nlohmann JSON: Object has changed (signal)

[fem_dev]

I'm using a external JSON API called nlohmann JSON inside my Qt GUI Application.
Github link: https://github.com/nlohmann/json

I would like to connect a JSON object to a class method in a way that, every time that this JSON object has changed I receive a signal in a custom slot.

Below I wrote a simple not working example just to illustrate what I would like to do:

void myFunc(nlohman::json& json)
{
    // Update JSON object:
    json["newValue"] = 5;
}

void App::Init()
{
    ....
    nlohman::json myJson;
    ....

    myJson["newValue"] = 0;

    // May be, some connect syntax that I don't know...
    connect(myJson,  nlohman::json::someMethod, this, mySlot)

    myFunc(myJson);
}

void App::mySlot()
{
    qDebug() << "myJson has changed!";
}

So, if I call the App::Init() method, I would like to see the qDebug() output:

myJson has changed!

Is there possible?
If yes, how can I do it?

My system:

• Ubuntu 20.04
• Qt 5.15
• Qt Creator IDE 4.12.3

[Pablo J. Rogina]

@fem_dev you may want to read the signal and slots documentation.
A key requirement for this feature to work is that your class derives from QObject and it has the Q_OBJECT macro in the definition (see small example from documentation)

On the other hand, is a rather compelling reason to use such an external library for JSON when Qt framework does support JSON indeed.

[Bonnie]

@fem_dev said in nlohmann JSON: Object has changed (signal):

Is there possible?

I would vote for NO since it doesn't use Qt at all.

[fem_dev]

@Pablo-J-Rogina Thank you...

I know that Qt JSON support...

I'm just using this external JSON API because I'm developing a C++ library that will be bind with PyBind11 library, which will enable to use my C++ library inside the Python environment.

Pybind11: https://github.com/pybind/pybind11

When my C++ library function return a JSON object, the Pybind11 understand the nlohmann JSON object and convert it to a Python dict native type.

So I can use JSON data between C++ Library and Python environment.

I can't do that using the Qt JSON API. Right?

[Pablo J. Rogina]

@fem_dev said in nlohmann JSON: Object has changed (signal):

So I can use JSON data between C++ Library and Python environment.
I can't do that using the Qt JSON API. Right?

I really cannot help with that, i.e. C++ and Python bindings...

[sprokuda]

it seems you will have to add some signal emitting in your nlohman::json::someMethod, nohow else...you need to add emit someSignal(); to your nlohman::json::someMethod, and add definition like public slots: void someSignal(); and only then you will be able to connect this signal to mySlot like connect(myJson, nlohman::json::someSignal, this, mySlot)

[SGaist]

@sprokuda hi, that is wrong. The nlohman::json is a powerful C++ json handling library. It does not need to be modified to add support for Qt signals and slots, it should rather be used to process JSON data and be used by a class that does the Qt part of the work.