Iterating XmlElement and calling removeChildElement

General JUCE discussion
1

I hit a little speed bump that I figured out a workaround for, but curious if I’m overlooking a simpler solution.

In short, I was iterating over an XmlElement using forEachXmlChildElement, checking each child element for a condition, and then possibly removing the child element. This was crashing when it would loop through the next iteration, following an iteration where removeChildElement was called (if I remember right, the next child pointer wasn’t null, but its attributes were).

Anyways, my solution was to make a copy of the XmlElement to iterate, but then reference the original XmlElement when I needed to remove a child element. Like so:

// ...XmlElement presetXml was passed into this method
auto const presetXmlCopy = presetXml;  // make a copy to iterate over, that we won't modify
forEachXmlChildElement(presetXmlCopy, child)
{
    String paramId = child->getStringAttribute("id");

    // check if param exists in APVTS
    if (auto param = parameters.getParameter(paramId))
    {
        DBG("Parameter " + paramId + " found in APVTS. Keep it.");
    }
    else
    {
        DBG("Parameter " + paramId + " NOT found in APVTS! Remove it.");

        // remove child from ORIGINAL presetXml element,
        if (auto badChild = presetXml.getChildByAttribute("id", paramId))
        {
            presetXml.removeChildElement (badChild, true);
        }
}

(This is in the context of a plug-in, to sanitize XML preset files that a user can save to/load from disk, and remove any PARAM child elements from the XML with unknown ids.)

This works fine as-is, but it relies on identifying the child to remove by using getChildByAttribute. So it means I can’t remove child elements by non-unique criteria, like
if ( ! child->hasTagName("PARAM"))

Is there a way to remove child elements from an XmlElement while iterating over it?

2

The two approaches I know are either iterating in reverse order, or collect all children and delete them afterwards in one go:

Array<XmlElement*> toBeDeleted;

forEachXmlChildElement(presetXmlCopy, child)
{
    const auto paramId = child->getStringAttribute("id");

    if (parameters.getParameter (paramId) == nullptr)
        toBeDeleted.add (presetXml.getChildByAttribute ("id", paramId));
}

for (auto* badChild : toBeDeleted)
    presetXml.removeChildElement (badChild, true);
2 Likes
3

Oh, yeah, that probably makes more sense to make an array of all the ones you want removed, and collecting their pointers directly, rather than identify them by an attribute like I was.

It definitely opens up the possibility of checking if ( ! child->hasTagName("PARAM")). Thanks!