Why is this a binding loop?

[mzimmers]

Thank you both for the help. To answer your question about why there was a signal on a get -- the way this app was written, the get not only returned the current value, but also updated it first (I know it's weird).

To fix this, I've created a setter for the property:

QStringList ChangeConsumables::GetPosterModalChecklist() {
  return m_posterModalChecklist;
}

void ChangeConsumables::SetPosterModalChecklist(QStringList qsl) {
  m_posterModalChecklist = qsl;
  emit EventPosterModalChecklistChangedForQML(m_posterModalChecklist);
}

The code is a little convoluted, as the setter is actually invoked by another object (the same one that's providing the QStringList to the getter), but it seems to work.

Thanks again...

[SGaist]

Hi,

There's one thing missing from your setter: the equality guard. Usually the first thing done in a setter is check if the value changes and stop there if not. This allows to avoid signal storms were each party connected re-emits the same values over and over.

[mzimmers]

@SGaist so, like this?

void ChangeConsumables::SetPosterModalChecklist(QStringList qsl) {
  if (m_posterModalChecklist != qsl) {
      m_posterModalChecklist = qsl;
      emit EventPosterModalChecklistChangedForQML(m_posterModalChecklist);
  }
}

[SGaist]

Either that or

void ChangeConsumables::SetPosterModalChecklist(QStringList qsl) {
  if (m_posterModalChecklist == qsl) {
      return;
}
  m_posterModalChecklist = qsl;
  emit EventPosterModalChecklistChangedForQML(m_posterModalChecklist);
}

Depending on your taste.
Note that I have seen this version more frequently. Also in Qt's own sources.

[mzimmers]

@SGaist I realize I'm old school, but I really don't like return statements in the middle of a function. I know it's done by far better coders than I, but I'm just not a fan.

[SGaist]

I understand your point.

IIRC, early returns like these may allow some optimization by the compiler.

It's technically also at least one less operation because != is usually implemented as ! ==.

[mzimmers]

Not to be argumentative, but I'd venture that a routine that is comparing QStringList objects probably leaves much to be desired in the optimization department anyway.

Your comment about !== is interesting. Is there a way for the programmer to explicitly effect a similar comparison?

[SGaist]

@mzimmers said in Why is this a binding loop?:

Not to be argumentative, but I'd venture that a routine that is comparing QStringList objects probably leaves much to be desired in the optimization department anyway.

I was more in the generic case of simple type comparison :-)

@mzimmers said in Why is this a binding loop?:

Your comment about !== is interesting. Is there a way for the programmer to explicitly effect a similar comparison?

I am not sure I am following you on that one.

[mzimmers]

@SGaist said in Why is this a binding loop?:

I am not sure I am following you on that one.

Well, of course C++ doesn't have a "!==" operator, so I assumed you meant that it first compares type, then content (a la Javascript). I was just curious how a C++ coder might perform that type check explicitly.

[SGaist]

You missed the space between the ! and the == :-)

I meant it as: "!(a == b)"

[mzimmers]

Oh...I hate it when I do that (heh).

But now my statement on efficiency is even more true...

EDIT:

For those who really wish to exact that last bit of optimization, you could always do something like this:

void ChangeConsumables::SetPosterModalChecklist(QStringList qsl) {
  do {
    if (m_posterModalChecklist == qsl) {
      continue;
    }
    if (any other reasons to exclude further processing) {
      continue;
    }
    m_posterModalChecklist = qsl;
    emit EventPosterModalChecklistChangedForQML(m_posterModalChecklist);
  } while (false);
}

This technique is also very handy for avoiding multiple if statements (with their annoying attendant indentation).