How can I globally access a QT dialog from any class

Question

I have a QT dialog which I need to have access to from anywhere in the program. Basically what I need to do is something like creating a static instance of it somewhere in my program, something like:

'''Note''': This is just an example of what I am trying to do, not actual code (which is too long to post here)

class Core
{
  public:
    static DialogType *MyDialog;
};

DialogType *Core::MyDialog = NULL;

// later in main.cpp

int main(int argc, char *argv[])
{
    try
    {
        Core::Init();
        QApplication a(argc, argv);
        Core::MyDialog = new DialogType();
        ...

However, despite this would work for any other type, it just doesn't work for classes that are inherited from QDialog. The compiler just return: DialogType does not name a type (and yes I do #include that .h file with declaration of DialogType)

What am I doing wrong? Why QT doesn't allow that? How could I make it possible to access single instance of my dialog from ANY class anywhere in the program?

Answer 1

If you genuinely do need just one single, always available instance of that specific class then you could build it off the Singleton pattern so it either creates a pointer and returns it, or just returns a pointer if it's created. Singletons are often recommended as there's plenty of faults with them, but for something along these lines it's probably easier than setting up a static reference to a QDialog inherited class.

Answer 2

If you are getting an error that the compiler doesn't know what type you are using then you must either insert a forward declaration, or #include a header file that contains either a forward declaration or a definition.

A forward declaration is sufficient if your member type is a pointer or a reference but if it is any other ADT, an #include becomes required.

In the code you posted, you could have:

Core.h:

class DialogType; // forward declaration.

class Core
{
  public:
    static DialogType *MyDialog;
};

In your source file, you could then have:

Core.cpp:

#include "Core.h"
#include "DialogType.h"

DialogType *Core::MyDialog = new DialogType();