[Interest] Converting std::string.c_str() code to use QString methods

Michael Jackson imikejackson at gmail.com
Fri Aug 30 22:00:17 CEST 2013

On Aug 30, 2013, at 3:45 PM, Andreas Pakulat <apaku at gmx.de> wrote:

> Hi,
> On Fri, Aug 30, 2013 at 9:08 PM, Michael Jackson <imikejackson at gmail.com> wrote:
> I have a large code base that we are migrating to more fully utilize Qt classes. Many places in the code have lines like the following:
> std::string path("/path/to/foo.txt");
> FILE* f = fopen(path.c_str(), "wb");
> If the "path" variable is now declared as a QString which QString method would be me the equivalent as the "c_str()" of std::string?
> Not sure if I should be using "toASCII()" or "data()" or "toUtf8()" or what exactly.
> Google didn't really give me a definitive answer.
> In this particular case you want QFile::encode(path).data() to get the string encoded in a way that fopen can handle - assuming the path actually comes from the user originally. Otherwise http://qt-project.org/doc/qt-4.8/qfile.html#encodeName says all hardcoded paths should be ascii anyway so you could use toAscii().data().
> In other places you may need other function, depending on what encoding the function the char* is passed to expects. For example for the various print-functions you'd usually use toLocal8Bit().data() to make sure that the text is printed in the users current locale. Beyond that you'll have to consult the documentation/implementation of whatever function you want to pass the string data to wether it expects a certain encoding or requires ascii etc.
> The only thing to watch out for is that toUtf8() gives you a temporary ByteArray and hence calling data() on it yields a char-pointer to a temporary memory location. So if you pass this to a function that holds onto the data beyond its execution you'll need to use something like this to keep the QByteArray around long enough:
> QString path("...");
> QByteArray ba = path.toUtf8();
> functionThatKeepsReferenceToChar(ba.data());
> ...
> Andreas

Thanks for the nice explanations. In some places we use FILE*, some iostream, some custom file. There are other places that we use the HDF5 library and it is "C" based and takes "char*" for all of its "string" type arguments. Was hoping for a nice global "search and replace" but I'll have to take it line by line.

Thanks All.
Mike Jackson

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