Bonita Montero wrote:
> Am 26.08.2023 um 07:05 schrieb Pavel:
>
>> Visitor does not need anything, it simply solves a particular class
>> of  problems that happens to include a structure of objects of
>> multiple types. Your visitor implementation is invalid if it cannot
>> solve a valid visitor problem.
>
> Either I didn't describe the difference to what you imagine correctly
> or you just didn't understand me.
None of the above it is just you are not trying to read.

> In the Visitor pattern, the Visitor
> needs a base class with a virtual method to derive from, and the derived
> object is passed by reference to the Visitor function of the objects
> being visited.
No. Visitor needs no types. The **problems that Visitor solves** include
data structures of multiple types. The applicability of Visitor is
exactly when the programmer is limited to small backward-compatible
changes to the type hierarchy **given in the problem, not "needed by
Visitor"**, ideally, no changes at all.

Try to read what's written, not what you want to see.

I will try one more time, in negative terms, in case it makes it easier
for you to comprehend:

No type hierarchy in the problem -- no need in Visitor in the solution.

Now, "a base class with a virtual method to derive from" are just
possible implementation details for the type hierarchy, so not 100%
needed. The type hierarchy could be a variant or another type hierarchy
where the dynamic type is detectable at run-time. There must be,
however, a comprehensive type hierarchy for the Visitor to be useful.

What you are trying to demonstrate, on the other hand, is called a
"higher-order function" in functional programming. And, no, it does not
need a new name because it already has an industry-standard name.

> However, the derived and the base class are superfluous
> with functional programming. Since C++11 you can simply request a
> function<> object and conveniently you just pass a lambda that when
> called, a temporary function<> object is created that represents the
> interface to the object that can be visited.

Am 30.08.2023 um 04:14 schrieb Pavel:

> No. Visitor needs no types. The **problems that Visitor solves** include
> data structures of multiple types. The applicability of Visitor is
> exactly when the programmer is limited to small backward-compatible
> changes to the type hierarchy **given in the problem, not "needed by
> Visitor"**, ideally, no changes at all.

You simply don't understand what I wrote.

Bonita Montero wrote:
> Am 30.08.2023 um 04:14 schrieb Pavel:
>
>> No. Visitor needs no types. The **problems that Visitor solves**
>> include data structures of multiple types. The applicability of
>> Visitor is exactly when the programmer is limited to small
>> backward-compatible changes to the type hierarchy **given in the
>> problem, not "needed by Visitor"**, ideally, no changes at all.
>
> You simply don't understand what I wrote.
>
Of course if you say so.