> On Oct 13, 2015, at 1:46 PM, Marc Mutz <marc.m...@kdab.com> wrote: > > Hi, > > After looking quite a bit into the current state of string handling in Qt for > my QtWS talk last week, I have become frustrated by the state of string > handling in Qt. > > We have such powerful tools for string handling (QStringRef, QStringBuilder), > but all APIs outside QString and its immediate surroundings only deal in > QString. The correct way would be to overload every function taking QString > with QLatin1String and QStringRef versions, and then, for some other rare > cases, const QChar *, int size. Let alone std::basic_string<char16_t>. > > I would therefore like to propose to abandon QString for new API (and over > time phase it out of existing API), and only provide (const QChar*, size_t) > as > the most general form. I would propose to package the two into a class, > called > - you guessed it - QStringView. > > =FAQ= > > Q: Why not just use QStringRef? > > A: QStringRef is tied to QString. E.g. you can't create a QStringRef from a > pair of QChar*, int. It also is kind of stuck in historic mistakes making it > undesireable as a cheap-to-pass parameter type. > > Q: What mistakes do you refer to? > > A: The fact that it has copy ctor and assignment operator, so it's not a > trivally-copyable type and thus cannot efficiently passed by-value. It may > also > be too large for pass-by-value due to the rather useless QString pointer > (should have been QStringData*, if any). Neither can be fixed before Qt 6. > > Q: Why size_t? > > A: The intent of QStringView (and std::experimental::string_view) is to act > as > an interface between modules written with different compilers and different > flags. A std::string will never be compatible between compilers or even just > different flags, but a simple struct {char*, size_t} will always be, by way > of > it's C compatibility. > > So the goal is not just to accept QString, QStringRef, and (QChar*,int) (and > QVarLengthArray<QChar>!) as input to QStringView, but also > std::basic_string<char16_t> and std::vector<char16_t>. > > Q: What about the plans to make QString UTF-8-backed? > > A: QStringView-using code will need to be ported just as QString-using code > will. > > Q: What future do you have in mind for QStringRef? > > A: None in particular, though I have found a need for an owning QStringRef in > some places. But I expect Qt 6' QString to be able to provide a restricted > view on shared data, such that it would subsume QStringRef completely. > > Q: What about QLatin1String? > > A: Once QString is backed by UTF-8, latin-1 ceases to be a special charset. > We > might want something like QUsAsciiString, but it would just be a UTF-8 > string, > so it could be packed into QStringView. > > Q: What about QByteArray, QVector? > > A: I'm unsure about QByteArrayView. It might not pull its weight compared to > std::(experimental::)string_view, but I also note that we're currently > missing > a QByteArrayRef, so a QBAView might make sense while we wait for the std one > to become available to us. > > I'm actively opposed to a QArrayView, because I don't think it provides us > with anything std::(experimental::)array_view doesn't already. > > Q: What about a rope? > > A: A rope is a more complex string that can provide complex views on existing > data as well as store rules for generating stretches of data (as opposed to > the data itself). > > A rope is a very complex data structure and would not work as a universal > interface type. It would be cool if Qt had a rope, but that is outside the > scope of my proposal. > > Q: What do you mean when you say "abandon QString"? > > A: I mean that functions should not take QStrings as arguments, but > QStringViews. Then users can transparently pass QString, QStringRef and any > of > a number of other "string" types without overloading the function on each of > them. > > I do not mean to abandon QString, the class. Only QString, the interface type. > > Q: What API should QStringView have? > > A: Since it's mainly an interface type, it should have implicit conversions > from all kinds of "string" types, but explicit conversion _to_ those string > types. It should carry all the API from QString that can be implemented on > just a (QChar*, size_t) (e.g. trimmed(), left(), mid(), section(), split(), > but not append(), replace() (except maybe the (QChar,QChar) overload. > Corresponding QString/Ref API could (eventually) just forward to the > QStringView one. > > Thanks, now fire away, > Marc > > -- > Marc Mutz <marc.m...@kdab.com> | Senior Software Engineer > KDAB (Deutschland) GmbH & Co.KG, a KDAB Group Company > Tel: +49-30-521325470 > KDAB - The Qt Experts > _______________________________________________ > Development mailing list > Development@qt-project.org > http://lists.qt-project.org/mailman/listinfo/development
In general this sounds like a dangerous idea because it carries over all the old API concepts (i.e. (QChar *, size_t) is an extremely broken abstraction). You need to read and truly comprehend https://developer.apple.com/swift/blog/?id=30 before suggesting any changes to string-related APIs for the next major version of Qt, because if anything, THAT is what it should look like. Anything but that is a near-useless wrapper around binary data, not a true string class. -- Jake Petroules - jake.petroules at petroules.com
_______________________________________________ Development mailing list Development@qt-project.org http://lists.qt-project.org/mailman/listinfo/development