CODEWITHSUNDEEP
c++
user2011931
user2011931

Reputation: 21

member function pointer - how to do this?

I have the following:

    class NetRequest: public QObject {
        Q_OBJECT
    public:

            NetRequest(QString postData, void(BrowserApp::*f)(QByteArray));
            QString postData;
            void (BrowserApp::*fn)(QByteArray);
        public slots:
            void requestFinished(QNetworkReply *r);
        }

and the cpp is:

        NetRequest::NetRequest(QString postData, void(BrowserApp::*f)(QByteArray)) {
            this->postData = postData;
            this->fn = f;
        }

        void NetRequest::requestFinished(QNetworkReply *r) {
            QByteArray data;
                fn(data);<--- this doesn't work
        }

I am trying to call the function "fn" in requestFinished but the compiler doesn't let me - what am I please doing wrong?

Thank you for all your help!

Upvotes: 0

Views: 74

Answers (2)

John Dibling
John Dibling

Reputation: 101456

When you have a pointer-to-member-function, in order to call through it you must also have a pointer to the object you are using for the call.

But your code:

fn(data);

Has no such object pointer. You probably are assuming that an object is associated with the member function pointer -- but it isn't.

Try this:

(that->*fn)(data);

that is a pointer to a BrowserApp object. It will need to be either passed in to the requestFinished function, or somehow stored in the NetRequest object -- probably at the same time when fn is set.

Upvotes: 2

Some programmer dude
Some programmer dude

Reputation: 409176

I suggest you look into std::function and std::bind. They will help you in many ways when it comes to "function pointers".

Then it would look something like this instead:

class NetRequest: public QObject {
    std::function<void(QByteArray)> fn;

    // ...

public:
    NetRequest::NetRequest(QString postData, std::function<void(QByteArray)> f) {
        // ...
        fn = f;
    }

    // ...
};

Now when creating an instance of the NetRequest object, you must use std::bind to bind a function from the BrowserApp class to the std::function object:

BrowserApp app;

NetRequest request("some string", std::bind(&BrowserApp::SomeFunction, app));

The nice thing about using std::function is that the callback function no longer have to be a BrowserApp member function, it can by any function, including lambda expressions.

Upvotes: 0

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