QXmlQuery :: evaluateTo（）函数返回空列表

Question

我正在尝试使用QXmlQuery解析XML文件。但是，当我运行QXmlQuery::evaluateTo()函数时，我得到一个空列表。 我的XML如下：

<?xml version="1.0" encoding="UTF-8"?>
<gpx>
    <trk>
        <trkseg>
            <trkpt lat="42.45612" lon="-5.45623">
                <ele>50.202291</ele>
                <time>...</time>
                <desc>lat.=42.45612, lon.=-5.45623, Alt.=50.202291m. Speed=12.225666m/h.</desc>
                <speed>4.121307</speed>
            </trkpt>
            <trkpt lat="50.166598" lon="-2.811355">
                <ele>47.236732</ele>
                <time>...</time>
                <desc>lat.=50.166598, lon.=-2.811355, Alt.=47.236732m. Speed=11.134823m/h.</desc>
                <speed>2.123456</speed>
            </trkpt>
         </trkseg>
    </trk>
</gpx>

我的代码如下：

    QFile file(fileName);
    if (!file.open(QFile::ReadOnly | QFile::Text)) {
       qDebug() << "Error: Cannot read file " << qPrintable(fileName)
                  << ": " << qPrintable(file.errorString())
                  << "\n";
    }
    QXmlQuery q;
    q.setFocus(&file);
    q.setQuery("//gpx/string()");
    bool valid = q.isValid();

    QStringList items;
    q.evaluateTo(&items);
    qDebug() << items;

任何帮助都会受到赞赏。谢谢。