Crash in QString toUtf8() - c++

Question

I have a slot:
void Foo::func(QString str1, const QString& str2, int width, int height)
{
std::unique_lock<std::mutex> _lock(m_mutex);
#ifdef _DEBUG
MEMORYSTATUSEX statex;
statex.dwLength = sizeof (statex);
if (GlobalMemoryStatusEx(&statex)) {
qDebug() << QString("There are %1 free MB of physical memory.\n").arg(statex.ullAvailPhys / 1024 / 1024);
}
#endif
BaseClass::someFunc(
str1.toStdString(),
str2.toUtf8().constData(),
width,
height
);
}
It seems to be correct and it works. But if program works for a long time (night for example) it crashes in this function at line
str2.toUtf8().constData()
I first thought that this is a thread-based error, but my lock didn't work. All the local variables and class members are ok. The crash error is:
First-chance exception at 0x76E45B68 foo.exe: Microsoft C++ exception: std::bad_alloc at memory location 0x00EFC6EC
But from the code above last MEMORYSTATUSEX show that available memory is
There are 2314 free MB of physical memory
I also thought that str2 is too big, but it works fine with length of 49152, 168441 etc. What is the problem? Did I miss something?
Stack Trace:
KernelBase.dll!_RaiseException#16() Unknown
[External Code]
Qt5Cored.dll!qBadAlloc() Line 2849 C++
Qt5Cored.dll!QByteArray::QByteArray(int size, Qt::Initialization __formal) Line 1409 C++
Qt5Cored.dll!QUtf8::convertFromUnicode(const QChar * uc, int len) Line 151 C++
Qt5Cored.dll!QString::toUtf8_helper(const QString & str) Line 4373 C++
Qt5Cored.dll!QString::toUtf8() Line 56 C++
foo.exe!Foo::func(QString str1, const QString & str2, int width, int height) Line 3600 C++
foo.exe!Foo::qt_static_metacall(QObject * _o, QMetaObject::Call _c, int _id, void * * _a) Line 1126 C++
Qt5Cored.dll!QMetaObject::activate(QObject * sender, int signalOffset, int local_signal_index, void * * argv) Line 3717 C++
Qt5Cored.dll!QMetaObject::activate(QObject * sender, const QMetaObject * m, int local_signal_index, void * * argv) Line 3582 C++
foo.exe!SomeClass::func(QString _t1, const QString & _t2, int _t3, int _t4) Line 137 C++
foo.exe!SomeClass::qt_static_metacall(QObject * _o, QMetaObject::Call _c, int _id, void * * _a) Line 81 C++
Qt5Cored.dll!QMetaCallEvent::placeMetaCall(QObject * object) Line 485 C++
Qt5Cored.dll!QObject::event(QEvent * e) Line 1246 C++
Qt5Widgetsd.dll!QApplicationPrivate::notify_helper(QObject * receiver, QEvent * e) Line 3720 C++
Qt5Widgetsd.dll!QApplication::notify(QObject * receiver, QEvent * e) Line 3164 C++
Qt5Cored.dll!QCoreApplication::notifyInternal(QObject * receiver, QEvent * event) Line 935 C++
Qt5Cored.dll!QCoreApplication::sendEvent(QObject * receiver, QEvent * event) Line 228 C++
Qt5Cored.dll!QCoreApplicationPrivate::sendPostedEvents(QObject * receiver, int event_type, QThreadData * data) Line 1552 C++
Qt5Cored.dll!QCoreApplication::sendPostedEvents(QObject * receiver, int event_type) Line 1410 C++
qwindowsd.dll!QWindowsGuiEventDispatcher::sendPostedEvents() Line 81 C++
Qt5Cored.dll!qt_internal_proc(HWND__ * hwnd, unsigned int message, unsigned int wp, long lp) Line 414 C++
[External Code]
Qt5Cored.dll!QEventDispatcherWin32::processEvents(QFlags<enum QEventLoop::ProcessEventsFlag> flags) Line 807 C++
qwindowsd.dll!QWindowsGuiEventDispatcher::processEvents(QFlags<enum QEventLoop::ProcessEventsFlag> flags) Line 73 C++
Qt5Cored.dll!QEventLoop::processEvents(QFlags<enum QEventLoop::ProcessEventsFlag> flags) Line 129 C++
Qt5Cored.dll!QEventLoop::exec(QFlags<enum QEventLoop::ProcessEventsFlag> flags) Line 204 C++
Qt5Cored.dll!QCoreApplication::exec() Line 1188 C++
Qt5Guid.dll!QGuiApplication::exec() Line 1508 C++
Qt5Widgetsd.dll!QApplication::exec() Line 2957 C++
foo.exe!WinMain(HINSTANCE__ * __formal, HINSTANCE__ * __formal, char * lpCmdLine, int __formal) Line 348 C++
[External Code]
Thanks in advance!

Answer 1

For those who, like me, just stumbled on this thread because they wanted to convert a QString into a char * with a call to str.toUtf8().constData() or str.toUtf8().data(), but that was leading a program to crash or to throw an exception like EXC_BAD_ACCESS on XCode, here's the answer:
in a call like foo = str.toUtf8().constData();, the str.toUtf8() part creates a temp object.
a call to tmp.data() or tmp.constData() return a pointer to an internal buffer managed by the object itself, and whose lifetime is tied to the temp object's lifetime. source.
once that statement is executed, the lifetime of the object whose address was assigned to foo, once returned by data() or constData(), will end.
therefore, as expected, after that statement foo will hold an address to an object whose lifetime ended, thus causing crashes and exceptions.