Question

class A
{
  pointer* p;
  template<class T> f(T a)
  {
    p->property()->doSomething(a); // doSomething is a template function
  }
};

Above won’t compile, but I want to access the member pointer in the template function even though it’s unsafe. The reason I want to do this is because ‘p->property()->doSomething(a)’ is actually really really long, and I’m tried of rewriting it (and I don’t want to use #define). So, how do I do something like that?

Edit: doSomething is a 3rd party function… but I think it is something like this:

template<class T>
void doSomething(T a)
{
  // something
}

Edit: here’s the actual code:

#include <QMainWindow>
#include <QtNetwork>
#include <QtWebKit>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

private slots:
    void replyFinished(QNetworkReply*);


private:
    Ui::MainWindow *ui;
    QNetworkAccessManager *nam;
    QUrl _url;

    void outputHeader(QNetworkReply*);

    template<class T> void writeToOuput(T arg)
    {
        ui->textEdit->append(QString("%1").arg(arg));
    }
};

Edit: Here’s the error I’m getting: ‘invalid use of incomplete type ui::MainWindow’
When I click it, it goes to this line ‘ui->textEdit->append(QString(“%1”).arg(arg));’ in the code

Edit: Here’s the other error: ‘Forward declaration of Ui::MainWindow’
When I click it, it goes no where.

Answer 1

With the way the question is worded as is right now, I’m really having trouble making out what the problem is. However, if this is the code giving you trouble:

template<class T> void writeToOuput(T arg)
{
    ui->textEdit->append(QString("%1").arg(arg));
}

1. Try including <QTextEdit>
2. Does Ui::MainWindow actually define a member called textEdit?

It’d help if you could post the build error.

And finally, just a suggestion, but I’d recommend moving this to your cpp file as a non-member function:

template<class T> void writeToOuput(Ui::MainWindow* ui, T arg)
{
    ui->textEdit->append(QString("%1").arg(arg));
}

Rationale:

1. Less compile-time dependencies, faster builds.
2. Decouples the logic from your class as it does not need to be a member to function correctly.

Update: op posted the error.

Edit: Here’s the error I’m getting: ‘invalid use of incomplete type
ui::MainWindow’

You need to include the header that defines ui::MainWindow instead of just working with a forward declaration. For MOC-generated files like this, that’s generally going to be called ui_MainWindow.h.