Robust way to map QComboBox items to internal types

Justin Sayne

I'd like to fill a QComboBox with various types and map them to different logic in my application. What's the most robust way to do this?

I could do string comparison:

if(box->currentText() == "Item 1")
  do this;
else if(box->currentText() == "Item 2")
  do that;

but that breaks as soon as I rename my items and - worst of all - I might not even notice I've broken sth.

Another solution is to do an index mapping:

if(box->currentIndex() == 1)
  do this;
else if(box->currentIndex() == 2)
  do that;

or even

enum ComboBoxTypes { Item_1, Item_2 }
if(box->currentIndex() == Item)
  do this;
else if(box->currentIndex() == Item)
  do that;

but this again breaks without notice if I reorder, change or delete items.
Is there a better way to do this?

Chris Kawa

There's setItemData() for exactly this purpose. You can associate any kind of data with the items and then add/reorder/delete them and the data stays and can be queried with itemData() or currentData()
For example:

enum ComboBoxTypes { Foo, Bar }
box->setItemData(0, Foo);
box->setItemData(1, Bar);

//and then

if (box->currentData().toInt() == Foo)
    //do stuff...

If you need to associate more then one piece of data you can do that by specifying the extra param, e.g.

box->setItemData(0, someStuff, Qt::UserRole);
box->setItemData(0, someOtherStuff, Qt::UserRole + 1);

and same for retrieval.

It's also common to put a pointer to some data structure of yours as the item data. This way you get an instant access to data related to the item.

Justin Sayne

This is a nice solution but it only works in code, right? There is not a way to do this in QtDesigner by adding a QVariant to each item?!

Chris Kawa

No, you can't set this up in the designer.