QIntValidator gotcha

The problem I’m going to present is actually dirt-simple, and its causes are even described in the official docs. Yet the Google itself couldn’t find the solution when I needed it, so let’s help the tech giant out by blogging about it.

A few days ago I was working on a seemingly simple task: given a description of an integer type (e.g. signed 32-bit, unsigned 64-bit), ensure that a QLineEdit will only accept numbers that fit into that type. Luckily, Qt already has some input-checking facilities, letting us attach a so-called validator to an input field. Furthermore, there’s a standard QIntValidator that determines if an integer is in the given range—just what I needed!

I tested it with unsigned 8-bit first, and everything went well. But then I tried signed type and immediately discovered that I can input “128” even though the upper bound is 127. But why? Maybe QIntValidator’s range is actually a half-open interval? Nope, it’s a closed one all right. Have I made an error calculating the bounds? A few qDebug()s confirmed I didn’t, and hard-coding the upper limit didn’t change a thing either. What the hell?

Going through the customary ritual of losing faith first in myself, then in my compiler, and finally in my machine, I printed out the result the validator returns to QLineEdit. Sure enough, it was Acceptable for -128, and Intermediate for 128. Because, y’know, “128” is just “-128” without a minus; the validator deems it okay for the user to input their numbers in whichever order they please.

So if you’ve come here from Google, don’t worry: QIntValidator’s lower bound is inclusive; and QIntValidator’s upper bound is inclusive, too. You just have a range that’s “sloped” towards the negative numbers, and thus your validator seemingly accepts values bigger than it should. Connect to QLineEdit::​textEdited(const QString&) and change input’s border color to red or something if the result is not Acceptable, and you’ll be fine.