How to translate a function name in QJsonObject from string to function pointer?

[SPlatten]

I have a QJsonObject containing:

decoder: "decoderFn"

To extract the function name as a string:

QJsonObject::iterator itrDecoder = objDecoder.find(clsJSON::mscszDecoder);

if ( itrDecoder == objDecoder.end() ) {
    continue;
}
QString strDecoder = itrDecoder.value().toString();

Can I translate strDecoder to a function pointer or do I need to change the string contents to a full prototype of the function?

And use eval? Will try out whilst waiting for a comment / answer...

[sierdzio]

If your target object is a QObject, and your method is marked as slot or Q_INVOKABLE, you can call that method by name (taken from JSON) using invokeMethod().

[SPlatten]

@sierdzio , thank you, can you show an example? My functions in the JSON aren't part of a class, something like:

bool blnDecodeHeartbeat(const QJsonObject& crobjJSON) {
    clsModule* pModule = pDecodeModuleAndPort(crobjJSON);
    if ( pModule != nullptr ) {
        if ( pModule->blnIsMsgSndrSet() != true ) {
            QJsonObject::const_iterator citrFound = crobjJSON.find(clsJSON::mscszPort);

            if ( citrFound == crobjJSON.end() ) {
    //No port in message, cannot set port, abort decode!
                return false;
            }
        }            
        pModule->updateHearbeat();
    //Extract the message type from the message
        clsJSON::sendAck(crobjJSON);
        return true;
    }
    return false;
}

If wrapping them up in a class would help, I can do that...can you help with an example? All the functions I want to use like this will have the same prototype, I mean same return and parameters.

[sierdzio]

Hm in that case I'd rather do an if-else cascade and call proper function depending on JSON data.

Anyway, coming back to invokeMethod() idea:

class MyClass : public QObject
{
  Q_OBJECT

public:
  Q_INVOKABLE int decoderFn(int something);
};

// And now, in your JSON parser:
MyClass myObject = ...;
QString strDecoder = itrDecoder.value().toString();
int returnValue = 0;
bool isOk = QMetaObject()::invokeMethod(
  myObject, strDecoder.constData(),
  Qt::DirectConnection,
  Q_RETURN_ARG(int, returnValue), Q_ARG(int, 123));

qDebug() << "Result is:" << returnValue << "is ok?" << isOk;

[SPlatten]

@sierdzio , I was trying:

        QMetaObject::invokeMethod(nullptr, cpszDecode, Qt::DirectConnection
                                 ,Q_RETURN_ARG(bool, blnRet)
                                 ,Q_ARG(const QJsonObject&, "crobjJSON"));

With the first parameter to see if it would work but I get:

no matching constructor for invalidation of 'QArgument<const QJsonObject&>'

If I move my decode functions into a class and pass the type to the first parameter how do I satisfy the argument type?

[KroMignon]

@SPlatten said in How to translate a function name in QJsonObject from string to function pointer?:

no matching constructor for invalidation of 'QArgument<const QJsonObject&>'

First, "crobjJSON" is a string not a QJsonObject!

Second, Q_ARG() should be something like Q_ARG(QJsonObject, const &crobjJSON)

[sierdzio]

Object is needed, I don't think it will work with nullptr. Argument - try with just QJsonObject, without const reference. If that still fails, maybe you need to register QJsonObject with with meta object system.

Or use if-else as mentioned.

[SPlatten]

@KroMignon I was using the example:

QString retVal;
QMetaObject::invokeMethod(obj, "compute", Qt::DirectConnection,
                          Q_RETURN_ARG(QString, retVal),
                          Q_ARG(QString, "sqrt"),
                          Q_ARG(int, 42),
                          Q_ARG(double, 9.7));

on:
https://doc.qt.io/qt-5/qmetaobject.html#invokeMethod

Which is why I thought the second parameter of Q_ARG had to be a string.