member function pointer - how to do this? - c++

Question

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!

Answer 1

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.

Answer 2

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.