Question

I’m trying to set a variable that can be accessed from multiple children at the MainWindow (QMainWindow) level. The issue is whenever I try to access it from a child (or grandchild), I get a segmentation fault.

Here’s a mock up of the involved code:

//MainWindow.h
...
public:
    int getVariable();
    void setVariable(int i);
...
private:
    int globalInt;
    SomeWidget *myWidget;


//MainWindow.cpp
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
    ...
    this->globalInt = 10;
    myWidget = new SomeWidget();
    myWidget->setParent(this);
    ....
}
int getVariable()
{
    return this->globalInt;
}
void setVariable(int i)
{
    this->globalInt = i;
}
...



//SomeWidget.cpp
...
int x = (static_cast<MainWindow*>(this->parent()))->getVariable(); //Causes Segfault
...

I honestly have no idea what I’m doing wrong. I’ve tried creating a new MainWindow* pointer to the parent and casting it, I’ve tried making the “global” int public and directly accessing it, etc. etc. Any ideas what I need to do?

Answer 1

You could use reinterpret_cast. Note that this is not a secure way, because you can make pointers “that point to an object of an incompatible class, and thus dereferencing it is unsafe.” (Info)

I just arrived at work so I only had time for a little (and ugly) example, a small console program.

class some_widget
{
public:
    some_widget(){ m_parent = 0;}

    void set_parent( void* p_parent ){m_parent= p_parent;}
    void* get_parent(){return m_parent;}

    void do_something();

private:
    void* m_parent;
};

class main_window
{
public:
    main_window(void);
    ~main_window(void){ delete myWidget; myWidget = 0;}

    int getVariable(){return global_int;}
    void setVariable(int i){global_int = i;}

    some_widget* get_widget(){return myWidget;}

private:
    int global_int;
    some_widget *myWidget;
};

main_window::main_window(void)
{
    global_int = 10;
    myWidget = new some_widget();
    myWidget->set_parent(this);
}

void some_widget::do_something()
{
    if( this->get_parent() != 0 )
    {
        int x = reinterpret_cast<main_window*>(this->get_parent())->getVariable();
    }
}

int main( int argc, char* argv[] )
{
    main_window* mw = new main_window();

    mw->get_widget()->do_something();

    return 0;
}