QT程序hello

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void QApplication::setMainWidget ( QWidget * mainWidget ) [static]
Sets the application's main widget to mainWidget.
In most respects the main widget is like any other widget, except that if it is closed, the application exits. QApplication does not take ownership of the mainWidget, so if you create your main widget on the heap you must delete it yourself.
You need not have a main widget; connecting lastWindowClosed() to quit() is an alternative.
On X11, this function also resizes and moves the main widget according to the -geometry command-line option, so you should set the default geometry (using QWidget::setGeometry()) before calling setMainWidget().

这样试一试#include <qapplication.h>#include <qpushbutton.h>int main( int argc, char **argv ){    QApplication a( argc, argv );    QPushButton hello( "Hello world!", 0 );    hello.resize( 100, 30 );    a.setMainWidget( &hello );    hello.show();    return a.exec();}