Qt access mainWindow in a class without giving the object to it - c++

Question

I need to access the mainWindow object in a different class. The problem is, that I can not give mainWindow to this class (I don't want to do this, it would make everything much more complicated). Question: Is there any way in C++ or Qt to put an object in something like a local "database" or sth where every other class in the project can look into and communicate with the objects.
What I want to have in the end is something like this:
// A.h
#ifndef A_H
#define A_H
class A{
public:
A() { /*here comes the ting*/ myMainWindow->sayHi(); }
};
#endif // A_H
// MainWindow.cpp
#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "a.h"
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
A *a = new A;
}
MainWindow::~MainWindow(){delete ui;}
MainWindow::sayHi(){
// just do anything
}
I don't think, this is possible, but I give it a try...
Thanks for answers!

Answer 1

I need to access the mainWindow object in a different class. The
problem is, that I can not give mainWindow to this class (I don't want
to do this, it would make everything much more complicated).
That is doable. The author don't want to expose the "main window" variable holding the reference or pointer to the object. And, obviously the author wants that UI object to be callable from other objects. In Qt that implies either both objects on UI thread or communication is via queued signal-slot connection only. But the direct call wanted, hence on the same thread.
Is there any way in C++ or Qt to put an object in something like a
local "database" or sth where every other class in the project can
look into and communicate with the objects.
Local thread storage is a known pattern to implement things like that. Qt has own implementation of it called QThreadStorage. You can attempt something like that:
// myLts.h
void ltsRegisterObject(const QString &key, QObject *object);
void ltsRemoveObject(const QString &key);
QObject* ltsGetObject(const QString &key);
template <typename T> T* ltsGet(const QString &key) {
return qobject_cast<T*>(ltsGetObject(key));
}
// myLts.cpp
static QThreadStorage<QMap<QString, QObject*> > s_qtObjects;
void ltsRegisterObject(const QString &key, QObject *object)
{
s_qtObjects.localData().insert(key, object);
}
void ltsRemoveObject(const QString &key)
{
if (!s_qtObjects.hasLocalData())
return;
s_qtObjects.localData().remove(key);
}
QObject* ltsGetObject(const QString &key)
{
QObject *object;
auto it = s_qtObjects.localData().find(key);
return it != s_qtObjects.localData().end() ? it.value() : nullptr;
}
Register main window object in LTS:
#include "myLts.h"
// ...
// register main window in LTS
ltsRegisterObject("mainWindow", &mainWindow);
Find and use the object:
#include "myLts.h"
// ...
// use main window from LTS
auto* pMainWnd = ltsGet<QMainWindow>("mainWindow");
if (pMainWnd)
pMainWnd->show();
P.S. I did not compile this. But it is not hard to fix if so.