forked from boostorg/smart_ptr
-
Notifications
You must be signed in to change notification settings - Fork 0
/
Copy pathweak_ptr.htm
240 lines (223 loc) · 12 KB
/
weak_ptr.htm
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
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>weak_ptr</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
</head>
<body text="#000000" bgcolor="#ffffff" link="#0000ff" vlink="#0000ff">
<h1><img height="86" alt="boost.png (6897 bytes)" src="../../boost.png"
width="277" align="middle" border="0">weak_ptr class template</h1>
<p><A href="#Introduction">Introduction</A><br>
<A href="#Synopsis">Synopsis</A><br>
<A href="#Members">Members</A><br>
<A href="#functions">Free Functions</A><br>
<A href="#FAQ">Frequently Asked Questions</A>
</p>
<h2><a name="Introduction">Introduction</a></h2>
<p>The <b>weak_ptr</b> class template stores a "weak reference" to an object that's
already managed by a <b>shared_ptr</b>. To access the object, a <STRONG>weak_ptr</STRONG>
can be converted to a <STRONG>shared_ptr</STRONG> using <A href="shared_ptr.htm#constructors">
the <STRONG>shared_ptr</STRONG> constructor</A> or the member function <STRONG><A href="#lock">
lock</A></STRONG>. When the last <b>shared_ptr</b> to the object goes
away and the object is deleted, the attempt to obtain a <STRONG>shared_ptr</STRONG>
from the <b>weak_ptr</b> instances that refer to the deleted object will fail:
the constructor will throw an exception of type <STRONG>boost::bad_weak_ptr</STRONG>,
and <STRONG>weak_ptr::lock</STRONG> will return an <EM>empty</EM> <STRONG>shared_ptr</STRONG>.</p>
<p>Every <b>weak_ptr</b> meets the <b>CopyConstructible</b> and <b>Assignable</b> requirements
of the C++ Standard Library, and so can be used in standard library containers.
Comparison operators are supplied so that <b>weak_ptr</b> works with the
standard library's associative containers.</p>
<P><STRONG>weak_ptr</STRONG> operations never throw exceptions.</P>
<p>The class template is parameterized on <b>T</b>, the type of the object pointed
to.</p>
<P>Compared to <STRONG>shared_ptr</STRONG>, <STRONG>weak_ptr</STRONG> provides a
very limited subset of operations since accessing its stored pointer is often
dangerous in multithreaded programs, and sometimes unsafe even within a single
thread (that is, it may invoke undefined behavior.) Pretend for a moment that <b>weak_ptr</b>
has a <b>get</b> member function that returns a raw pointer, and consider this
innocent piece of code:</P>
<pre>shared_ptr<int> p(new int(5));
weak_ptr<int> q(p);
// some time later
if(int * r = q.get())
{
// use *r
}
</pre>
<P>Imagine that after the <STRONG>if</STRONG>, but immediately before <STRONG>r</STRONG>
is used, another thread executes the statement <code>p.reset()</code>. Now <STRONG>r</STRONG>
is a dangling pointer.</P>
<P>The solution to this problem is to create a temporary <STRONG>shared_ptr</STRONG>
from <STRONG>q</STRONG>:</P>
<pre>shared_ptr<int> p(new int(5));
weak_ptr<int> q(p);
// some time later
if(shared_ptr<int> r = q.<A href="#lock" >lock</A>())
{
// use *r
}
</pre>
<p>Now <STRONG>r</STRONG> holds a reference to the object that was pointed by <STRONG>q</STRONG>.
Even if <code>p.reset()</code> is executed in another thread, the object will
stay alive until <STRONG>r</STRONG> goes out of scope or is reset. By obtaining
a <STRONG>shared_ptr</STRONG> to the object, we have effectively locked it
against destruction.</p>
<h2><a name="Synopsis">Synopsis</a></h2>
<pre>namespace boost {
template<class T> class weak_ptr {
public:
typedef T <A href="#element_type" >element_type</A>;
<A href="#default-constructor" >weak_ptr</A>();
template<class Y> <A href="#constructors" >weak_ptr</A>(shared_ptr<Y> const & r);
<A href="#constructors" >weak_ptr</A>(weak_ptr const & r);
template<class Y> <A href="#constructors" >weak_ptr</A>(weak_ptr<Y> const & r);
<A href="#destructor" >~weak_ptr</A>();
weak_ptr & <A href="#assignment" >operator=</A>(weak_ptr const & r);
template<class Y> weak_ptr & <A href="#assignment" >operator=</A>(weak_ptr<Y> const & r);
template<class Y> weak_ptr & <A href="#assignment" >operator=</A>(shared_ptr<Y> const & r);
long <A href="#use_count" >use_count</A>() const;
bool <A href="#expired" >expired</A>() const;
shared_ptr<T> <A href="#lock" >lock</A>() const;
void <A href="#reset" >reset</A>();
void <A href="#swap" >swap</A>(weak_ptr<T> & b);
};
template<class T, class U>
bool <A href="#comparison" >operator<</A>(weak_ptr<T> const & a, weak_ptr<U> const & b);
template<class T>
void <A href="#free-swap" >swap</A>(weak_ptr<T> & a, weak_ptr<T> & b);
}
</pre>
<h2><a name="Members">Members</a></h2>
<h3><a name="element_type">element_type</a></h3>
<pre>typedef T element_type;</pre>
<blockquote>
<p>Provides the type of the template parameter T.</p>
</blockquote>
<h3><a name="constructors">constructors</a></h3>
<pre><a name="default-constructor">weak_ptr();</a></pre>
<blockquote>
<p><b>Effects:</b> Constructs an <EM>empty</EM> <b>weak_ptr</b>.</p>
<p><b>Postconditions:</b> <code>use_count() == 0</code>.</p>
<p><b>Throws:</b> nothing.</p>
</blockquote>
<pre>template<class Y> weak_ptr(shared_ptr<Y> const & r);
weak_ptr(weak_ptr const & r);
template<class Y> weak_ptr(weak_ptr<Y> const & r);</pre>
<blockquote>
<p><b>Effects:</b> If <STRONG>r</STRONG> is <EM>empty</EM>, constructs an <EM>empty</EM>
<STRONG>weak_ptr</STRONG>; otherwise, constructs a <b>weak_ptr</b> that <EM>shares
ownership</EM> with <STRONG>r</STRONG> as if by storing a copy of the
pointer stored in <b>r</b>.</p>
<p><b>Postconditions:</b> <code>use_count() == r.use_count()</code>.</p>
<p><b>Throws:</b> nothing.</p>
</blockquote>
<h3><a name="destructor">destructor</a></h3>
<pre>~weak_ptr();</pre>
<BLOCKQUOTE>
<P><B>Effects:</B> Destroys this <b>weak_ptr</b> but has no effect on the object
its stored pointer points to.</P>
<P><B>Throws:</B> nothing.</P>
</BLOCKQUOTE>
<h3><a name="assignment">assignment</a></h3>
<pre>weak_ptr & operator=(weak_ptr const & r);
template<class Y> weak_ptr & operator=(weak_ptr<Y> const & r);
template<class Y> weak_ptr & operator=(shared_ptr<Y> const & r);</pre>
<BLOCKQUOTE>
<P><B>Effects:</B> Equivalent to <code>weak_ptr(r).swap(*this)</code>.</P>
<P><B>Throws:</B> nothing.</P>
<P><B>Notes:</B> The implementation is free to meet the effects (and the implied
guarantees) via different means, without creating a temporary.</P>
</BLOCKQUOTE>
<h3><a name="use_count">use_count</a></h3>
<pre>long use_count() const;</pre>
<blockquote>
<p><b>Returns:</b> 0 if <STRONG>*this</STRONG> is <EM>empty</EM>; otherwise, the
number of <b>shared_ptr</b> objects that <EM>share ownership</EM> with <STRONG>*this</STRONG>.</p>
<p><b>Throws:</b> nothing.</p>
<P><B>Notes:</B> <code>use_count()</code> is not necessarily efficient. Use only
for debugging and testing purposes, not for production code.</P>
</blockquote>
<h3><a name="expired">expired</a></h3>
<pre>bool expired() const;</pre>
<blockquote>
<p><b>Returns:</b> <code>use_count() == 0</code>.</p>
<p><b>Throws:</b> nothing.</p>
<P><B>Notes:</B> <code>expired()</code> may be faster than <code>use_count()</code>.</P>
</blockquote>
<h3><a name="lock">lock</a></h3>
<pre>shared_ptr<T> lock() const;</pre>
<BLOCKQUOTE>
<P><B>Returns:</B> <code>expired()? shared_ptr<T>(): shared_ptr<T>(*this)</code>.</P>
<P><B>Throws:</B> nothing.</P>
</BLOCKQUOTE>
<h3><a name="reset">reset</a></h3>
<pre>void reset();</pre>
<BLOCKQUOTE>
<P><B>Effects:</B> Equivalent to <code>weak_ptr().swap(*this)</code>.</P>
</BLOCKQUOTE>
<h3><a name="swap">swap</a></h3>
<pre>void swap(weak_ptr & b);</pre>
<blockquote>
<p><b>Effects:</b> Exchanges the contents of the two smart pointers.</p>
<p><b>Throws:</b> nothing.</p>
</blockquote>
<h2><a name="functions">Free Functions</a></h2>
<h3><a name="comparison">comparison</a></h3>
<pre>template<class T, class U>
bool operator<(weak_ptr<T> const & a, weak_ptr<U> const & b);</pre>
<blockquote>
<p><b>Returns:</b> an unspecified value such that</p>
<UL>
<LI>
<b>operator<</b> is a strict weak ordering as described in section 25.3 <code>[lib.alg.sorting]</code>
of the C++ standard;
<LI>
under the equivalence relation defined by <STRONG>operator<</STRONG>, <code>!(a
< b) && !(b < a)</code>, two <STRONG>weak_ptr</STRONG> instances
are equivalent if and only if they <EM>share ownership</EM> or are both <EM>empty</EM>.</LI></UL>
<p><b>Throws:</b> nothing.</p>
<P><B>Notes:</B> Allows <STRONG>weak_ptr</STRONG> objects to be used as keys in
associative containers.</P>
</blockquote>
<h3><a name="free-swap">swap</a></h3>
<pre>template<class T>
void swap(weak_ptr<T> & a, weak_ptr<T> & b)</pre>
<BLOCKQUOTE>
<P><B>Effects:</B> Equivalent to <code>a.swap(b)</code>.</P>
<P><B>Throws:</B> nothing.</P>
<P><B>Notes:</B> Matches the interface of <B>std::swap</B>. Provided as an aid to
generic programming.</P>
</BLOCKQUOTE>
<h2><a name="FAQ">Frequently Asked Questions</a></h2>
<P><B>Q.</B> Can an object create a <STRONG>weak_ptr</STRONG> to itself in its
constructor?</P>
<P><b>A.</b> No. A <STRONG>weak_ptr</STRONG> can only be created from a <STRONG>shared_ptr</STRONG>,
and at object construction time no <STRONG>shared_ptr</STRONG> to the object
exists yet. Even if you could create a temporary <STRONG>shared_ptr</STRONG> to <STRONG>
this</STRONG>, it would go out of scope at the end of the constructor, and
all <STRONG>weak_ptr</STRONG> instances would instantly expire.</P>
<P>The solution is to make the constructor private, and supply a factory function
that returns a <STRONG>shared_ptr</STRONG>:<BR>
</P>
<pre>
class X
{
private:
X();
public:
static shared_ptr<X> create()
{
shared_ptr<X> px(new X);
// create weak pointers from px here
return px;
}
};
</pre>
<hr>
<p>$Date$</p>
<p><small>Copyright 1999 Greg Colvin and Beman Dawes. Copyright 2002 Darin Adler.
Copyright 2002-2005 Peter Dimov. Distributed under the Boost Software License, Version
1.0. See accompanying file <A href="../../LICENSE_1_0.txt">LICENSE_1_0.txt</A> or
copy at <A href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</A>.</small></p>
</body>
</html>