Qt5 Quickly convert json to an object without converting field by field

Xrtero

#qt5 #http #json
In Qt5, when I want to convert a JSON to an object, I must get a QJsonObject and know each the object's field name, then use the key-value pairs to retrieve the object's properties, like this:

obj.setId(json["data"]["id"].toInt());

Is there a better way to quickly convert a json ?

thanks :)

Christian Ehrlicher

I don't see how this should be done quicker - you have to pass the keys somehow.

Xrtero

@Christian-Ehrlicher said in Qt5 Quickly convert json to an object without converting field by field:

I don't see how this should be done quicker - you have to pass the keys somehow.

like this:
a http get request .

......
In http request
.....
QNetworkReply* reply; /*initialized*/

QByteArray      data = reply->readAll().data();
QJsonParseError jsonError;
QJsonDocument   json = QJsonDocument::fromJson(data, &jsonError);

MyObject* myObject = MyObject::fromJson(json.object());

......
In MyObject  fromJson function
.....
myObject.setId(json["data"]["id"].toInt());

Christian Ehrlicher

And what should setId() do? How should it magically know where to put the value to?

Xrtero

@Christian-Ehrlicher
just set my object field

.....
in MyObject class 
id is MyObject field
.....
void setId(quint32 newId){
id = newId
}

Christian Ehrlicher

Sorry I don't understand the problem.
What's wrong with

myObject.setId(json["data"]["id"].toInt());

?

JonB

@Xrtero
Like @Christian-Ehrlicher I'm not sure what your issue is.

You are apparently receiving the JSON text over the network. I would assume that will take waayyy longer than any in-memory manipulation anyway, and the second-most expensive (though far behind) might be the fromJson() text parsing.

Xrtero

@Christian-Ehrlicher said in Qt5 Quickly convert json to an object without converting field by field:

Sorry I don't understand the problem.
What's wrong with

myObject.setId(json["data"]["id"].toInt());

?

@JonB said in Qt5 Quickly convert json to an object without converting field by field:

@Xrtero
Like @Christian-Ehrlicher I'm not sure what your issue is.

You are apparently receiving the JSON text over the network. I would assume that will take waayyy longer than any in-memory manipulation anyway, and the second-most expensive (though far behind) might be the fromJson() text parsing.

Send an HTTP GET request using QNetworkAccessManager and receive a response in the format of Application/json. I need to parse this response (JSON) into an object.

Pl45m4

@Xrtero

Maybe this topic helps:

https://stackoverflow.com/questions/19822211/qt-parsing-json-using-qjsondocument-qjsonobject-qjsonarray

Christian Ehrlicher

@Xrtero said in Qt5 Quickly convert json to an object without converting field by field:

. I need to parse this response (JSON) into an object.

You already do this...

Xrtero

@Christian-Ehrlicher

Therefore, I'm wondering if there's a more convenient method. For instance, if the JSON is large and the object to be converted has many fields, manually writing the code would involve a significant amount of work.

Christian Ehrlicher

What should be more convenient here? How should some magic code know which attribute you want? Noone else but you can know what you want to retrieve from the json.

JonB

@Xrtero IMO you are worrying about something you should not be worrying about.

hskoglund

With a bit of luck reflection will be included in C++26 and then you can write something like this:

MyObject myObject;
for (auto d : std::meta::nonstatic_data_members_of(^MyObject))
    myObject.[:d:] = json["data"][[:d:]].toInt();

I.e. steps through all your different data members in a loop (assuming they are all of the type int).
More info here

Xrtero

@Christian-Ehrlicher said in Qt5 Quickly convert json to an object without converting field by field:

What should be more convenient here? How should some magic code know which attribute you want? Noone else but you can know what you want to retrieve from the json.

My current idea is to implement this feature using Qt's reflection, which would allow us to access each field and convert it into the corresponding type.

Xrtero

@JonB

just to make convert json convenient and reduce some repetitive work.

Xrtero

@hskoglund said in Qt5 Quickly convert json to an object without converting field by field:

With a bit of luck reflection will be included in C++26 and then you can write something like this:
MyObject myObject;
for (auto d : std::meta::nonstatic_data_members_of(^MyObject))
    myObject.[:d:] = json["data"][[:d:]].toInt();
I.e. steps through all your different data members in a loop (assuming they are all of the type int).
More info here

Qt also has the same feature, and that's what I was planning to do.
Finally, someone understands my idea. Thank you!

JonB

@Xrtero
So you will use a feature of C++ 26, and it will only work if all your properties are of type int. You could do something like this easily if you changed to a language like Python. Anyway if this is what you are looking for and you are happy that is good.