QTextCodec canEncode, what is the expected behavior?

[Thuan_Firelight]

I am testing a few string conversions for use with an external non-unicode program on Windows using Qt 5.4.1. I have set my windows non-unicode program locale to Japanese and testing the 2 different strings using the code below:

    foreach(const QString& arg, arguments)
    {
        QTextCodec* codec = QTextCodec::codecForLocale();
        QByteArray localizedArg = arg.toLocal8Bit();
        if(QString::fromLocal8Bit(localizedArg) != arg)
        {
            qDebug() << arg << "codec->canEncode" << codec->canEncode(arg) << "QString::fromLocal8Bit(localizedArg) != arg";
        }
        else
        {
            qDebug() << arg << "codec->canEncode" << codec->canEncode(arg);
        }
    }

The variable "arguments" contains the following two strings:

1. d:/でｱﾋｨﾝ/1.10/Amber/新しいバンク 2.file
2. d:/1.10/Amber/你你.file

The first string contains a mixture of English and Japanese characters and the second string contains a mixture of English and Chinese characters.

Running through the loop above produces the following result in the output window:

"d:/でｱﾋｨﾝ/1.10/Amber/新しいバンク 2.file" codec->canEncode true
"d:/1.10/Amber/你你.file" codec->canEncode true QString::fromLocal8Bit(localizedArg) != arg

If characters are lost or incorrectly converted such that reversing the operation produces a different string, I would expect QTextCodec::canEncode to return false (i.e. if it's going to convert the foreign characters to "?").

Stepping through the code, I got into QTextCodec::canEncode which the return result is based on state.invalidChars == 0, and just before it, it invoked the QWindowsLocalCodec::convertFromUnicode which doesn't seem to do anything to the ConverterState passed in. Is this correct or a bug?

[aha_1980]

@Thuan_Firelight please search the forum, I remember a similar question some. I just dont remember the outcome.

[Thuan_Firelight]

@aha_1980 I did, and the closest one I found was this: https://forum.qt.io/topic/93921/unexpected-result-from-qtextcodec-canencode-qstring/7.

However, it seems the "solved" response was that "US-ASCII" is not recommended. I am not using "US-ASCII" however. The last response was from the OP and he reckon it is not restricted to "US-ASCII". I wanted to bump the thread, but the forum mechanism recommend I start a new thread as that one is quite old.

And there is also this bug report: https://bugreports.qt.io/browse/QTBUG-6925, the last comment state it was closed. So I checking what's the expected behavior before I comment further on the bug report to bump it up.

[aha_1980]

@Thuan_Firelight

I would recommend you to place a comment and vote on QTBUG-6925, which was re-opened by the way.

If you can break down your problem to a minimal, compile and testable example, please attach it there also - it helps debugging and fixing the problem.

You should of course try that with the latest release (which is 5.12-RC by now) - maybe there is already some improvement for your case.