[Development] Updating holdover API from Qt 1 times

[Kevin Kofler]

Sze Howe Koh wrote:
> 3) Pick one of the options above, but go one step further and use
> std::optional (C++17) instead of returning null objects. I imagine this
> should apply more broadly to the whole of Qt, not just the functions
> discussed here.

IMHO, std::optional conceptually makes sense only for types that are not 
able to natively represent a null state. This is not the case for 
implicitly-shared data objects, which have a natural native null state 
(where the shared d pointer is null). So std::optional is redundant and 
introduces an unwanted distinction between a null optional and a null 
object. And it also means that at least a boolean flag is unnecessarily 
added somewhere in memory.

        Kevin Kofler