Pimpl + QSharedPointer语言 - Destructor = Disaster

昨天我遇到了痛苦，这让我沮丧了24小时。问题归结为在随机基础上发生的意外崩溃。更复杂的是，调试报告也有绝对的随机模式。更复杂的是，所有的调试跟踪都指向随机的 Qt源代码或本机dll，也就是说，每次都证明问题不是在我这边。

这里有一些这样可爱的报告的例子:

Program received signal SIGSEGV, Segmentation fault.
0x0000000077864324 in ntdll!RtlAppendStringToString () from C:Windowssystem32ntdll.dll
(gdb) bt
#0 0x0000000077864324 in ntdll!RtlAppendStringToString () from C:Windowssystem32ntdll.dll
#1 0x000000002efc0230 in ?? ()
#2 0x0000000002070005 in ?? ()
#3 0x000000002efc0000 in ?? ()
#4 0x000000007787969f in ntdll!RtlIsValidHandle () from C:Windowssystem32ntdll.dll
#5 0x0000000000000000 in ?? ()

warning: HEAP: Free Heap block 307e5950 modified at 307e59c0 after it was freed
Program received signal SIGTRAP, Trace/breakpoint trap.
0x00000000778bf0b2 in ntdll!ExpInterlockedPopEntrySListFault16 () from C:Windowssystem32ntdll.dll
(gdb) bt
#0 0x00000000778bf0b2 in ntdll!ExpInterlockedPopEntrySListFault16 () from C:Windowssystem32ntdll.dll
#1 0x000000007786fd34 in ntdll!RtlIsValidHandle () from C:Windowssystem32ntdll.dll
#2 0x0000000077910d20 in ntdll!RtlGetLastNtStatus () from C:Windowssystem32ntdll.dll
#3 0x00000000307e5950 in ?? ()
#4 0x00000000307e59c0 in ?? ()
#5 0x00000000ffffffff in ?? ()
#6 0x0000000000220f10 in ?? ()
#7 0x0000000077712d60 in WaitForMultipleObjectsEx () from C:Windowssystem32kernel32.dll
#8 0x0000000000000000 in ?? ()

Program received signal SIGSEGV, Segmentation fault.
0x0000000000a9678a in QBasicAtomicInt::ref (this=0x8) at ../../include/QtCore/../../../qt-src/src/corelib/arch/qatomic_x86_64.h:121
121 : "memory");
(gdb) bt
#0 0x0000000000a9678a in QBasicAtomicInt::ref (this=0x8) at ../../include/QtCore/../../../qt-src/src/corelib/arch/qatomic_x86_64.h:121
#1 0x00000000009df08e in QVariant::QVariant (this=0x21e4d0, p=...) at d:/Distributions/qt-src/src/corelib/kernel/qvariant.cpp:1426
#2 0x0000000000b4dde9 in QList<QVariant>::value (this=0x323bd480, i=1) at ../../include/QtCore/../../../qt-src/src/corelib/tools/qlist.h:666
#3 0x00000000009ccff7 in QObject::property (this=0x3067e900,
name=0xa9d042a <QCDEStyle::drawPrimitive(QStyle::PrimitiveElement, QStyleOption const*, QPainter*, QWidget const*) const::pts5+650> "_q_stylerect")
at d:/Distributions/qt-src/src/corelib/kernel/qobject.cpp:3742
#4 0x0000000000000000 in ?? ()

正如你所看到的，这个东西是非常讨厌的，它没有提供任何有用的信息。但是，有一件事我没有注意到。这是编译过程中一个奇怪的警告，也很难用眼睛捕捉到:

In file included from d:/Libraries/x64/MinGW-w64/4.7.2/Qt/4.8.4/include/QtCore/qsharedpointer.h:50:0,
                 from d:/Libraries/x64/MinGW-w64/4.7.2/Qt/4.8.4/include/QtCore/QSharedPointer:1,
                 from ../../../../source/libraries/Project/sources/Method.hpp:4,
                 from ../../../../source/libraries/Project/sources/Slot.hpp:4,
                 from ../../../../source/libraries/Project/sources/Slot.cpp:1:
d:/Libraries/x64/MinGW-w64/4.7.2/Qt/4.8.4/include/QtCore/qsharedpointer_impl.h: In instantiation of 'static void QtSharedPointer::ExternalRefCount<T>::deref(QtSharedPointer::ExternalRefCount<T>::Data*, T*) [with T = Project::Method::Private; QtSharedPointer::ExternalRefCount<T>::Data = QtSharedPointer::ExternalRefCountData]':
d:/Libraries/x64/MinGW-w64/4.7.2/Qt/4.8.4/include/QtCore/qsharedpointer_impl.h:336:11:   required from 'void QtSharedPointer::ExternalRefCount<T>::deref() [with T = Project::Method::Private]'
d:/Libraries/x64/MinGW-w64/4.7.2/Qt/4.8.4/include/QtCore/qsharedpointer_impl.h:401:38:   required from 'QtSharedPointer::ExternalRefCount<T>::~ExternalRefCount() [with T = Project::Method::Private]'
d:/Libraries/x64/MinGW-w64/4.7.2/Qt/4.8.4/include/QtCore/qsharedpointer_impl.h:466:7:   required from here
d:/Libraries/x64/MinGW-w64/4.7.2/Qt/4.8.4/include/QtCore/qsharedpointer_impl.h:342:21: warning: possible problem detected in invocation of delete operator: [enabled by default]
d:/Libraries/x64/MinGW-w64/4.7.2/Qt/4.8.4/include/QtCore/qsharedpointer_impl.h:337:28: warning: 'value' has incomplete type [enabled by default]

实际上，我把这个警告只是作为最后的手段，因为在如此绝望地寻找错误时，代码已经感染了日志死亡。

之后，我回忆起，例如，如果对Pimpl使用std::unique_ptr或std::scoped_ptr，则必须提供析构函数，否则代码甚至无法编译。然而，我也记得std::shared_ptr并不关心析构函数，没有它也可以工作得很好。这是我没有注意到这个奇怪警告的另一个原因。长话短说，当我添加析构函数时，这种随机崩溃就停止了。看起来Qt的QSharedPointer与std::shared_ptr相比有一些设计缺陷。我想如果Qt开发人员将这个警告转换为错误会更好，因为像这样的调试马拉松根本不值得一个人的时间，精力和神经。

我的问题是:

1. QSharedPointer怎么了?为什么析构函数如此重要?
为什么没有析构函数就会崩溃?这些对象(使用Pimpl + QSharedPointer)是在堆栈上创建的，在它们死后没有其他对象可以访问它们。然而，崩溃发生在他们死亡后的一段随机时期。以前有人遇到过这样的问题吗?请分享你的经验。
2. 还有其他陷阱吗就像Qt一样——那些我必须知道的东西肯定会留下来未来安全吗?

希望这些问题和我的帖子能帮助其他人避免我在过去24小时里所经历的地狱。