You need to implement all methods from 'customctrl.h' file in 'customctrl.cpp' file:
customctrl.cpp
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#include "customctrl.h"
customctrl::customctrl(QWidget *parent = 0) : QWidget(parent)
{
resize(50, 50);
setWindowTitle("Qt");
}
customctrl::~customctrl() {;}
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