Create an instance of QScopedPointer later - qt

Question

For example here is my code
QScopedPointer<QTimer> timer2(new QTimer);
But I want to define
QScopedPointer<QTimer> timer2;
in mainwindow.h and create an instance
timer2(new QTimer);
in the mainwindow.cpp
How?

Answer 1

Try the following:
// mainwindow.h
class MainWindow : public QMainWindow
{
private:
QScopedPointer<QTimer> timer2;
};
If you want to create the instance in the constructor, use the following:
// mainwindow.cpp
MainWindow::MainWindow()
:timer2(new QTimer)
{
}
Alternately, if you want to create the instance in some arbitrary member function of MainWindow, use this:
// mainwindow.cpp
void MainWindow::someFunction()
{
timer2.reset(new QTimer);
}
It's also worth reviewing initialization lists in C++ and the documentation for QScopedPointer.

Answer 2

Use method reset of QScopedPointer
timer2.reset(new QTimer());

Answer 3

What you're doing amounts to a premature pessimization. You're creating members of a MainWindow class separately and individually on the heap, when you should be simply putting them into the class as members:
// interface
#include <QMainWindow>
#include <QTimer>
class MainWindow : public QMainWindow {
Q_OBJECT
QTimer m_timer;
public:
MainWindow(QWidget * parent = 0, Qt::WindowFlags flags = 0);
};
// implementation
MainWindow::MainWindow(QWidget * parent, Qt::WindowFlags flags) :
QMainWindow(parent, flags),
m_timer()
{
...
}
Of course, you would ordinarily not want to expose all the details of the MainWindow's implementation in the interface (header) file. Thus you would leverage the PIMPL idiom:
// interface
#include <QMainWindow>
class MainWindowPrivate;
class MainWindow : public QMainWindow {
Q_OBJECT
Q_DECLARE_PRIVATE(MainWindow)
QScopedPointer<MainWindowPrivate> const d_ptr;
public:
MainWindow(QWidget * parent = 0, Qt::WindowFlags flags = 0);
}
// implementation
#include "MainWindow.h"
#include <QTimer>
class MainWindowPrivate {
public:
QTimer timer;
}
MainWindow::MainWindow(QWidget * parent, Qt::WindowFlags flags) :
QMainWindow(parent, flags),
d_ptr(new(MainWindowPrivate())
{
Q_D(MainWindow);
d->timer.start( ... );
...
}