Out of range condition on Q_PROPERTY

[A Former User]

I am new to QT and was wondering how are you supposed to handle errors in the QT property system, more specifically on set the functions?

class SomeClass : public QObject
{
Q_OBJECT
Q_PROPERTY(qint16 temperature READ temperature WRITE setTemperature)

private:
qint16 mTemperature;
...
...
...

void SomeClass::temperature()
{
return mTemperature;
}

void SomeClass::setTemperature(qint16 _temperature)
{
if (_temperature < 10 || _temperature > 100)
{
// argument is out of range, what do you do here?
return;
}
mTemperature = _temperature;
}

Are you supposed to throw an exception, invoke a signal or what?
Any help would be greatly appreciated.

[mcosta]

HI and welcome to devnet,

Are you supposed to throw an exception, invoke a signal or what?

For sure Qt does not use exceptions.

What usually I do (is not an official Qt guideline) is to force the value to be in the range; you have 2 options depending of what do you need:

1. Don't change the value if the proposed value is out-of range;

void MyObject::setValue (int newValue) 
{
    if (newValue < MIN_VALUE || newValue > MAX_VALUE)
        return;

    if (_v != newValue) {
        _v = newValue;

        Q_EMIT valueChanged(_v);
    }
}

2. Force the value to the limit values.

void MyObject::setValue (int newValue) 
{
    if (newValue < MIN_VALUE)
        newValue = MIN_VALUE;

    if (newValue > MAX_VALUE)
        newValue = MAX_VALUE;

    if (_v != newValue) {
        _v = newValue;

        Q_EMIT valueChanged(_v);
    }
}

Anyway if you have a range (or only one of lower/upper limit) I suggest to define READ ONLY properties for the limits

[A Former User]

Mcosta,

Thanks for the input, I'll probably use coercion as you suggest seems like the logical thing to do.