[Interest] Confused about QtConcurrent mappedReduced() documentation

[Thiago Macieira]

On Friday, 3 June 2022 14:04:55 PDT Sean Murphy via Interest wrote:
> The issue I'm seeing is that there doesn't appear to be any way to
> initialize the "collage" QImage BEFORE the first call to addToCollage().
> Instead, it seems like behind the scenes, the mappedReduced() function
> creates a default constructed QImage, and then passes that to
> addToCollage(). Then the call to QPainter::drawImage() inside
> addToCollage() can't actually do anything because the underlying QImage is
> null. Each successive call to addToCollage() passes the previously
> "modified" QImage in to be modified further, but since it's still a null
> QImage, nothing actually happens.

Then you can detect the situation and initialise it to the right size on first 
use.

But no, this is a limitation in the API. If we take the prototypical example 
of a reduction, std::accumulate, it does have an initial value:
https://en.cppreference.com/w/cpp/algorithm/accumulate

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Cloud Software Architect - Intel DCAI Cloud Engineering