Qt：将对象传递到每个窗口

Question

Im a beginner at C++ since I'm a Java-Dev. I'm learning Qt right now. Now I want in my widget application on central object for holding special functionality (for example a bunch of methods for sending different network-command or something like that). In my mainwindow.cpp I've got this

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    //...
    BusinessLogic* bl = new BusinessLogic;
    bl->setMyValue(44);
    this->setBusinessLogic(bl);
    this->getBusinessLogic();

    //...
}

BusinessLogic* MainWindow::getBusinessLogic()
{
    return this->bl;
}

void MainWindow::setBusinessLogic(BusinessLogic* newbl)
{
    this->bl = newbl;
}

Where BusinessLogic.cpp is the class which I want to access from all windows in my application. So in my Dialog which get opend when the pushButton is clicked, I try to use the businesslogic instance like this in the constructor

Dialog::Dialog(QDialog *parent) :
    QDialog(parent),
    ui(new Ui::Dialog)
{
    //...   
    ((MainWindow*)parent)->getBusinessLogic();
}

Unfortunately the program crashes everytime the dialog get instanciated (the line with ((MainWindow*)parent)->getBusinessLogic(); get called.

What can I do?

Answer 1

There are several methods for achieving your goal(all depends on your requirements) but one approach is you can create Singleton class of your BusinessLogic and then access it anywhere. have a look here . OR you can also pass object of class one class to another. OR you can use signal slot mechanism.

For you best Solution can be

Class BLogic{
public:
    static BLogic* instance(){
       static BLogic ins;
       return &ins;
    }
    void setVal(int v){ 
        val = v;
    }
private:
    BLogic(){}
    int val;
};

Now you can call functions of BLogic from any class like

BLogic::instance()->setVal(10);

OR Example of passing objects can be

class ClassA
{
public:
   void setClassB(ClassB *ob){
      m_classB = ob;
   }
   ClassB* getClassB(){
      return m_classB;
   }
private:
   ClassB* m_classB;
};

class ClassB
{
public:
   void setClassA(ClassA *ob){
      m_classA = ob;
   }
   ClassA* getClassA(){
      return m_classA;
   }
private:
   ClassA* m_classA;
};

Now from main.cpp

ClassA* obA = new ClassA;
ClassB* obB = new ClassB;
obA->setClassB(obB);
obB->setClassA(obA);

Answer 2

Don't pass a zero parent or a dangling pointer to the dialog.

But more seriously, verify your assumptions:

qDebug() << "the parent object is" << parent;
auto p = qobject_cast<MainWindow*>(parent);
if (p)
    p->getBusinessLogic();

Debug output will give you some idea of whether a parent is set, and of what type it is at the moment. Remember that during the construction of a MainWindow instance, it goes through being different types (quite literally). So depending on when you construct the child, you might get a parent of type QWidget* or QMainWindow* or MainWindow* . During the destruction you could even get a QObject* parent!