passing arguments to C++ functions

[Christian Ehrlicher]

@mzimmers said in passing arguments to C++ functions:

bool EquipmentModel::changesExist(Equipment e) {

This is creating a copy by calling the Equipment copy ctor so no chance to do any cast to vspStruct.
Pass it by const ref or pointer - everything else will not work.

[JonB]

@Christian-Ehrlicher I have appended a new paragraph to my earlier response as a result of this conversation with you.

[mzimmers]

@JonB thanks for the in-depth response. I've actually tried the virtual method approach you suggest, but I must be doing something wrong:

// equipment.h
    virtual QVariant data(int role);
// vsp.h
    QVariant data(int role) override;
// equipmentModel.cpp
bool EquipmentModel::changesExist(Equipment e) {
    e.data(256); // calls the method in equipment.h, not vsp.h.
    ...

BTW: my equipment struct isn't abstract; I actually use it for some items. I'm not sure whether this makes a difference.)

[JonB]

@mzimmers
Please re-read the discussion. The point is that if you have a formal parameter Equipment e this has to be passed in as a copy by value to a temporary of base-type Equipment. You pass a vspStruct as your parameter, but that does not get through to the function, it gets just an Equipment copy. And so e.data() is Equipment::data(), the vspStruct original has got lost.

This would not happen if your method were either of

bool EquipmentModel::changesExist([const] Equipment *e)    // pointer
bool EquipmentModel::changesExist([const] Equipment &e)    // reference

Then it would pass and receive your original vspStruct. You need to change over to one of these, not Equipment e. I believe @Axel-Spoerl & @Christian-Ehrlicher have said QML does not prevent you from doing so.

[mpergand]

@JonB
I confirm, i 've just made a test, it doesn't work with a copy passed by value.

[JonB]

@Christian-Ehrlicher said in passing arguments to C++ functions:

This is creating a copy by calling the Equipment copy ctor so no chance to do any cast to vspStruct.
Pass it by const ref or pointer - everything else will not work.

[mzimmers]

@JonB said in passing arguments to C++ functions:

You need to change over to one of these, not Equipment e. I believe @Axel-Spoerl & @Christian-Ehrlicher have said QML does not prevent you from doing so.

Well, I tried that, and this is the result:

Notice that e is still an Equipment. By contrast, listEntry is a (pointer to a) Vsp.

Or, am I still not understanding you?

[mzimmers]

So, after a little trial and error, here's where I stand:

1. according to @Axel-Spoerl, since my Equipment class isn't derived from QObject, QML is limited to passing it as a const reference to C++ functions.
2. There are (at least) 3 options for coding my C++ function:

• bool EquipmentModel::changesExist(Equipment e)
• bool EquipmentModel::changesExist(const Equipment *e)
• bool EquipmentModel::changesExist(const Equipment &e)
  As @Christian-Ehrlicher pointed out, the first option won't work for me. The second option produces a run time error about incompatible JS arguments. The last option builds and runs, but the behavior is somewhat baffling:
  
  My cast doesn't seem to have had the desired effect of "recapturing" the subclass. Perhaps I'm doing something wrong with this?

3. Interestingly enough, if I do a direct JS comparison in my QML instead of calling this routine, it works. So, if my assumptions are correct, it's really the QML-to-C++ interface that is the limitation here, and this is probably due to the fact that I'm trying to use a non-QObject based struct.

Unless someone has any QML-based ideas on this, I'm going to consider the matter closed. I do have more questions, but they're more appropriate for the C++ forum.

Thanks to everyone who helped with this.

[JonB]

@mzimmers said in passing arguments to C++ functions:

bool EquipmentModel::changesExist(const Equipment &e)

That is the only correct one of the three.

If you want you can discuss elsewhere whether/how to achieve what you want.

[mzimmers]

@JonB if I understood @Axel-Spoerl correctly, the pointer version would work if I derived my struct from QQuickItem, but that's not a desired option at this time.

And yes, I think it's time to move this discussion to the C++ forum. Thanks again for the help.