michael@0: // Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
michael@0: // Use of this source code is governed by a BSD-style license that can be
michael@0: // found in the LICENSE file.
michael@0: //
michael@0: // A "smart" pointer type with reference tracking.  Every pointer to a
michael@0: // particular object is kept on a circular linked list.  When the last pointer
michael@0: // to an object is destroyed or reassigned, the object is deleted.
michael@0: //
michael@0: // Used properly, this deletes the object when the last reference goes away.
michael@0: // There are several caveats:
michael@0: // - Like all reference counting schemes, cycles lead to leaks.
michael@0: // - Each smart pointer is actually two pointers (8 bytes instead of 4).
michael@0: // - Every time a pointer is released, the entire list of pointers to that
michael@0: //   object is traversed.  This class is therefore NOT SUITABLE when there
michael@0: //   will often be more than two or three pointers to a particular object.
michael@0: // - References are only tracked as long as linked_ptr<> objects are copied.
michael@0: //   If a linked_ptr<> is converted to a raw pointer and back, BAD THINGS
michael@0: //   will happen (double deletion).
michael@0: //
michael@0: // A good use of this class is storing object references in STL containers.
michael@0: // You can safely put linked_ptr<> in a vector<>.
michael@0: // Other uses may not be as good.
michael@0: //
michael@0: // Note: If you use an incomplete type with linked_ptr<>, the class
michael@0: // *containing* linked_ptr<> must have a constructor and destructor (even
michael@0: // if they do nothing!).
michael@0: //
michael@0: // Thread Safety:
michael@0: //   A linked_ptr is NOT thread safe. Copying a linked_ptr object is
michael@0: //   effectively a read-write operation.
michael@0: //
michael@0: // Alternative: to linked_ptr is shared_ptr, which
michael@0: //  - is also two pointers in size (8 bytes for 32 bit addresses)
michael@0: //  - is thread safe for copying and deletion
michael@0: //  - supports weak_ptrs
michael@0: 
michael@0: #ifndef BASE_LINKED_PTR_H_
michael@0: #define BASE_LINKED_PTR_H_
michael@0: 
michael@0: #include "base/logging.h"  // for CHECK macros
michael@0: 
michael@0: // This is used internally by all instances of linked_ptr<>.  It needs to be
michael@0: // a non-template class because different types of linked_ptr<> can refer to
michael@0: // the same object (linked_ptr<Superclass>(obj) vs linked_ptr<Subclass>(obj)).
michael@0: // So, it needs to be possible for different types of linked_ptr to participate
michael@0: // in the same circular linked list, so we need a single class type here.
michael@0: //
michael@0: // DO NOT USE THIS CLASS DIRECTLY YOURSELF.  Use linked_ptr<T>.
michael@0: class linked_ptr_internal {
michael@0:  public:
michael@0:   // Create a new circle that includes only this instance.
michael@0:   void join_new() {
michael@0:     next_ = this;
michael@0:   }
michael@0: 
michael@0:   // Join an existing circle.
michael@0:   void join(linked_ptr_internal const* ptr) {
michael@0:     next_ = ptr->next_;
michael@0:     ptr->next_ = this;
michael@0:   }
michael@0: 
michael@0:   // Leave whatever circle we're part of.  Returns true iff we were the
michael@0:   // last member of the circle.  Once this is done, you can join() another.
michael@0:   bool depart() {
michael@0:     if (next_ == this) return true;
michael@0:     linked_ptr_internal const* p = next_;
michael@0:     while (p->next_ != this) p = p->next_;
michael@0:     p->next_ = next_;
michael@0:     return false;
michael@0:   }
michael@0: 
michael@0:  private:
michael@0:   mutable linked_ptr_internal const* next_;
michael@0: };
michael@0: 
michael@0: template <typename T>
michael@0: class linked_ptr {
michael@0:  public:
michael@0:   typedef T element_type;
michael@0: 
michael@0:   // Take over ownership of a raw pointer.  This should happen as soon as
michael@0:   // possible after the object is created.
michael@0:   explicit linked_ptr(T* ptr = NULL) { capture(ptr); }
michael@0:   ~linked_ptr() { depart(); }
michael@0: 
michael@0:   // Copy an existing linked_ptr<>, adding ourselves to the list of references.
michael@0:   template <typename U> linked_ptr(linked_ptr<U> const& ptr) { copy(&ptr); }
michael@0:   linked_ptr(linked_ptr const& ptr) { DCHECK_NE(&ptr, this); copy(&ptr); }
michael@0: 
michael@0:   // Assignment releases the old value and acquires the new.
michael@0:   template <typename U> linked_ptr& operator=(linked_ptr<U> const& ptr) {
michael@0:     depart();
michael@0:     copy(&ptr);
michael@0:     return *this;
michael@0:   }
michael@0: 
michael@0:   linked_ptr& operator=(linked_ptr const& ptr) {
michael@0:     if (&ptr != this) {
michael@0:       depart();
michael@0:       copy(&ptr);
michael@0:     }
michael@0:     return *this;
michael@0:   }
michael@0: 
michael@0:   // Smart pointer members.
michael@0:   void reset(T* ptr = NULL) { depart(); capture(ptr); }
michael@0:   T* get() const { return value_; }
michael@0:   T* operator->() const { return value_; }
michael@0:   T& operator*() const { return *value_; }
michael@0:   // Release ownership of the pointed object and returns it.
michael@0:   // Sole ownership by this linked_ptr object is required.
michael@0:   T* release() {
michael@0:     bool last = link_.depart();
michael@0:     CHECK(last);
michael@0:     T* v = value_;
michael@0:     value_ = NULL;
michael@0:     return v;
michael@0:   }
michael@0: 
michael@0:   bool operator==(const T* p) const { return value_ == p; }
michael@0:   bool operator!=(const T* p) const { return value_ != p; }
michael@0:   template <typename U>
michael@0:   bool operator==(linked_ptr<U> const& ptr) const {
michael@0:     return value_ == ptr.get();
michael@0:   }
michael@0:   template <typename U>
michael@0:   bool operator!=(linked_ptr<U> const& ptr) const {
michael@0:     return value_ != ptr.get();
michael@0:   }
michael@0: 
michael@0:  private:
michael@0:   template <typename U>
michael@0:   friend class linked_ptr;
michael@0: 
michael@0:   T* value_;
michael@0:   linked_ptr_internal link_;
michael@0: 
michael@0:   void depart() {
michael@0:     if (link_.depart()) delete value_;
michael@0:   }
michael@0: 
michael@0:   void capture(T* ptr) {
michael@0:     value_ = ptr;
michael@0:     link_.join_new();
michael@0:   }
michael@0: 
michael@0:   template <typename U> void copy(linked_ptr<U> const* ptr) {
michael@0:     value_ = ptr->get();
michael@0:     if (value_)
michael@0:       link_.join(&ptr->link_);
michael@0:     else
michael@0:       link_.join_new();
michael@0:   }
michael@0: };
michael@0: 
michael@0: template<typename T> inline
michael@0: bool operator==(T* ptr, const linked_ptr<T>& x) {
michael@0:   return ptr == x.get();
michael@0: }
michael@0: 
michael@0: template<typename T> inline
michael@0: bool operator!=(T* ptr, const linked_ptr<T>& x) {
michael@0:   return ptr != x.get();
michael@0: }
michael@0: 
michael@0: // A function to convert T* into linked_ptr<T>
michael@0: // Doing e.g. make_linked_ptr(new FooBarBaz<type>(arg)) is a shorter notation
michael@0: // for linked_ptr<FooBarBaz<type> >(new FooBarBaz<type>(arg))
michael@0: template <typename T>
michael@0: linked_ptr<T> make_linked_ptr(T* ptr) {
michael@0:   return linked_ptr<T>(ptr);
michael@0: }
michael@0: 
michael@0: #endif  // BASE_LINKED_PTR_H_