Safe to share a QCompleter?

[emobemo]

wysota, please specify the reason why you would not use one QCompleter for multiple widgets. I am using shared QCompleter now and it works but I want to know if there are some hidden dangers of doing so.

[wysota]

wysota, please specify the reason why you would not use one QCompleter for multiple widgets. I am using shared QCompleter now and it works but I want to know if there are some hidden dangers of doing so.

If you have more than one widget using the same completer then how are you going to share the completion prefix between them? If one widget contains text "abcd" and the other contains "klmnopq" then you have to ensure every focus change of the widget is going to reset the completion prefix.

Consider the following code (Qt5 + C++11):

#include <QtWidgets>
 
int main(int argc, char **argv) {
  QApplication app(argc, argv);
  QWidget w;
  QVBoxLayout *l = new QVBoxLayout(&w);
  QLineEdit *le1 = new QLineEdit;
  QLineEdit *le2 = new QLineEdit;
  l->addWidget(le1);
  l->addWidget(le2);
  QCompleter *cpl = new QCompleter(&w);
  le1->setCompleter(cpl);
  le2->setCompleter(cpl);
  QStringListModel model({"abcde", "abcdef", "abcdefg", "klmno", "klmnopq", "klmnopqr"});
  cpl->setModel(&model);
  QObject::connect(le1, SIGNAL(returnPressed()), cpl, SLOT(complete()));
  QObject::connect(le2, SIGNAL(returnPressed()), cpl, SLOT(complete()));
  le1->setText("abc");
  le2->setText("klm");
  w.show();
  return app.exec();
}

#include <QtWidgets>

int main(int argc, char **argv) {
  QApplication app(argc, argv);
  QWidget w;
  QVBoxLayout *l = new QVBoxLayout(&w);
  QLineEdit *le1 = new QLineEdit;
  QLineEdit *le2 = new QLineEdit;
  l->addWidget(le1);
  l->addWidget(le2);
  QCompleter *cpl = new QCompleter(&w);
  le1->setCompleter(cpl);
  le2->setCompleter(cpl);
  QStringListModel model({"abcde", "abcdef", "abcdefg", "klmno", "klmnopq", "klmnopqr"});
  cpl->setModel(&model);
  QObject::connect(le1, SIGNAL(returnPressed()), cpl, SLOT(complete()));
  QObject::connect(le2, SIGNAL(returnPressed()), cpl, SLOT(complete()));
  le1->setText("abc");
  le2->setText("klm");
  w.show();
  return app.exec();
}

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See what happens if you request completion (by pressing return key) after switching focus to the other widget -- it shows completions for a wrong widget. In addition to that even if you do update the prefix as the focus changes, you'll get a performance hit as the model has to be refiltered each time. QCompleter is just a couple of bytes of additional memory used, why bother "saving" those several bytes if it can get you in trouble?

[emobemo]

See what happens if you request completion (by pressing return key) after switching focus to the other widget -- it shows completions for a wrong widget. In addition to that even if you do update the prefix as the focus changes, you'll get a performance hit as the model has to be refiltered each time. QCompleter is just a couple of bytes of additional memory used, why bother "saving" those several bytes if it can get you in trouble?

When you setText() on the QLineEdit the completion prefix of the completer does not change since it changes on keyPressEvent(), so even if you remove the second QLineEdit from the sample code the full list will popup. The other issue about the completer is that it not just keeps couple of bytes of additional memory. The QCompleter has its own internal proxy model to filter and sort data from the source model and if you have a lot of completers you will also have a lot of internal proxy models. However the documentation doesn't say that it should be used only on one widget at a time, but it slightly implies so with the setWidget() method, which in my opinion is redundant. I was wondering if there is some posibility of chrash or other dangerous behaviour when sharing QCompleter.

[wysota]

When you setText() on the QLineEdit the completion prefix of the completer does not change since it changes on keyPressEvent(), so even if you remove the second QLineEdit from the sample code the full list will popup.

That's not the issue I meant (the one you say is easily avoided by connecting textChanged signal to setCompletionPrefix slot). Focus on first line edit, press "d", focus on second line edit, press return. You'll get completions for the first widget while being focused on the second widget. This cannot be countered using signals and slots (you can do it using event filters though however this makes the objects more tightly coupled).