qt error: variable `QPainter painter' has initializer but incomplete type

/home/yx/QTproject/helloWorld/mainwindow.cpp:39: error: variable ‘QPainter painter’ has initializer but incomplete type QPainter painter(this);

 QPainter painter(this);//这句话报错

^
解决方案：在.cpp文件中添加#include”qpainter.h”