QPainter

[phil_]

Hello,

I want to use QPainter to draw very simple lines and points. I checked the docs and wanted to copy the example, but the following code doesn't work:

int main(int argc, char *argv[])
{
	QApplication app(argc, argv);
	QWidget fenster1;
	fenster1.resize(400,400);
 
 
	QPainter achsen(&fenster1);
 
	achsen.setPen(QPen(Qt::blue,2));
 
	achsen.drawPoint(200,100);
 
	achsen.drawLine(100,100,301,301);
 
	fenster1.show();
 
    return app.exec();
}

int main(int argc, char *argv[])
{
	QApplication app(argc, argv);
	QWidget fenster1;
	fenster1.resize(400,400);


	QPainter achsen(&fenster1);

	achsen.setPen(QPen(Qt::blue,2));

	achsen.drawPoint(200,100);

	achsen.drawLine(100,100,301,301);

	fenster1.show();
    
    return app.exec();
}

To copy to clipboard, switch view to plain text mode 

It doesn't return an error, but I can neither see the line nor the point. It just displays an empty window. Where is the mistake?

Thanks.

[jacek]

You can't paint on hidden widgets, because these operations will be just ignored. Another problem is that everything you paint on a widget isn't remembered, so if that widget is hidden or covered by another window, you will loose everything you have drawn. Third problem is that you shouldn't draw anything outside paintEvent() in Qt4, because of "backing store".

The correct solution is to subclass QWidget and reimplement paintEvent() (see the attachment).

[phil_]

Thanks very much. After trying to compile your main.cpp I get a problem that most likely is caused by my qmake:

release/main.moc:35: error: `Test' has not been declared
release/main.moc:40: error: `Test' has not been declared
release/main.moc:41: error: non-member function `const QMetaObject* metaObject()
' cannot have `const' method qualifier
release/main.moc:45: error: `Test' has not been declared
release/main.moc: In function `void* qt_metacast(const char*)':
release/main.moc:49: error: `Test' has not been declared
release/main.moc:49: error: expected `>' before '*' token
release/main.moc:49: error: expected `(' before '*' token
release/main.moc:49: error: expected primary-expression before '>' token
release/main.moc:49: error: invalid use of `this' in non-member function
release/main.moc:49: error: expected `)' before ';' token
release/main.moc:50: error: cannot call member function `virtual void* QWidget::
qt_metacast(const char*)' without object
release/main.moc: At global scope:
release/main.moc:53: error: `Test' has not been declared
release/main.moc: In function `int qt_metacall(QMetaObject::Call, int, void**)':
 
release/main.moc:55: error: cannot call member function `virtual int QWidget::qt
_metacall(QMetaObject::Call, int, void**)' without object
mingw32-make[1]: *** [release\main.o] Error 1
mingw32-make[1]: Leaving directory `D:/projekte/Schaubild/cpp2'
mingw32-make: *** [release] Error 2

release/main.moc:35: error: `Test' has not been declared
release/main.moc:40: error: `Test' has not been declared
release/main.moc:41: error: non-member function `const QMetaObject* metaObject()
' cannot have `const' method qualifier
release/main.moc:45: error: `Test' has not been declared
release/main.moc: In function `void* qt_metacast(const char*)':
release/main.moc:49: error: `Test' has not been declared
release/main.moc:49: error: expected `>' before '*' token
release/main.moc:49: error: expected `(' before '*' token
release/main.moc:49: error: expected primary-expression before '>' token
release/main.moc:49: error: invalid use of `this' in non-member function
release/main.moc:49: error: expected `)' before ';' token
release/main.moc:50: error: cannot call member function `virtual void* QWidget::
qt_metacast(const char*)' without object
release/main.moc: At global scope:
release/main.moc:53: error: `Test' has not been declared
release/main.moc: In function `int qt_metacall(QMetaObject::Call, int, void**)':

release/main.moc:55: error: cannot call member function `virtual int QWidget::qt
_metacall(QMetaObject::Call, int, void**)' without object
mingw32-make[1]: *** [release\main.o] Error 1
mingw32-make[1]: Leaving directory `D:/projekte/Schaubild/cpp2'
mingw32-make: *** [release] Error 2

To copy to clipboard, switch view to plain text mode 

Is there perhaps any option I have to change in any settings?

[jacek]

If you have saved that file as "main.cpp" in an empty directory, you should be able to compile it using:

qmake -project
qmake
make

[phil_]

No, that doesn't work.

[jacek]

No, that doesn't work.

How exactly it "doesn't work"? Do you issue these commands from the "Qt 4.x.x Command Prompt"?