[Development] I ❤️ Qt containers! :) (was: How qAsConst and qExchange lead to qNN)

Volker Hilsheimer volker.hilsheimer at qt.io
Tue Nov 8 14:25:36 CET 2022

> On 7 Nov 2022, at 21:15, Marc Mutz via Development <development at qt-project.org> wrote:


> Anyway; to all those who disagree when I say Qt should concentrate on 
> its core competencies and stop meddling with container classes, shared 
> pointers, etc, I say this: which of the two universes above would you 
> rather have lived in these past 30 years? A or B? Be truthful!

Historically, STL implementations were unusable and unreliable for cross platform development (we supported HP-UX, AIX, SGI, Sun back in those days), and generally incomplete (only a few associative containers pre-C++11). So, 30 or 20 years ago, or perhaps up to C++11 and until we could drop commercial Unix systems as irrelevant (esp for Nokia’s plans; although no idea about the quality of the STL for Symbian C++), the STL wasn’t really much of an option.

However, this is a more fundamental question about what we try to achieve with Qt. Qt has never tried to be a C++ library that follows the design principals of the std library. In many cases, we don’t even care that much about the single responsibility principle (hello, QWidget). Qt container classes have always been more than a dumb storage of data on top of which algorithms operate. QString is a very rich class with tons of functionality, specific to text handling. std::string is a sequence of characters. Working with QString vs std::string to deal with user-provided text input requires rather completely different mindsets.

Our core competence of designing intuitive APIs does not exclude container classes. That’s why with Qt containers, we can write

if (list.contains(value)) {
    // …

rather than

if (std::find(list.begin(), list.end(), value) != list.end()) {
    // ...

Perhaps it makes me an inferior C++ developer, but I rather prefer the former. Well, std::map got a contains(), and std::string a starts_with in C++ 20, only 25 years late(r).

Indeed, sometimes that convenience means that our users, and we ourselves, can do something silly like:

if (map.contains(key)) {
    value = map.value(key);
    // do stuff with value

Convenience is no excuse for us as developers of Qt to be sloppy. It is also no excuse for ignoring the new features we get into our toolbox when we move to C++ 20 or 23. But that C++ 20 finally introduces std::map::contains (but not std::vector::contains…), or adds std::span, is also no excuse for us to toss our own designs out of the window, and to demand that all Qt users must embrace the STL instead.

One of Qt’s goals has always been to make C++ accessible for people that come from other languages, with a programming model that is not rooted in how the C++ standard library does things. That programming language used to be Java - hence our Java-style iterators in Qt containers. Today, people perhaps rather learn programming with Python in school. There will always be more people that have learned programming with other languages, than those that have carefully studied the C++ standard and the impact of various constructs in Compiler Explorer. We must make it easy for the former, while also enabling the latter.

And there are the practical reasons why I don’t want to replace QList with std::vector and QHash with std::unordered_map: we store our data structures in the d-pointer, and we want to stay flexbile wrt the actual stored type. So copy-elision and return-value-optimization don't buy us much: we need to return copies of containers from our property getters. Not const references to containers, and not temporary lists that can be moved out. So we do need reference counting.

For the here and now, and the last 25 years of Qt and C++, it’s not helpful to argue that we will soon be able to return a type-erased span and get rid of “horrible and inefficient” APIs returning owning containers. std::span is a new tool, opening up new opportunities; the expressiveness of e.g. C++ ranges might even make it much easier for someone coming from e.g. Python to use Qt, while allowing us to write much more efficient code. So we do need to consider how we name and design our APIs today so that we can add new APIs to unlock that power in the future. And we need to keep looking for ways to improve what we have - with extra awareness of what potential changes mean for our users and co-contributors.

Those improvements cannot require that we force everyone to change significant amounts of existing code, all the time; or that they have to regularly unlearn established Qt patterns; or that they have to live without the convenience. Yes, I’m biased, but I honestly don’t see any universe where a Qt without our implicitly shared, owning, old-fashioned containers, and instead with only STL containers and programming paradigms, would have been as easy, or as much fun, to use.


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