buildToolsVersion '28.0.3'

buildToolsVersion androidBuildToolsVersion

engine.rootContext()->setContextProperty("applicationDirPath", QGuiApplication::applicationDirPath());

static void foo()
{
...
}

int main(int argc, char *argv[])
{
	QGuiApplication app(argc, argv);
	foo();  // 1
	const int result = QCoreApplication::exec();
	foo();  // 2
	return result;

}

#include <QSet>
#include <iostream>

using namespace std;

typedef int CData;
using CDataRef = reference_wrapper<CData>;

QSet<CData*>* mSelection = new QSet<CData*>;

QSet<CDataRef>*& SelectionAsRefs( void )
{
	static_assert(sizeof(CData*) == sizeof(CDataRef));
	return *(QSet<CDataRef> **) &mSelection;
}

static QSet<CData*>* makeUB(QSet<CData*>** src, QSet<CDataRef>** alias)
{
	static QSet<CData*> data = { new CData };  // set with one element

	*src = &data;
	*alias = nullptr;

	return *src;  // what will be there: &data or nullptr?
}

int main(int, char *[])
{
	mSelection = makeUB(&mSelection, &SelectionAsRefs());
	std::cout << (!mSelection ? "OK" : "UB") << std::endl;
}

int i;
void* p;  // declaration
p = &i;  // p begins its lifetime with dynamic type int*

// Packed data
struct A {
  int a; // 2 bits
  int b; // 15
};

// State less business-logic class
class UnpackedA
{
public:
	int a() const { return *reinterpret_cast<const int*>(buf()) & 0x3; }
	int b() const { return *reinterpret_cast<const int*>(buf() + sizeof(int)) & 0x7FFF; }
	
private:
// Returns buffer of packed data
	const char* buf() const { return reinterpret_cast<const char*>(this); }
};

int foo() {
	A a;  // e.g. got from network
	auto ua = new (&a) UnpackedA;
}

void* p;          // declaration
p = new int(5);   // p begins its lifetime with dynamic type int*

QSet<std::reference_wrapper<CData>>& mSelectionAsRefs() 
{
	static_assert(sizeof(CData*) == sizeof(std::reference_wrapper<CData>));
	return std::bit_cast<QSet<std::reference_wrapper<CData>>&>(mSelection);
}

QSet<std::reference_wrapper<CData>>& mSelectionAsRefs() 
{
	static_assert(sizeof(CData*) == sizeof(std::reference_wrapper<CData>));
	return reinterpret_cast<QSet<std::reference_wrapper<CData>>&>(mSelection);
}

QSet<std::reference_wrapper<CData>>& mSelectionAsRefs()
{
	static_assert(sizeof(CData*) == sizeof(std::reference_wrapper<CData>));

	QSet<CData*>* src = &mSelection;
	QSet<std::reference_wrapper<CData>>* alias;
		
	std::memcpy(&alias, &src, sizeof(alias));  // starts lifetime of alias as QSet<CData*>*
	return *alias;
}

QSet<std::reference_wrapper<CData>>& mSelectionAsRefs()
{
	static_assert(sizeof(CData*) == sizeof(std::reference_wrapper<CData>));

	QSet<CData*>* src = &mSelection;
	void* alias;

	std::memcpy(&alias, &src, sizeof(alias));  // starts lifetime of alias as QSet<CData*>*
	return *static_cast<QSet<std::reference_wrapper<CData>>*>(alias);
}

struct CBigClass {
...
 QList<CData*> mData;
 QSet<CData *> mSelection; 
...
}

или

struct CBigClass {
...
 QList<CData> mData;
 QSet<reference_wrapper<CData>> mSelection; 
...
};