Unable to add items when function called from other class?

adalovegirls

I have a function that adds some items to a ListWidget (filenames link to the source code in my GitHub repo):

void Debugger::addRegisterViewItem() {
    for (int i = 0; i < 16; i++) {
        ui->leftRegisterListWidget->addItem(QString("%1").arg(chip8->V[i], 4, 16, QChar{'0'}).toUpper());
    }
}

when I call this from within the Debugger class it works as expected. However, when I try calling it from another class:

SDL2Widget.cpp:

void SDL2Widget::mainLoop() {
// snip
    debug->updateWidgets();
// snip
}

the program panics with a segmentation fault, and upon inspection, the ui pointer in the Debugger class is empty when called from SDL2Widget.cpp.

I feel like I just have a fundamental lack of knowledge of how classes interact, but I'm not even sure where to look to learn how I should be doing it... I'm sorry for this probably very beginner question, but if someone could point me in the right direction I would be very grateful. :)

Christian Ehrlicher

@adalovegirls said in Unable to add items when function called from other class?:

debug

Make sure this is a valid pointer.

adalovegirls

@Christian-Ehrlicher Ahhh... I hadn't realized getDebugContext() was getting called before the Debugger window was actually created. I moved debug = Debugger::getDebugContext(); into mainLoop to test it, and it works. The thing I'm not sure to do, is how could I make sure getDebugContext() is called after the Debugger window is created and shown? I have a feeling reassigning it in every iteration of the loop isn't the best practice.

Christian Ehrlicher

First you can use QPointer<QWidget> instead Widget* for your 'debug' member - then you will see if it's still valid when you call SDL2Widget::mainLoop()

adalovegirls

@Christian-Ehrlicher How can I do that exactly? I changed the definition to QPointer<QWidget> debug; and it gives the error implicit instantiation of undefined template 'QPointer<QWidget>'

Thank you for the help so far!

Christian Ehrlicher

First you should not use QWidget but your derived class (so 'Debugger' in your case) and than you should also include <QPointer>

adalovegirls

@Christian-Ehrlicher Oh. It wasn't indicating I was missing QPointer but including it fixed it, along with the change from QWidget to Debugger. After doing that however, the pointer is null when the time comes to call updateWidgets().

Christian Ehrlicher

So my assumption is correct - the pointer is invalid and therefore the app is crashing.

adalovegirls

@Christian-Ehrlicher How can I make it valid when I need it, then?

Christian Ehrlicher

@adalovegirls said in Unable to add items when function called from other class?:

How can I make it valid when I need it, then?

I don't know your code - make sure to create a proper widget, don't delete it afterwards.

kshegunov

@adalovegirls said in Unable to add items when function called from other class?:

How can I make it valid when I need it, then?

Out of curiosity, is there a reason that everybody and their mother is a global variable?
Anyway, I peeked, and you're almost certainly calling updateWidgets() when there's no Debugger object created.
Piece of advice - fix this code pronto. Globals are nasty, everything global is everything nasty ... you get the idea ...

adalovegirls

@kshegunov No reason! I'm just a beginner who doesn't really know what they're doing. :)
And yes, I am aware now that I was calling updateWidgets() when Debugger had yet to be created, I'm asking how I can solve this in a more elegant way than calling it every mainLoop iteration:

void SDL2Widget::mainLoop() {
    debug = Debugger::getDebugContext();

    if (debug)
        debug->updateWidgets();
}

It works, but I'm sure it's not the best way to do it.

kshegunov

@adalovegirls said in Unable to add items when function called from other class?:

And yes, I am aware now that I was calling updateWidgets() when Debugger had yet to be created, I'm asking how I can solve this in a more elegant way than calling it every mainLoop iteration:

If you keep the Debugger as a pseudo-singleton (i.e. a glorified global), this is exactly how you do it.
Alternatively, you can enforce that a debugger is created before whatever widget by passing its address through the widget's constructor (and saving it in a member variable). Then you can't create a widget without first having a debugger and you don't get/check anything in the loop. If you're in the loop then Debugger has been passed to you/exists already.

adalovegirls

@kshegunov Alright. I guess I'll stick with the current method then, because I don't want it to be a requirement that the debugger is created before the emulator runs.

kshegunov

@adalovegirls said in Unable to add items when function called from other class?:

Alright. I guess I'll stick with the current method then, because I don't want it to be a requirement that the debugger is created before the emulator runs.

Then I suggest you put the pointer into the class and set it to the widget through a setter, instead of coupling the classes through the Debugger::getDebugContext or pass it through the constructor, but with a default value (of nullptr). As a rule of thumb you want to rely on as few application-global states (a.k.a. global variables) as possible.

adalovegirls

@kshegunov I don't know how to do that without an example...

kshegunov

https://www.geeksforgeeks.org/constructors-c/
also
https://www.w3schools.com/cpp/cpp_encapsulation.asp

adalovegirls

This post is deleted!

adalovegirls

@kshegunov Alright. I tried making a setter function in SDL2Widget, but in the header for it I'm getting the error Debugger has not been declared even though I have debugger.h included:

SDL2Widget.h

#ifndef SDL2WIDGET_H
#define SDL2WIDGET_H

#include "chip8.h"
#include "SDL.h"
#include "debugger.h"

#include <QDebug>
#include <QWidget>
#include <QTimer>
#include <vector>

class SDL2Widget : public QWidget
{
	Q_OBJECT    
public:
    SDL2Widget(QWidget* parent = nullptr);
	~SDL2Widget();
    static void loadRom(std::vector<unsigned char> rom);
    static void recKey(int key, bool state);
    static Chip8* getC8Context();
    static SDL2Widget* getSDLContext();
    void setDebugContext(Debugger* dbg);
};
#endif

SDL2Widget.cpp

void SDL2Widget::setDebugContext(Debugger* dbg) {
    debug = dbg;
}

I'm also getting the error no matching function for call to SDL2Widget::setDebugContext(Debugger*) when calling the setter from the Debugger class:

debugger.cpp

sdl2->setDebugContext(this);

kshegunov

You probably have a cyclic dependency. Forward declare the class(es) and include the headers in the source files only is the solution.