// Copyright (c) 2011 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_MEMORY_LINKED_PTR_H_
#define BASE_MEMORY_LINKED_PTR_H_
#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_MEMORY_LINKED_PTR_H_