[Development] QList for Qt 6

Edward Welbourne edward.welbourne at qt.io
Mon May 20 11:35:23 CEST 2019

On Thu, May 16, 2019 at 9:35 PM Vitaly Fanaskov <vitaly.fanaskov at qt.io> wrote:
>>   If a user needs a regular container, range might be simply assigned to it.

NIkolai Marchenko (16 May 2019 20:38) replied
> It depends on what you expect the average usecase to be.
> If we assume that a regular container is generally more used then you are preventing ppl from "almost always auto"

First: I don't believe we've committed to "almost always auto" as a Qt
coding style (it leaves the reader to work out what everything is, which
I - as a reviewer - really don't like).  Being explicit about what type
of container you want to use has its virtues (albeit, as Marc points
out, auto's ambiguity is good when the API is in flux).

Second: if we return a container, the API designer has to decide which
type of container to return, which forces the caller to do a conversion
if that wasn't the type they wanted.  Returning a range lets the caller
chose how to store the values.

However, that's only a win if the supplier wasn't already holding the
values in a container, which CoW lets us return cheaply.

The win (assuming C++ ranges work enough like python generators) comes
when you're traversing some population of things and selecting some to
return, while skipping the rest; classically that might be implemented
as a traversal with a call-back object to act on each item; but a range
lets the caller just iterate over the results found, turning the
call-back's code into a loop's body, which is far easier to follow; or
collecting the results into a container of the caller's choosing.


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