Must the QGLWidget::makeCurrent () be called inside of the constructor?

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Must the QGLWidget::makeCurrent () be placed inside of the constructor?

such as:

QGLWidget::QGLWidget(QWidget *parent)
{
   ......
   makeCurrent();
  ......
      glBegin(GL_LINES);
    glVertex3f(+1.0, +1.0, -1.0);
    ...
}

QGLWidget::QGLWidget(QWidget *parent)
{
   ......
   makeCurrent();
  ......
      glBegin(GL_LINES);
    glVertex3f(+1.0, +1.0, -1.0);
    ...
}

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I tried this:

QGLWidget *canvas;
...
canvas->makeCurrent();
  ......
      glBegin(GL_LINES);
    glVertex3f(+1.0, +1.0, -1.0);
    ...

QGLWidget *canvas;
...
canvas->makeCurrent();
  ......
      glBegin(GL_LINES);
    glVertex3f(+1.0, +1.0, -1.0);
    ...

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However, in this way, it's blank, nothing is shown on the QGLWidget.