void Calculator::customButtonClicked()
{
    QToolButton *clickedButton = qobject_cast<QToolButton *>(sender());
    QFile file(clickedButton->property("scriptFileName").toString());
    if (!file.open(QIODevice::ReadOnly)) {
        abortOperation();
        return;
    }
    QTextStream in(&file);
    in.setCodec("UTF-8");
    QString script = in.readAll();
    file.close();
    QScriptEngine interpreter;
    QScriptValue operand(&interpreter, display->text().toDouble());
    interpreter.globalObject().setProperty("x", operand);
    QScriptValue result = interpreter.evaluate(script);
    if (!result.isNumber()) {
        abortOperation();
        return;
    }
    setDisplayValue(result.toNumber());
    waitingForOperand = true;
}
从QScriptValue operand(&interpreter, display->text().toDouble());
这里 operand的值就为空了,查看QScriptValue的构造函数没有参数是两个的构造函数。。。