c++/src/kj/refcount.h - toolchain/capnproto - Git at Google

 // 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.

 #pragma once

 #include "memory.h"

 #if _MSC_VER
 #if _MSC_VER < 1910
 #include <intrin.h>
 #else
 #include <intrin0.h>
 #endif
 #endif

 KJ_BEGIN_HEADER

 namespace kj {

 // =======================================================================================
 // Non-atomic (thread-unsafe) refcounting

 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:
   Refcounted() = default;
   virtual ~Refcounted() noexcept(false);
   KJ_DISALLOW_COPY(Refcounted);

   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);
 }

 // =======================================================================================
 // Atomic (thread-safe) refcounting
 //
 // Warning: Atomic ops are SLOW.

 #if _MSC_VER && !defined(__clang__)
 #if _M_ARM
 #define KJ_MSVC_INTERLOCKED(OP, MEM) _Interlocked##OP##_##MEM
 #else
 #define KJ_MSVC_INTERLOCKED(OP, MEM) _Interlocked##OP
 #endif
 #endif

 class AtomicRefcounted: private kj::Disposer {
 public:
   AtomicRefcounted() = default;
   virtual ~AtomicRefcounted() noexcept(false);
   KJ_DISALLOW_COPY(AtomicRefcounted);

   inline bool isShared() const {
 #if _MSC_VER && !defined(__clang__)
     return KJ_MSVC_INTERLOCKED(Or, acq)(&refcount, 0) > 1;
 #else
     return __atomic_load_n(&refcount, __ATOMIC_ACQUIRE) > 1;
 #endif
   }

 private:
 #if _MSC_VER && !defined(__clang__)
   mutable volatile long refcount = 0;
 #else
   mutable volatile uint refcount = 0;
 #endif

   bool addRefWeakInternal() const;

   void disposeImpl(void* pointer) const override;
   template <typename T>
   static kj::Own<T> addRefInternal(T* object);
   template <typename T>
   static kj::Own<const T> addRefInternal(const T* object);

   template <typename T>
   friend kj::Own<T> atomicAddRef(T& object);
   template <typename T>
   friend kj::Own<const T> atomicAddRef(const T& object);
   template <typename T>
   friend kj::Maybe<kj::Own<const T>> atomicAddRefWeak(const T& object);
   template <typename T, typename... Params>
   friend kj::Own<T> atomicRefcounted(Params&&... params);
 };

 template <typename T, typename... Params>
 inline kj::Own<T> atomicRefcounted(Params&&... params) {
   return AtomicRefcounted::addRefInternal(new T(kj::fwd<Params>(params)...));
 }

 template <typename T>
 kj::Own<T> atomicAddRef(T& object) {
   KJ_IREQUIRE(object.AtomicRefcounted::refcount > 0,
       "Object not allocated with kj::atomicRefcounted().");
   return AtomicRefcounted::addRefInternal(&object);
 }

 template <typename T>
 kj::Own<const T> atomicAddRef(const T& object) {
   KJ_IREQUIRE(object.AtomicRefcounted::refcount > 0,
       "Object not allocated with kj::atomicRefcounted().");
   return AtomicRefcounted::addRefInternal(&object);
 }

 template <typename T>
 kj::Maybe<kj::Own<const T>> atomicAddRefWeak(const T& object) {
   // Try to addref an object whose refcount could have already reached zero in another thread, and
   // whose destructor could therefore already have started executing. The destructor must contain
   // some synchronization that guarantees that said destructor has not yet completed when
   // attomicAddRefWeak() is called (so that the object is still valid). Since the destructor cannot
   // be canceled once it has started, in the case that it has already started, this function
   // returns nullptr.

   const AtomicRefcounted* refcounted = &object;
   if (refcounted->addRefWeakInternal()) {
     return kj::Own<const T>(&object, *refcounted);
   } else {
     return nullptr;
   }
 }

 template <typename T>
 kj::Own<T> AtomicRefcounted::addRefInternal(T* object) {
   AtomicRefcounted* refcounted = object;
 #if _MSC_VER && !defined(__clang__)
   KJ_MSVC_INTERLOCKED(Increment, nf)(&refcounted->refcount);
 #else
   __atomic_add_fetch(&refcounted->refcount, 1, __ATOMIC_RELAXED);
 #endif
   return kj::Own<T>(object, *refcounted);
 }

 template <typename T>
 kj::Own<const T> AtomicRefcounted::addRefInternal(const T* object) {
   const AtomicRefcounted* refcounted = object;
 #if _MSC_VER && !defined(__clang__)
   KJ_MSVC_INTERLOCKED(Increment, nf)(&refcounted->refcount);
 #else
   __atomic_add_fetch(&refcounted->refcount, 1, __ATOMIC_RELAXED);
 #endif
   return kj::Own<const T>(object, *refcounted);
 }

 }  // namespace kj

 KJ_END_HEADER
	// 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.

	#pragma once

	#include "memory.h"

	#if _MSC_VER
	#if _MSC_VER < 1910
	#include <intrin.h>
	#else
	#include <intrin0.h>
	#endif
	#endif

	KJ_BEGIN_HEADER

	namespace kj {

	// =======================================================================================
	// Non-atomic (thread-unsafe) refcounting

	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:
	Refcounted() = default;
	virtual ~Refcounted() noexcept(false);
	KJ_DISALLOW_COPY(Refcounted);

	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);
	}

	// =======================================================================================
	// Atomic (thread-safe) refcounting
	//
	// Warning: Atomic ops are SLOW.

	#if _MSC_VER && !defined(__clang__)
	#if _M_ARM
	#define KJ_MSVC_INTERLOCKED(OP, MEM) _Interlocked##OP##_##MEM
	#else
	#define KJ_MSVC_INTERLOCKED(OP, MEM) _Interlocked##OP
	#endif
	#endif

	class AtomicRefcounted: private kj::Disposer {
	public:
	AtomicRefcounted() = default;
	virtual ~AtomicRefcounted() noexcept(false);
	KJ_DISALLOW_COPY(AtomicRefcounted);

	inline bool isShared() const {
	#if _MSC_VER && !defined(__clang__)
	return KJ_MSVC_INTERLOCKED(Or, acq)(&refcount, 0) > 1;
	#else
	return __atomic_load_n(&refcount, __ATOMIC_ACQUIRE) > 1;
	#endif
	}

	private:
	#if _MSC_VER && !defined(__clang__)
	mutable volatile long refcount = 0;
	#else
	mutable volatile uint refcount = 0;
	#endif

	bool addRefWeakInternal() const;

	void disposeImpl(void* pointer) const override;
	template <typename T>
	static kj::Own<T> addRefInternal(T* object);
	template <typename T>
	static kj::Own<const T> addRefInternal(const T* object);

	template <typename T>
	friend kj::Own<T> atomicAddRef(T& object);
	template <typename T>
	friend kj::Own<const T> atomicAddRef(const T& object);
	template <typename T>
	friend kj::Maybe<kj::Own<const T>> atomicAddRefWeak(const T& object);
	template <typename T, typename... Params>
	friend kj::Own<T> atomicRefcounted(Params&&... params);
	};

	template <typename T, typename... Params>
	inline kj::Own<T> atomicRefcounted(Params&&... params) {
	return AtomicRefcounted::addRefInternal(new T(kj::fwd<Params>(params)...));
	}

	template <typename T>
	kj::Own<T> atomicAddRef(T& object) {
	KJ_IREQUIRE(object.AtomicRefcounted::refcount > 0,
	"Object not allocated with kj::atomicRefcounted().");
	return AtomicRefcounted::addRefInternal(&object);
	}

	template <typename T>
	kj::Own<const T> atomicAddRef(const T& object) {
	KJ_IREQUIRE(object.AtomicRefcounted::refcount > 0,
	"Object not allocated with kj::atomicRefcounted().");
	return AtomicRefcounted::addRefInternal(&object);
	}

	template <typename T>
	kj::Maybe<kj::Own<const T>> atomicAddRefWeak(const T& object) {
	// Try to addref an object whose refcount could have already reached zero in another thread, and
	// whose destructor could therefore already have started executing. The destructor must contain
	// some synchronization that guarantees that said destructor has not yet completed when
	// attomicAddRefWeak() is called (so that the object is still valid). Since the destructor cannot
	// be canceled once it has started, in the case that it has already started, this function
	// returns nullptr.

	const AtomicRefcounted* refcounted = &object;
	if (refcounted->addRefWeakInternal()) {
	return kj::Own<const T>(&object, *refcounted);
	} else {
	return nullptr;
	}
	}

	template <typename T>
	kj::Own<T> AtomicRefcounted::addRefInternal(T* object) {
	AtomicRefcounted* refcounted = object;
	#if _MSC_VER && !defined(__clang__)
	KJ_MSVC_INTERLOCKED(Increment, nf)(&refcounted->refcount);
	#else
	__atomic_add_fetch(&refcounted->refcount, 1, __ATOMIC_RELAXED);
	#endif
	return kj::Own<T>(object, *refcounted);
	}

	template <typename T>
	kj::Own<const T> AtomicRefcounted::addRefInternal(const T* object) {
	const AtomicRefcounted* refcounted = object;
	#if _MSC_VER && !defined(__clang__)
	KJ_MSVC_INTERLOCKED(Increment, nf)(&refcounted->refcount);
	#else
	__atomic_add_fetch(&refcounted->refcount, 1, __ATOMIC_RELAXED);
	#endif
	return kj::Own<const T>(object, *refcounted);
	}

	} // namespace kj

	KJ_END_HEADER