qt openGL-绘制三角形

Question

So my goal is to (first) draw a triangle with openGL.

my questions:

1) How/when do both of my functions get called? I see that only one gets called. ie void MyGLWidget::paintGL. I am confused because as you can see I never call this function, it gets called automatically.I added a widget on my ui which I promoted to MyGLWidget. But when/why/how does it get (not) called?

my code: myglwidget.cpp

#include "myglwidget.h"
#include <QtWidgets>
#include <QtOpenGL>
#include <GL/glu.h>


MyGLWidget::MyGLWidget(QWidget *parent)
    : QGLWidget(QGLFormat(QGL::SampleBuffers), parent)
{
}

void MyGLWidget::initializeGL()
{
    glClearColor(1,1,0,1);
    qDebug("init"); //<-------never gets printed
}

void MyGLWidget::paintGL()
{
    glClear(GL_COLOR_BUFFER_BIT);
    qDebug("painting"); //<---- does get printed
    glColor3f(1,0,0);
    glBegin(GL_TRIANGLES);
        glVertex3f(-0.5,-0.5,0);
        glVertex3f(0.5,-0.5,0);
        glVertex3f(0.0,0.5,0);
    glEnd();
}

myglwidget.h

#ifndef MYGLWIDGET_H
#define MYGLWIDGET_H
#include <QGLWidget>

class MyGLWidget : public QGLWidget
{
    Q_OBJECT
public:
   explicit MyGLWidget(QWidget *parent = 0);

   void initializeGL();
   void paintGL();
   void resizeGL(int width, int height);
private:

};

#endif // MYGLWIDGET_H

main.cpp

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    Widget w;
    w.setWindowTitle("OpenGL with Qt DAO");
    w.show();

    return a.exec();
}

Answer 1

I do not see a class called Widget made by you which is connected to the class MyGlWidget. Maybe I am wrong but shouldn't you have to make a instance of MyGlWidget (Call it's constructor instead of Widget?)

Answer 2

Both functions are called internally by the QGLWidget super class. See QT Docs

In the documentation you also see that these virtual functions are protected . In your code they are public . So you have to change that in order to make it work.