NaN seems to be defined in the XSD standard:

Yeah, understood. Its just that QXmlStreamWriter::writeTextElement() only knows about text (not floats), while QString::number() knows nothing about XML.

What you really want, is something like this:

QString xsdNumber(const double number, const char format, const int precision)
{
    if (qIsInf(number)) {
        return (number < 0) ? QStringLiteral("-INF") : QStringLiteral("INF");
    }

    if (qIsNaN(number)) return QStringLiteral("NaN");

    return QString::number(number, format, precision);
}

static void writeDoubleToXMLElement(QXmlStreamWriter & stream, const QString & elementName, const double value) {
        stream.writeTextElement(elementName, xsdNumber(value, 'g', 10));
}

Note, I considered templating xsdNumber()'s number type so you could use float, double or long double, but since QString::number() only supports double (and integers), there's currently no value in adding that complexity.

Cheers.

@ChrisW67 - NaN seems to be defined in the XSD standard:
The ·lexical space· of double is the set of all decimal numerals with or without a decimal point, numerals in scientific (exponential) notation, and the ·literals· 'INF', '+INF', '-INF', and 'NaN'
In https://www.w3.org/TR/xmlschema11-2/.
I wanted to benefit from that.

Ok, I will check this case by hand!

As @ChrisW67 said, the nan is hard-coded. For example, you can see it in Qt 6.4 here:

    } else if (qt_is_nan(d)) {
        if (bufSize >= 3) {
            buf[0] = 'n';
            buf[1] = 'a';
            buf[2] = 'n';
            length = 3;
    } ...

Assuming you cannot adjust the XML schema (maybe its provided by someone else?), you could simply handle NaN's manually, like:

stream.writeTextElement(elementName,
    qIsNan(value) ? QString("NaN") : QString::number(value, 'g', 10));

Cheers.

QXmlSchemaValidator checks your XML against the schema you provide. It is this schema that is rejecting "nan" in whatever element you have written it. Are you sure the schema will accept any non-number in that element?