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
|
** Understanding transitional VCL lifecycle **
---------- How it used to look ----------
All VCL classes were explicitly lifecycle managed; so you would
do:
Dialog aDialog(...); // old - on stack allocation
aDialog.Execute(...);
or:
Dialog *pDialog = new Dialog(...); // old - manual heap allocation
pDialog->Execute(...);
delete pDialog;
or:
boost::shared_ptr<Dialog> xDialog(new pDialog()); // old
xDialog->Execute(...);
// depending who shared the ptr this would be freed sometime
In several cases this lead to rather unpleasant code, when
various shared_ptr wrappers were used, the lifecycle was far less than
obvious. Where controls were wrapped by other ref-counted classes -
such as UNO interfaces, which were also used by native Window
pointers, the lifecycle became extremely opaque. In addition VCL had
significant issues with re-enterancy and event emission - adding
various means such as DogTags to try to detect destruction of a window
between calls:
ImplDelData aDogTag( this ); // 'orrible old code
Show( true, SHOW_NOACTIVATE );
if( !aDogTag.IsDead() ) // did 'this' go invalid yet ?
Update();
Unfortunately use of such protection is/was ad-hoc, and far
from uniform, despite the prevelance of such potential problems.
When a lifecycle problem was hit, typically it would take the
form of accessing memory that had been freed, and contained garbage due
to lingering pointers to freed objects.
---------- Where we are now: ----------
To fix this situation we now have a VclPtr - which is a smart
reference-counting pointer (include/vcl/vclptr.hxx) which is
designed to look and behave -very- much like a normal pointer
to reduce code-thrash. VclPtr is used to wrap all OutputDevice
derived classes thus:
VclPtr<Dialog> pDialog( new Dialog( ... ), SAL_NO_ACQUIRE );
...
pDialog.disposeAndClear();
However - while the VclPtr reference count controls the
lifecycle of the Dialog object, it is necessary to be able to
break reference count cycles. These are extremely common in
widget hierarchies as each widget holds (smart) pointers to
its parents and also its children.
Thus - all previous 'delete' calls are replaced with 'dispose'
method calls:
** What is dispose ?
Dispose is defined to be a method that releases all references
that an object holds - thus allowing their underlying
resources to be released. However - in this specific case it
also releases all backing graphical resources. In practical
terms, all destructor functionality has been moved into
'dispose' methods, in order to provide a minimal initial
behavioral change.
As such a VclPtr can have three states:
VclPtr<PushButton> pButton;
...
assert (pButton == nullptr || !pButton); // null
assert (pButton && !pButton->IsDisposed()); // alive
assert (pButton && pButton->IsDisposed()); // disposed
** ScopedVclPtr - making disposes easier
While replacing existing code with new, it can be a bit
tiresome to have to manually add 'disposeAndClear()'
calls to VclPtr<> instances.
Luckily it is easy to avoid that with a ScopedVclPtr which
does this for you when it goes out of scope.
** One extra gotcha - an initial reference-count of 1
In the normal world of love and sanity, eg. creating UNO
objects, the objects start with a ref-count of zero. Thus
the first reference is always taken after construction by
the surrounding smart pointer.
Unfortunately, the existing VCL code is somewhat tortured,
and does a lot of reference and de-reference action on the
class -during- construction. This forces us to construct with
a reference of 1 - and to hand that into the initial smart
pointer with a SAL_NO_ACQUIRE.
To make this easier, we have 'Instance' template wrappers
that make this apparently easier, by constructing the
pointer for you.
** How does my familiar code change ?
Lets tweak the exemplary code above to fit the new model:
- Dialog aDialog(... dialog params ... );
- aDialog.Execute(...);
+ ScopedVclPtrInstance<Dialog> pDialog(... dialog params ... );
+ pDialog->Execute(...); // VclPtr behaves much like a pointer
or:
- Dialog *pDialog = new Dialog(... dialog params ...);
+ VclPtrInstance<Dialog> pDialog(... dialog params ...);
pDialog->Execute(...);
- delete pDialog;
+ pDialog.disposeAndClear(); // done manually - replaces a delete
or:
- boost::shared_ptr<Dialog> xDialog(new Dialog(...));
+ ScopedVclPtrInstance<Dialog> xDialog(...);
xDialog->Execute(...);
+ // depending how shared_ptr was shared perhaps
+ // someone else gets a VclPtr to xDialog
or:
- VirtualDevice aDev;
+ ScopedVclPtrInstance<VirtualDevice> pDev;
** Why are these 'disposeOnce' calls in destructors ?
This is an interim measure while we are migrating, such that
it is possible to delete an object conventionally and ensure
that its dispose method gets called. In the 'end' we would
instead assert that a Window has been disposed in it's
destructor, and elide these calls.
As the object's vtable is altered as we go down the
destruction process, and we want to call the correct dispose
methods we need this disposeOnce(); call for the interim in
every destructor. This is enforced by a clang plugin.
The plus side of disposeOnce is that the mechanics behind it
ensure that a dispose() method is only called a single time,
simplifying their implementation.
---------- Who owns & disposes what ? ----------
** referencing / ownership inheritance / hierarchy.
** VclBuilder
+ and it's magic dispose method.
---------- What remains to be done ? ----------
* Cleanup DogTags and LazyDelete.
* Expand the VclPtr pattern to many other less
than safe VCL types.
* create factory functions for VclPtr<> types and privatize
their constructors.
* Pass 'const VclPtr<> &' instead of pointers everywhere
+ add 'explicit' keywords to VclPtr constructors to
accelerate compilation etc.
* Cleanup common existing methods such that they continue to
work post-dispose.
* Dispose functions should be audited to:
+ not leave dangling pointsr
+ shrink them - some work should incrementally
migrate back to destructors.
---------- FAQ / debugging hints ----------
** Compile with dbgutil
This is by far the best way to turn on debugging and
assertions that help you find problems. In particular
there are a few that are really helpful:
vcl/source/window/window.cxx (Window::dispose)
"Window ( N4sfx27sidebar20SidebarDockingWindowE (Properties))
^^^ class name window title ^^^
with live children destroyed: N4sfx27sidebar6TabBarE ()
N4sfx27sidebar4DeckE () 10FixedImage ()"
You can de-mangle these names if you can't read them thus:
$ c++filt -t N4sfx27sidebar20SidebarDockingWindowE
sfx2::sidebar::SidebarDockingWindow
In the above case - it is clear that the children have not been
disposed before their parents. As an aside, having a dispose chain
separate from destructors allows us to emit real type names for
parents here.
To fix this, we will need to get the dispose ordering right,
occasionally in the conversion we re-ordered destruction, or
omitted a disposeAndClear() in a ::dispose() method.
=> If you see this, check the order of disposeAndClear() in
the sfx2::Sidebar::SidebarDockingWindow::dispose() method
=> also worth git grepping for 'new sfx::sidebar::TabBar' to
see where those children were added.
|
