1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
|
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
// 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_
#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_
|
