libzypp 17.31.23
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A simple filter is a function or functor matching the signature:
A simple filter is a function or functor matching the signature:
bool
. Anything which is convertible into a bool
will do;Besides basic filter functors which actually evaluate the ResObject
(e.g. ByKind, ByName) you may use Functors for building compex queries. to build more complex filters.
As you can see in the last example there is no difference in using a filter or an action functor, as both have the same signature. A difference of course is the way forEach interprets the returned value.
Consequently you can netgate and chain actions as well. Thus PrintAndCount(counter)
could be chain(Print(),Count(counter))
, if these functors are provided.
PrintAndCount
is an example how a functor can return data collected during the query. You ca easily write a collector, that takes a std:list<ResObject::Ptr>&
and fills it with the matches found.
But as a rule of thumb, a functor should be lightweight. If you want to get data out, pass references to variables in (and assert these variables live as long as the query lasts). Or use std::ref.
Internally all functors are passed by value. Thus it would not help you to create an instance of some collecting functor, and pass it to the query. The query will then fill a copy of your functor, you won't get the data back. (Well, you probabely could, by using boosr::ref).
Why functors and not plain functions?
You can use plain functions if they don't have to deliver data back to the application. The C-style
approach is having functions that take a void * data
as last argument. This data
pointer is then passed arround and casted up and down.
If you look at a functor, you'll see that it contains both, the function to call (its operator()
) and the data you'd otherwise pass as void * data
. That's nice and safe.