Smart pointers are a great thing. When used properly, they can really help make life easier, and simpler. But things can, and do, occasionally go wrong - and that is when the hurt comes in. Qt provides
a number of smart pointer classes, some might say too many, but that's a topic for a whole different discussion, one of which is QSharedPointer.
From the documentation:
"
The QSharedPointer class holds a strong reference to a shared pointer
The QSharedPointer is an automatic, shared pointer in C++. It behaves
exactly like a normal pointer for normal purposes, including respect
for constness.
QSharedPointer will delete the pointer it is holding when it goes out
of scope, provided no other QSharedPointer objects are referencing it.
A QSharedPointer object can be created from a normal pointer, another
QSharedPointer object or by promoting a QWeakPointer object to a strong
reference.
Essentially, QSharedPointer works through
reference counting, which means if you somehow make a mistake with cleaning up your references, the object your shared pointer refers to won't be deleted, and you've got a hard to track memory leak on your hands. This is precisely what happened to me recently
at work, amongst a jungle of a few different libraries, so tracing the problem by hand was really not going to happen, so I needed a miracle, or short of that, a reliable way to track reference count changes on a QSharedPointer instance.
Reading up on QSharedPointer's internals, it became obvious that the reference counting was stored in
the dpointer of each QSharedPointer instance. The dpointer is shared amongst QSharedPointer instances referring to the same pointer. So, we should be able to set a watch in gdb to break whenever the refcount changes.
First, we need to find out the address of a QSharedPointer instance, so set a breakpoint just after we first create it:
(gdb) break main.cpp:22
Breakpoint 1 at 0x8048806: file main.cpp, line 22.
(gdb) r
Starting program: /home/burchr/qsharedpointer/qsharedpointer.
[Thread debugging using libthread_db enabled]
Breakpoint 1, main (argc=1, argv=0xbffff444) at main.cpp:22
22>- QSharedPointer<MyClass> copy(initial);
(gdb) p initial
$1 = {<QtSharedPointer::ExternalRefCount<MyClass>> = {<QtSharedPointer::Basic<MyClass>> = { value = 0x804c438}, d = 0x804c448}, <No data fields>}
Now we have the address, we can set a watch on the QBasicAtomicInt in the dpointer, we can watch the refcount for changes:
(gdb) watch $1.d->weakref
Hardware watchpoint 2: $1.d->weakref
Continue debugging, and gdb will break whenever the refcount changes, telling us the old and new values, like so:
(gdb) c
Continuing.
Hardware watchpoint 2: $1.d->weakref
Old value = {_q_value = 1}
New value = {_q_value = 2}
0x08048953 in QBasicAtomicInt::ref (this=0x804c44c)
at /usr/include/QtCore/qatomic_i386.h:120
120> : "memory");
Much thanks to:
Labels: coding, debugging, kde, maemo, meego, qt, work
