_header
Chris@50: #endif
Chris@50: 
Chris@50: namespace kj {
Chris@50: 
Chris@50: class Refcounted: private Disposer {
Chris@50:   // Subclass this to create a class that contains a reference count. Then, use
Chris@50:   // `kj::refcounted<T>()` to allocate a new refcounted pointer.
Chris@50:   //
Chris@50:   // Do NOT use this lightly.  Refcounting is a crutch.  Good designs should strive to make object
Chris@50:   // ownership clear, so that refcounting is not necessary.  All that said, reference counting can
Chris@50:   // sometimes simplify code that would otherwise become convoluted with explicit ownership, even
Chris@50:   // when ownership relationships are clear at an abstract level.
Chris@50:   //
Chris@50:   // NOT THREADSAFE:  This refcounting implementation assumes that an object's references are
Chris@50:   // manipulated only in one thread, because atomic (thread-safe) refcounting is surprisingly slow.
Chris@50:   //
Chris@50:   // In general, abstract classes should _not_ subclass this.  The concrete class at the bottom
Chris@50:   // of the hierarchy should be the one to decide how it implements refcounting.  Interfaces should
Chris@50:   // expose only an `addRef()` method that returns `Own<InterfaceType>`.  There are two reasons for
Chris@50:   // this rule:
Chris@50:   // 1. Interfaces would need to virtually inherit Refcounted, otherwise two refcounted interfaces
Chris@50:   //    could not be inherited by the same subclass.  Virtual inheritance is awkward and
Chris@50:   //    inefficient.
Chris@50:   // 2. An implementation may decide that it would rather return a copy than a refcount, or use
Chris@50:   //    some other strategy.
Chris@50:   //
Chris@50:   // TODO(cleanup):  Rethink above.  Virtual inheritance is not necessarily that bad.  OTOH, a
Chris@50:   //   virtual function call for every refcount is sad in its own way.  A Ref<T> type to replace
Chris@50:   //   Own<T> could also be nice.
Chris@50: 
Chris@50: public:
Chris@50:   virtual ~Refcounted() noexcept(false);
Chris@50: 
Chris@50:   inline bool isShared() const { return refcount > 1; }
Chris@50:   // Check if there are multiple references to this object. This is sometimes useful for deciding
Chris@50:   // whether it's safe to modify the object vs. make a copy.
Chris@50: 
Chris@50: private:
Chris@50:   mutable uint refcount = 0;
Chris@50:   // "mutable" because disposeImpl() is const.  Bleh.
Chris@50: 
Chris@50:   void disposeImpl(void* pointer) const override;
Chris@50:   template <typename T>
Chris@50:   static Own<T> addRefInternal(T* object);
Chris@50: 
Chris@50:   template <typename T>
Chris@50:   friend Own<T> addRef(T& object);
Chris@50:   template <typename T, typename... Params>
Chris@50:   friend Own<T> refcounted(Params&&... params);
Chris@50: };
Chris@50: 
Chris@50: template <typename T, typename... Params>
Chris@50: inline Own<T> refcounted(Params&&... params) {
Chris@50:   // Allocate a new refcounted instance of T, passing `params` to its constructor.  Returns an
Chris@50:   // initial reference to the object.  More references can be created with `kj::addRef()`.
Chris@50: 
Chris@50:   return Refcounted::addRefInternal(new T(kj::fwd<Params>(params)...));
Chris@50: }
Chris@50: 
Chris@50: template <typename T>
Chris@50: Own<T> addRef(T& object) {
Chris@50:   // Return a new reference to `object`, which must subclass Refcounted and have been allocated
Chris@50:   // using `kj::refcounted<>()`.  It is suggested that subclasses implement a non-static addRef()
Chris@50:   // method which wraps this and returns the appropriate type.
Chris@50: 
Chris@50:   KJ_IREQUIRE(object.Refcounted::refcount > 0, "Object not allocated with kj::refcounted().");
Chris@50:   return Refcounted::addRefInternal(&object);
Chris@50: }
Chris@50: 
Chris@50: template <typename T>
Chris@50: Own<T> Refcounted::addRefInternal(T* object) {
Chris@50:   Refcounted* refcounted = object;
Chris@50:   ++refcounted->refcount;
Chris@50:   return Own<T>(object, *refcounted);
Chris@50: }
Chris@50: 
Chris@50: }  // namespace kj
Chris@50: 
Chris@50: #endif  // KJ_REFCOUNT_H_