Qt中PaintWidget在MainWindow下不能绘制

Question

main.cpp

 #include "mainwindow.h"
#include <QApplication>
#include "paintwidget.h"
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
        w.showchild();
    return a.exec();
}

mainwindow.cpp

 #include "mainwindow.h"
#include "ui_mainwindow.h"
#include "paintwidget.h"
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
resize(800,600);
}
void MainWindow::showchild(){
    PaintWidget widget(this);
    widget.show();
}
MainWindow::~MainWindow()
{
    delete ui;
}

paintwidget.cpp

 #include "paintwidget.h"
#include <Qpainter>
#include "qpainter.h"

PaintWidget::PaintWidget(QWidget *parent) : QWidget(parent)
{
 resize(800,600);
 setWindowTitle(tr("paint demo"));
}
void PaintWidget::paintEvent(QPaintEvent *){
    QPainter painter(this);
    painter.drawLine(80,100,650,500);
    painter.setPen(Qt::red);
    painter.drawRect(10,10,100,400);
    painter.setPen(QPen(Qt::green,5));
    painter.setBrush(Qt::blue);
    painter.drawEllipse(50,150,400,200);
}

没有报错，但是什么也没绘制出来。

Answer 1

你的widget对象要动态分配出来才可以