[Development] Views

Mutz, Marc marc at kdab.com
Wed Jun 5 17:49:46 CEST 2019

On 2019-05-16 20:18, Mutz, Marc wrote:
> [1] Paraphrasing what Alex Stepanov teaches in his A9 courses: No C
> programmer would _ever_ get the idea to use a self-rebalancing
> red-black tree for something that holds a dozen elements. Because once
> you understand what is required to implement one, you'd shy away from
> the sheer complexity. Yet, in C++, just typing QMap makes the compiler
> do all that stuff for you. Don't use a map or a hash just because you
> can and the API is convenient. Use it when it makes sense, given what
> data is expected to be stored. And you will invariably end up with
> using vectors all over the place. According to Stepanov, developers
> wishing to use a map should seek a face-to-face meeting with their
> manager to explain why they need it :)

If you ever needed more convincing than an Alex Stepanov quote:


I repeat here what I said in the commit message, only stronger:

As a library implementer, you are simply not _allowed_ the freedom to 
use a convenient tool over the most efficient one. That is, to put it 
mildly, a disservice to users and a disgrace to the profession of 
programmers. 8KiB just to look up a pointer in a map of {string, int}? 
That's 1/4th of the icache size of many processors!

And I'm not git-blaming to look up who wrote that QMap in the first 
place. Qt is *full* of this stuff and you all should really understand 
one thing: It's not about _your_ convenience. You are working for your 
_users_. And this isn't rocket science. Almost _twenty_ years ago, Scott 
Meyers wrote in Effective STL: "Prefer to use a sorted vector".

Is this code performance-critical? Probably not. But ... I mean ... 8 
fscking Kilobytes. That's two pages that don't need to be paged in. 
Multiply that by a 100 or so uses of QMap in a normal Qt application and 
you see where this is going.

Please, think of yours truly what you want, but keep your _users_ in 
mind. Users who need to strip down Qt so it fits on their devices 
because ... well, because you were too lazy to pick the right data 


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