求教：first(),last()两个函数怎么用

今天看到QT两个字符串函数first()和last()，在自己机器的QT环境报错无此函数。环境：deepin 20.9，QT 5.15。
/home/hrhh-zengfc/qttest/sampl04_02_01/widget.cpp:-1: In member function ‘void Widget::on_pushButton_23_clicked()’:
/home/hrhh-zengfc/qttest/sampl04_02_01/widget.cpp:62: error: ‘class QString’ has no member named ‘first’
     QString str3 = str1.first(N + 1);            
                         ^~~~~
/home/hrhh-zengfc/qttest/sampl04_02_01/widget.cpp:64: error: ‘class QString’ has no member named ‘last’; did you mean ‘at’?
     str3 = str1.last(str1.size() - N - 1);        
                 ^~~~
请大神指教。