QJsonObject isUndefined is never undefined when applied to a copy
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I use Json files for initialisation of my program. Some variables are predefined by default, so if a certain tag is not in the init file I want to use these.
Now, if I use QJsonObject init["sometag"].isUndefined() to check for existence of an initialisation value, this works if init is the original object.
But after QJsonObject copy = init;
copy["sometag"],isUndefined() returns always false, and "sometag": Null is inserted into copy.
Same happens with
QJsonObject subpart = init["subpart"]Is this a bug or a feature, and how can I work around this?
Uli
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I use Json files for initialisation of my program. Some variables are predefined by default, so if a certain tag is not in the init file I want to use these.
Now, if I use QJsonObject init["sometag"].isUndefined() to check for existence of an initialisation value, this works if init is the original object.
But after QJsonObject copy = init;
copy["sometag"],isUndefined() returns always false, and "sometag": Null is inserted into copy.
Same happens with
QJsonObject subpart = init["subpart"]Is this a bug or a feature, and how can I work around this?
Uli
@lurchi
isUndefined()does something different from what you are thinking it does.You cannot index ause bool QJsonObject::contains(const QString &key) const to check whether a key exists before trying to index by it.QJsonObjectby["some arbitrary string"], so it won't get as far as the followingisUndefined().EDIT
I realise this contradicts https://doc.qt.io/qt-5/qjsonobject.html#operator-5b-5dThe returned QJsonValue is QJsonValue::Undefined if the key does not exist.
and you are asking a question about behaviour after copying via
QJsonObject copy = init;being different, is that right? I am rethinking.....EDIT2
OK, if I understand right, your issue is going to be connected to this method: QJsonValueRef QJsonObject::operator[](const QString &key)Returns a reference to the value for key. If there is no value with key key in the object, one is created with a QJsonValue::Null value and then returned.
Now, I think you will find that you calling
init["sometag"].isUndefined()activates this.
init["sometag"]had no such key initially, but executing this creates one withQJsonValue::Nullvalue in theinitobject. (Verify that for yourself before versus after the call by usingcontains("sometag").) And then that gets copied tocopy....Think about using what I said above about
QJsonObject::contains(const QString &key)to avoid creating the non-existent key-value.P.S.
If I am not mistaken: you have an initialQJsonObject,init. If you never wish to modify it and only make changes incopytry to declareinit(or something referring to it) asconst. That way should prevent you (compiler) callinginit["sometag"]and having it create that key ininitas an unwanted side-effect --- only the variousconstoverrides ofQJsonObject::operator[]etc. will be allowed?
