How does designer create a Line widget?

In Qt 5.7 the code generated by Qt Designer for a Horizontal Line (which can be checked in the menu using "Form/View Code...") is:

QFrame *line;
line = new QFrame(Form);
line->setFrameShape(QFrame::HLine);
line->setFrameShadow(QFrame::Sunken);

This will create the lines you see in Qt Designer.

The current answers do not seem to give working solutions, here is a comparison of all answers (this solution is the first line):

Full code:

#include <QtWidgets>

int main(int argc, char **argv) 
{
  QApplication app(argc, argv);

  QWidget widget;
  auto layout = new QVBoxLayout;
  widget.setLayout(layout);
  widget.resize(200, 200);

  auto lineA = new QFrame;
  lineA->setFrameShape(QFrame::HLine);
  lineA->setFrameShadow(QFrame::Sunken);
  layout->addWidget(lineA);

  QWidget *lineB = new QWidget;
  lineB->setFixedHeight(2);
  lineB->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Fixed);
  lineB->setStyleSheet(QString("background-color: #c0c0c0;"));
  layout->addWidget(lineB);

  auto lineC = new QFrame;
  lineC->setFixedHeight(3);
  lineC->setFrameShadow(QFrame::Sunken);
  lineC->setLineWidth(1);
  layout->addWidget(lineC);

  QFrame* lineD = new QFrame;
  lineD->setFrameShape(QFrame::HLine);
  layout->addWidget(lineD);

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

I guess you mean a horizontal / vertical line widget: it's just a simple QWidget with a gray background color and the horizontal is a fix height (1-3 pixel) and expanding width widget, the vertical is a fix width expanding height widget.

Horizontal example code:

QWidget *horizontalLineWidget = new QWidget;
horizontalLineWidget->setFixedHeight(2);
horizontalLineWidget->setSizePolicy(QSizePolicy::Expanding, QSizePolicy::Fixed);
horizontalLineWidget->setStyleSheet(QString("background-color: #c0c0c0;"));

Check out QFrame::setFrameShape(). To get a line, use either QFrame::HLine or QFrame::VLine as the function's argument.

// Create a horizontal line by creating a frame and setting its shape to QFrame::HLine:
QFrame* hFrame = new QFrame;
hFrame->setFrameShape(QFrame::HLine);

// Create a vertical line by creating a frame and setting its shape to QFrame::VLine:
QFrame* vFrame = new QFrame;
vFrame->setFrameShape(QFrame::VLine);

It is a QFrame with height 3, sunken shadow and line width equal to 1. You can see it if examine header generated by uic tool.