How to make xml parser more clear - c++

Question

I have an xml file built in this way:
<?xml version="1.0" encoding="UTF-8"?>
<Draw>
<Input>
<Cells>
<width>100</width>
<height>28</height>
</Cells>
<Column>custom</Column>
<Custom>
<header id="0">one</header>
<header id="1">two</header>
<header id="2">three</header>
<header id="3">four</header>
<header id="4">five</header>
</Custom>
</Input>
<Output>
<Cells>
<width>82</width>
<height>20</height>
</Cells>
<Column>upper</Column>
<Custom>
<header id="0">alfa</header>
<header id="1">beta</header>
<header id="2">gamma</header>
<header id="3">delta</header>
<header id="4">epsilon</header>
</Custom>
</Output>
</Draw>
And I’m trying to extrapolate the values ​​of the header tag: since we have two sets of header tags (Input and Output) the only working code that I managed to work for now is this:
void MainWindow::readXmlFile() {
QString target;
QFile* file = new QFile("System/Settings.xml");
/* If we can't open it, let's show an error message. */
if (!file->open(QIODevice::ReadOnly | QIODevice::Text)) return;
QXmlStreamReader xmlReader(file);
/* We'll parse the XML until we reach end of it.*/
while(!xmlReader.atEnd() && !xmlReader.hasError()) {
QXmlStreamReader::TokenType token = xmlReader.readNext();
if(token == QXmlStreamReader::StartDocument) {
continue;
}
/* If token is StartElement, we'll see if we can read it.*/
if (token == 4) {
if (xmlReader.name() == "Input" || xmlReader.name() == "Output") {
target = xmlReader.name().toString();
while (!(xmlReader.tokenType() == QXmlStreamReader::EndElement && xmlReader.name() == target)) {
if (xmlReader.tokenType() == QXmlStreamReader::StartElement) {
qDebug() << xmlReader.name().toString();
if (xmlReader.name() == "width") {
QString num = xmlReader.readElementText();
//input->horizontalHeader()->setDefaultSectionSize(num.toInt());
}
if (xmlReader.name() == "height") {
QString num = xmlReader.readElementText();
//input->verticalHeader()->setDefaultSectionSize(num.toInt());
}
if (xmlReader.name() == "header") {
//headers->append(xmlReader.readElementText());
//qDebug() << xmlReader.readElementText();
}
//input->setHorizontalHeaderLabels(header);
}
xmlReader.readNext();
}
}
}
}
/* Error handling. */
if(xmlReader.hasError()) {
QMessageBox::critical(this,
"QXSRExample::parseXML",
xmlReader.errorString(),
QMessageBox::Ok);
}
xmlReader.clear();
}
Since this code seems very repetitive, especially from line 15 to 18, could you help me to make it a little cleaner? The examples in the web are not very explanatory.
Thanks in advance

Answer 1

I usually use lambda expressions. You will need to add CONFIG += c++11 to your .pro file.
Then define utilities for most repetitive patterns: for instance
/* If token is StartElement, we'll see if we can read it.*/
if (token == 4) {
auto name = [&]() { return xmlReader.name().toString(); };
auto type = [&]() { return xmlReader.tokenType(); };
auto num = [&]() { return xmlReader.readElementText().toInt(); };
if (name() == "Input" || name() == "Output") {
target = name();
while (!(type() == QXmlStreamReader::EndElement && name() == target)) {
if (type() == QXmlStreamReader::StartElement) {
qDebug() << name();
if (name() == "width") {
input->horizontalHeader()->setDefaultSectionSize(num());
}
else if (name() == "height") {
input->verticalHeader()->setDefaultSectionSize(num());
}
else if (name() == "header") {
//headers->append(xmlReader.readElementText());
//qDebug() << xmlReader.readElementText();
}
//input->setHorizontalHeaderLabels(header);
}
xmlReader.readNext();
}
}
unrelated warning: your code is leaking memory, instead of allocating with new, consider the simpler stack allocation:
QFile file("System/Settings.xml");
/* If we can't open it, let's show an error message. */
if (!file.open(QIODevice::ReadOnly | QIODevice::Text)) return;
QXmlStreamReader xmlReader(&file);

Answer 2

I encountered the same problem few weeks ago, I had to read and write some xml file (arround 20/50 lines).
I started with QXmlStreamReader/Writer and finally give up and use QDomDocument.
The main difference (in terms of performance) between these two objects is QDomDocument loads all the xml file in memory. The syntax is also quite easier with QDomDoc !
See the doc for some written examples: http://qt-project.org/doc/qt-4.8/qdomdocument.html