[Development] How qAsConst and qExchange lead to qNN

Ulf Hermann ulf.hermann at qt.io
Mon Nov 14 17:37:15 CET 2022

Hi Marc,

> On 11.11.22 09:35, Ulf Hermann via Development wrote:
>> There is an undeniable benefit of _offering_ QSpan, QStringView, and
>> generator APIs in a few relevant cases:
>> 1. Users want to pass a "foreign" container to a Qt function that
>> doesn't only store it as QList or QString. It might merely iterate it or
>> store it as something else.
> The assumption that there's a problem only for "foreign containers" is
> incorrect: Take the native Qt container QString as an example. See
> assembly in my QAnyStringView blog post:
> https://www.qt.io/blog/qstringview-diaries-qanystringview You have this
> problem as soon as you pass constant data, which is a common enough
> use-case to warrant optimizing for.

My point is the "doesn't only store it as QList or QString" and 
"foreign" is in quotes quite on purpose. Maybe that wasn't clear enough, 

So, if the method immediately converts whatever it gets to QList or 
QString, then there is no point in passing it a span or view. Otherwise, 
since we don't know what people are going to do with the method (pass 
foreign containers, subsets, etc), a span or view may be adequate.


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