base/linked_ptr.h - chromium/src - Git at Google

 // Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // A "smart" pointer type with reference tracking.  Every pointer to a
 // particular object is kept on a circular linked list.  When the last pointer
 // to an object is destroyed or reassigned, the object is deleted.
 //
 // Used properly, this deletes the object when the last reference goes away.
 // There are several caveats:
 // - Like all reference counting schemes, cycles lead to leaks.
 // - Each smart pointer is actually two pointers (8 bytes instead of 4).
 // - Every time a pointer is released, the entire list of pointers to that
 //   object is traversed.  This class is therefore NOT SUITABLE when there
 //   will often be more than two or three pointers to a particular object.
 // - References are only tracked as long as linked_ptr<> objects are copied.
 //   If a linked_ptr<> is converted to a raw pointer and back, BAD THINGS
 //   will happen (double deletion).
 //
 // A good use of this class is storing object references in STL containers.
 // You can safely put linked_ptr<> in a vector<>.
 // Other uses may not be as good.
 //
 // Note: If you use an incomplete type with linked_ptr<>, the class
 // *containing* linked_ptr<> must have a constructor and destructor (even
 // if they do nothing!).
 //
 // Thread Safety:
 //   A linked_ptr is NOT thread safe. Copying a linked_ptr object is
 //   effectively a read-write operation.
 //
 // Alternative: to linked_ptr is shared_ptr, which
 //  - is also two pointers in size (8 bytes for 32 bit addresses)
 //  - is thread safe for copying and deletion
 //  - supports weak_ptrs

 #ifndef BASE_LINKED_PTR_H_
 #define BASE_LINKED_PTR_H_
 #pragma once

 #include "base/logging.h"  // for CHECK macros

 // This is used internally by all instances of linked_ptr<>.  It needs to be
 // a non-template class because different types of linked_ptr<> can refer to
 // the same object (linked_ptr<Superclass>(obj) vs linked_ptr<Subclass>(obj)).
 // So, it needs to be possible for different types of linked_ptr to participate
 // in the same circular linked list, so we need a single class type here.
 //
 // DO NOT USE THIS CLASS DIRECTLY YOURSELF.  Use linked_ptr<T>.
 class linked_ptr_internal {
  public:
   // Create a new circle that includes only this instance.
   void join_new() {
     next_ = this;
   }

   // Join an existing circle.
   void join(linked_ptr_internal const* ptr) {
     next_ = ptr->next_;
     ptr->next_ = this;
   }

   // Leave whatever circle we're part of.  Returns true iff we were the
   // last member of the circle.  Once this is done, you can join() another.
   bool depart() {
     if (next_ == this) return true;
     linked_ptr_internal const* p = next_;
     while (p->next_ != this) p = p->next_;
     p->next_ = next_;
     return false;
   }

  private:
   mutable linked_ptr_internal const* next_;
 };

 template <typename T>
 class linked_ptr {
  public:
   typedef T element_type;

   // Take over ownership of a raw pointer.  This should happen as soon as
   // possible after the object is created.
   explicit linked_ptr(T* ptr = NULL) { capture(ptr); }
   ~linked_ptr() { depart(); }

   // Copy an existing linked_ptr<>, adding ourselves to the list of references.
   template <typename U> linked_ptr(linked_ptr<U> const& ptr) { copy(&ptr); }

   linked_ptr(linked_ptr const& ptr) {
     DCHECK_NE(&ptr, this);
     copy(&ptr);
   }

   // Assignment releases the old value and acquires the new.
   template <typename U> linked_ptr& operator=(linked_ptr<U> const& ptr) {
     depart();
     copy(&ptr);
     return *this;
   }

   linked_ptr& operator=(linked_ptr const& ptr) {
     if (&ptr != this) {
       depart();
       copy(&ptr);
     }
     return *this;
   }

   // Smart pointer members.
   void reset(T* ptr = NULL) {
     depart();
     capture(ptr);
   }
   T* get() const { return value_; }
   T* operator->() const { return value_; }
   T& operator*() const { return *value_; }
   // Release ownership of the pointed object and returns it.
   // Sole ownership by this linked_ptr object is required.
   T* release() {
     bool last = link_.depart();
     CHECK(last);
     T* v = value_;
     value_ = NULL;
     return v;
   }

   bool operator==(const T* p) const { return value_ == p; }
   bool operator!=(const T* p) const { return value_ != p; }
   template <typename U>
   bool operator==(linked_ptr<U> const& ptr) const {
     return value_ == ptr.get();
   }
   template <typename U>
   bool operator!=(linked_ptr<U> const& ptr) const {
     return value_ != ptr.get();
   }

  private:
   template <typename U>
   friend class linked_ptr;

   T* value_;
   linked_ptr_internal link_;

   void depart() {
     if (link_.depart()) delete value_;
   }

   void capture(T* ptr) {
     value_ = ptr;
     link_.join_new();
   }

   template <typename U> void copy(linked_ptr<U> const* ptr) {
     value_ = ptr->get();
     if (value_)
       link_.join(&ptr->link_);
     else
       link_.join_new();
   }
 };

 template<typename T> inline
 bool operator==(T* ptr, const linked_ptr<T>& x) {
   return ptr == x.get();
 }

 template<typename T> inline
 bool operator!=(T* ptr, const linked_ptr<T>& x) {
   return ptr != x.get();
 }

 // A function to convert T* into linked_ptr<T>
 // Doing e.g. make_linked_ptr(new FooBarBaz<type>(arg)) is a shorter notation
 // for linked_ptr<FooBarBaz<type> >(new FooBarBaz<type>(arg))
 template <typename T>
 linked_ptr<T> make_linked_ptr(T* ptr) {
   return linked_ptr<T>(ptr);
 }

 #endif  // BASE_LINKED_PTR_H_
	// Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style license that can be
	// found in the LICENSE file.
	//
	// A "smart" pointer type with reference tracking. Every pointer to a
	// particular object is kept on a circular linked list. When the last pointer
	// to an object is destroyed or reassigned, the object is deleted.
	//
	// Used properly, this deletes the object when the last reference goes away.
	// There are several caveats:
	// - Like all reference counting schemes, cycles lead to leaks.
	// - Each smart pointer is actually two pointers (8 bytes instead of 4).
	// - Every time a pointer is released, the entire list of pointers to that
	// object is traversed. This class is therefore NOT SUITABLE when there
	// will often be more than two or three pointers to a particular object.
	// - References are only tracked as long as linked_ptr<> objects are copied.
	// If a linked_ptr<> is converted to a raw pointer and back, BAD THINGS
	// will happen (double deletion).
	//
	// A good use of this class is storing object references in STL containers.
	// You can safely put linked_ptr<> in a vector<>.
	// Other uses may not be as good.
	//
	// Note: If you use an incomplete type with linked_ptr<>, the class
	// containing linked_ptr<> must have a constructor and destructor (even
	// if they do nothing!).
	//
	// Thread Safety:
	// A linked_ptr is NOT thread safe. Copying a linked_ptr object is
	// effectively a read-write operation.
	//
	// Alternative: to linked_ptr is shared_ptr, which
	// - is also two pointers in size (8 bytes for 32 bit addresses)
	// - is thread safe for copying and deletion
	// - supports weak_ptrs

	#ifndef BASE_LINKED_PTR_H_
	#define BASE_LINKED_PTR_H_
	#pragma once

	#include "base/logging.h" // for CHECK macros

	// This is used internally by all instances of linked_ptr<>. It needs to be
	// a non-template class because different types of linked_ptr<> can refer to
	// the same object (linked_ptr<Superclass>(obj) vs linked_ptr<Subclass>(obj)).
	// So, it needs to be possible for different types of linked_ptr to participate
	// in the same circular linked list, so we need a single class type here.
	//
	// DO NOT USE THIS CLASS DIRECTLY YOURSELF. Use linked_ptr<T>.
	class linked_ptr_internal {
	public:
	// Create a new circle that includes only this instance.
	void join_new() {
	next_ = this;
	}

	// Join an existing circle.
	void join(linked_ptr_internal const* ptr) {
	next_ = ptr->next_;
	ptr->next_ = this;
	}

	// Leave whatever circle we're part of. Returns true iff we were the
	// last member of the circle. Once this is done, you can join() another.
	bool depart() {
	if (next_ == this) return true;
	linked_ptr_internal const* p = next_;
	while (p->next_ != this) p = p->next_;
	p->next_ = next_;
	return false;
	}

	private:
	mutable linked_ptr_internal const* next_;
	};

	template <typename T>
	class linked_ptr {
	public:
	typedef T element_type;

	// Take over ownership of a raw pointer. This should happen as soon as
	// possible after the object is created.
	explicit linked_ptr(T* ptr = NULL) { capture(ptr); }
	~linked_ptr() { depart(); }

	// Copy an existing linked_ptr<>, adding ourselves to the list of references.
	template <typename U> linked_ptr(linked_ptr<U> const& ptr) { copy(&ptr); }

	linked_ptr(linked_ptr const& ptr) {
	DCHECK_NE(&ptr, this);
	copy(&ptr);
	}

	// Assignment releases the old value and acquires the new.
	template <typename U> linked_ptr& operator=(linked_ptr<U> const& ptr) {
	depart();
	copy(&ptr);
	return *this;
	}

	linked_ptr& operator=(linked_ptr const& ptr) {
	if (&ptr != this) {
	depart();
	copy(&ptr);
	}
	return *this;
	}

	// Smart pointer members.
	void reset(T* ptr = NULL) {
	depart();
	capture(ptr);
	}
	T* get() const { return value_; }
	T* operator->() const { return value_; }
	T& operator() const { return value_; }
	// Release ownership of the pointed object and returns it.
	// Sole ownership by this linked_ptr object is required.
	T* release() {
	bool last = link_.depart();
	CHECK(last);
	T* v = value_;
	value_ = NULL;
	return v;
	}

	bool operator==(const T* p) const { return value_ == p; }
	bool operator!=(const T* p) const { return value_ != p; }
	template <typename U>
	bool operator==(linked_ptr<U> const& ptr) const {
	return value_ == ptr.get();
	}
	template <typename U>
	bool operator!=(linked_ptr<U> const& ptr) const {
	return value_ != ptr.get();
	}

	private:
	template <typename U>
	friend class linked_ptr;

	T* value_;
	linked_ptr_internal link_;

	void depart() {
	if (link_.depart()) delete value_;
	}

	void capture(T* ptr) {
	value_ = ptr;
	link_.join_new();
	}

	template <typename U> void copy(linked_ptr<U> const* ptr) {
	value_ = ptr->get();
	if (value_)
	link_.join(&ptr->link_);
	else
	link_.join_new();
	}
	};

	template<typename T> inline
	bool operator==(T* ptr, const linked_ptr<T>& x) {
	return ptr == x.get();
	}

	template<typename T> inline
	bool operator!=(T* ptr, const linked_ptr<T>& x) {
	return ptr != x.get();
	}

	// A function to convert T* into linked_ptr<T>
	// Doing e.g. make_linked_ptr(new FooBarBaz<type>(arg)) is a shorter notation
	// for linked_ptr<FooBarBaz<type> >(new FooBarBaz<type>(arg))
	template <typename T>
	linked_ptr<T> make_linked_ptr(T* ptr) {
	return linked_ptr<T>(ptr);
	}

	#endif // BASE_LINKED_PTR_H_