cannam@135: // Copyright (c) 2013-2014 Sandstorm Development Group, Inc. and contributors
cannam@135: // Licensed under the MIT License:
cannam@135: //
cannam@135: // Permission is hereby granted, free of charge, to any person obtaining a copy
cannam@135: // of this software and associated documentation files (the "Software"), to deal
cannam@135: // in the Software without restriction, including without limitation the rights
cannam@135: // to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
cannam@135: // copies of the Software, and to permit persons to whom the Software is
cannam@135: // furnished to do so, subject to the following conditions:
cannam@135: //
cannam@135: // The above copyright notice and this permission notice shall be included in
cannam@135: // all copies or substantial portions of the Software.
cannam@135: //
cannam@135: // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
cannam@135: // IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
cannam@135: // FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
cannam@135: // AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
cannam@135: // LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
cannam@135: // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
cannam@135: // THE SOFTWARE.
cannam@135: 
cannam@135: #include "memory.h"
cannam@135: 
cannam@135: #ifndef KJ_REFCOUNT_H_
cannam@135: #define KJ_REFCOUNT_H_
cannam@135: 
cannam@135: #if defined(__GNUC__) && !KJ_HEADER_WARNINGS
cannam@135: #pragma GCC system_header
cannam@135: #endif
cannam@135: 
cannam@135: namespace kj {
cannam@135: 
cannam@135: class Refcounted: private Disposer {
cannam@135:   // Subclass this to create a class that contains a reference count. Then, use
cannam@135:   // `kj::refcounted<T>()` to allocate a new refcounted pointer.
cannam@135:   //
cannam@135:   // Do NOT use this lightly.  Refcounting is a crutch.  Good designs should strive to make object
cannam@135:   // ownership clear, so that refcounting is not necessary.  All that said, reference counting can
cannam@135:   // sometimes simplify code that would otherwise become convoluted with explicit ownership, even
cannam@135:   // when ownership relationships are clear at an abstract level.
cannam@135:   //
cannam@135:   // NOT THREADSAFE:  This refcounting implementation assumes that an object's references are
cannam@135:   // manipulated only in one thread, because atomic (thread-safe) refcounting is surprisingly slow.
cannam@135:   //
cannam@135:   // In general, abstract classes should _not_ subclass this.  The concrete class at the bottom
cannam@135:   // of the hierarchy should be the one to decide how it implements refcounting.  Interfaces should
cannam@135:   // expose only an `addRef()` method that returns `Own<InterfaceType>`.  There are two reasons for
cannam@135:   // this rule:
cannam@135:   // 1. Interfaces would need to virtually inherit Refcounted, otherwise two refcounted interfaces
cannam@135:   //    could not be inherited by the same subclass.  Virtual inheritance is awkward and
cannam@135:   //    inefficient.
cannam@135:   // 2. An implementation may decide that it would rather return a copy than a refcount, or use
cannam@135:   //    some other strategy.
cannam@135:   //
cannam@135:   // TODO(cleanup):  Rethink above.  Virtual inheritance is not necessarily that bad.  OTOH, a
cannam@135:   //   virtual function call for every refcount is sad in its own way.  A Ref<T> type to replace
cannam@135:   //   Own<T> could also be nice.
cannam@135: 
cannam@135: public:
cannam@135:   virtual ~Refcounted() noexcept(false);
cannam@135: 
cannam@135:   inline bool isShared() const { return refcount > 1; }
cannam@135:   // Check if there are multiple references to this object. This is sometimes useful for deciding
cannam@135:   // whether it's safe to modify the object vs. make a copy.
cannam@135: 
cannam@135: private:
cannam@135:   mutable uint refcount = 0;
cannam@135:   // "mutable" because disposeImpl() is const.  Bleh.
cannam@135: 
cannam@135:   void disposeImpl(void* pointer) const override;
cannam@135:   template <typename T>
cannam@135:   static Own<T> addRefInternal(T* object);
cannam@135: 
cannam@135:   template <typename T>
cannam@135:   friend Own<T> addRef(T& object);
cannam@135:   template <typename T, typename... Params>
cannam@135:   friend Own<T> refcounted(Params&&... params);
cannam@135: };
cannam@135: 
cannam@135: template <typename T, typename... Params>
cannam@135: inline Own<T> refcounted(Params&&... params) {
cannam@135:   // Allocate a new refcounted instance of T, passing `params` to its constructor.  Returns an
cannam@135:   // initial reference to the object.  More references can be created with `kj::addRef()`.
cannam@135: 
cannam@135:   return Refcounted::addRefInternal(new T(kj::fwd<Params>(params)...));
cannam@135: }
cannam@135: 
cannam@135: template <typename T>
cannam@135: Own<T> addRef(T& object) {
cannam@135:   // Return a new reference to `object`, which must subclass Refcounted and have been allocated
cannam@135:   // using `kj::refcounted<>()`.  It is suggested that subclasses implement a non-static addRef()
cannam@135:   // method which wraps this and returns the appropriate type.
cannam@135: 
cannam@135:   KJ_IREQUIRE(object.Refcounted::refcount > 0, "Object not allocated with kj::refcounted().");
cannam@135:   return Refcounted::addRefInternal(&object);
cannam@135: }
cannam@135: 
cannam@135: template <typename T>
cannam@135: Own<T> Refcounted::addRefInternal(T* object) {
cannam@135:   Refcounted* refcounted = object;
cannam@135:   ++refcounted->refcount;
cannam@135:   return Own<T>(object, *refcounted);
cannam@135: }
cannam@135: 
cannam@135: }  // namespace kj
cannam@135: 
cannam@135: #endif  // KJ_REFCOUNT_H_