printf for qstring

s002wjh

what is the easiest way to convert qstring to use in printf? for example

qstring tmp = "test"

printf(" %s \n", test);

Christian Ehrlicher

Chris Kawa

On Windows you can also use printf("%ls\n", qUtf16Printable(tmp)) which has the nice benefit of (usually) avoiding conversion. Note it's not applicable to any platform where wchar_t is not 2 bytes.

Kent-Dorfman

generally not the best way to handle qstrings but in direct answer to your question:

QString mystring("some string");
printf("%s\n", mystring.toStdString().c_str());

There may be a Qstring direct method to get the const char* address of the stirng data but the method I show is pretty solid.

IMHO, you really shouldn't be using c style printf in C++ code. Learn to use the streams I/O.

Christian Ehrlicher

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Chris Kawa

@Kent-Dorfman said:

generally not the best way to handle qstrings

Agreed, especially that it requires two libraries to do a simple task. You can stay in Qt world with

printf("%s\n", mystring.toUtf8().data());

but that's essentially what qPrintable() does so what @Christian-Ehrlicher said.

s002wjh

@Christian-Ehrlicher said in printf for qstring:

See qPrintable() - but why?

oh, QT was compiled as lib on my machine so I wasn't able to use qDebug or any qWarning etc.

also I have a custom function that printout the file name, and line numbers using printf for debugging. unless there is some c++ or Qt way to print out the file name and line numbers,?

Christian Ehrlicher

@s002wjh said in printf for qstring:

so I wasn't able to use qDebug or any qWarning etc.

See qInstallMessageHandler() and/or QLoggingCategory or use a proper logging library (printf is no function to print out debug / logging stuff) like e.g. Log4Qt

aha_1980

@Chris-Kawa

Note it's not applicable to any platform where wchar_t is not 2 bytes.

I'm not sure this limitation exists.

qUtf16Printable is used in Qt itself on Linux and Windows at least. Which platforms did you have in mind?

Regards

Chris Kawa

@aha_1980 said in printf for qstring:

I'm not sure this limitation exists.

Isn't the size of wchar_t on most Linux distros and mac 4 bytes? %ls takes wchar_t* and as far as I can tell all qUtf16Printable does is some forceful casting to silence compiler warnings. Still, it uses the underlying QString data directly. QString uses 2 byte chars, so I don't see how this would work?
Unless there's more funky trickery going on on Linux, I don't know. I mostly keep to Windows so if you know how that works I'd be interested to learn.

aha_1980

@Chris-Kawa,

you are right: https://doc.qt.io/qt-5/qtglobal.html#qUtf16Printable states that the result is no valid wchar_t. so you can use the result with qDebug and QString::asprintf, but not with printf.

Regards

Unsolved printf for qstring