Re: [PFR] Some questions

From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2020-10-05 08:02:10

pon., 5 paź 2020 o 09:50 Andrzej Krzemienski <akrzemi1_at_[hidden]>
napisał(a):

>
>
> pon., 5 paź 2020 o 02:30 Gavin Lambert via Boost <boost_at_[hidden]>
> napisał(a):
>
>> This is not (yet?) a review, although I guess this could be counted as a
>> partial review towards the current state of the docs; but after reading
>> them I have several questions.
>>
>> 1. Why is the term "reflection" used at all?  As far as I am aware, this
>> is primarily used to refer to accessing member names from the structure,
>> which is not something that this library provides at all, so at best
>> this seems highly misleading.  The original "magic get" name seems more
>> appropriate since it is primarily about extending tuple-get to basic
>> structures without boilerplate macros.
>>
>> 2. An up-front clarification on the limitations of supported structures
>> would be nice.  "Aggregate initialisable" is not a concept that everyone
>> is familiar with.
>>
>> 3. What is the motivation for "flat" reflection to exist, at all?  I
>> can't find any explanation of why one might want to do it; other than
>> completely disregarding type safety, which seems like a bad thing.  (I
>> assume there is some reason that I'm not aware of, but that's why an
>> explanation would be nice.)
>>
>> 4. Flat reflection is stated to be non-portable, raising further
>> questions as to why it exists at all.
>>
>> 5. Many of the intro pages talk about "disabling loophole" with no
>> explanation of what that is.  The configuration macros page finally
>> presents a link that doesn't really explain anything anyway, other than
>> suggesting it is a Dark Magic that was intended to be banned but nobody
>> had gotten around to it yet.
>>
>> 6. Speaking of the configuration macros page, it doesn't indicate what
>> values are the defaults, other than it "auto-detects your compiler".  I
>> assume from the surrounding text that it would prefer to use C++17 and
>> would use "loophole" (whatever that is) otherwise, but it would be good
>> to make that (or whatever it actually does instead) explicit.
>>
>>
>> Granted #3 can't get you into *too* much trouble with the limitation on
>> only supporting aggregate-initialised types... but on the other hand,
>> type hierarchies are still significant for aggregates (it can be
>> important to distinguish a "handle" from a plain int, or a Boost.Units
>> value from another with different unit).  And it feels like you're doing
>> C++ wrong if you're using aggregate types much; they're only a little
>> better than PODs.
>>
>> (In all existing codebases I use, there are almost no aggregate types,
>> although there are a few almost-aggregates that have simple initialising
>> constructors, for example, or make member fields private and use a
>> get-set method pattern "just in case".  I imagine this is likely to be
>> true of most real-world codebases.)
>>
>> Having said that, I can see some value in aggregate types as DTOs (for
>> database/json/etc translation) and for reducing usage of std::pair and
>> std::tuple, which is a good thing, though only if used in limited scope.
>>   But that usage doesn't explain "flat" either; the type hierarchy still
>> should be important.
>>
>> Precise reflection, on the other hand, seems more potentially useful,
>> save for the unfortunate -- though understandable -- limitation on only
>> aggregates.  Having said that, I've personally never found a use-case
>> for a tuple-like get interface for anything, so perhaps I'm just not the
>> target audience for this library.
>>
>> (I also have a strong dislike for aggregate initialisation being
>> order-based in the first place; I would have preferred something like
>> C99's named initialisation.  C++20 is adding something that they're
>> calling that, but is utterly useless and crippled instead of doing it
>> properly.)
>>
>
> I think that the library serves a useful purpose -- admittedly, a small
> one -- but it fails to document it properly.
> The normal work flow would be something like the following. I am using a
> library similar to Boost.Spirit or a clever DB access library. They all
> need a user-provided tuple type or a type with tuple-like interface. So,
> basically, I could use `std::tuple<std::string, int, double>`. But if I do
> that, the individual types mean nothing, because there is no way to give
> them names. At this point I decide I will use a Simple Aggregate (the
> library should define this concept):
>
> ```
> struct Record {
>   std::string firstName;
>   std::string lastName;
>   int age;
>   double salary;
> };
> ```
>
> Its only purpose is to be a tuple with named individual fields. But now I
> have to provide index-based access. I could either specialize the
> tuple_size and the other type traits, or I can use the PFR library which
> does it for me.
>
> The library should really document the above flow as a motivating use
> case. Then everyone would be clear about its scope.
>
> I agree that the library should not use the term "reflection", I agree
> that "magic get" better reflected what it is doing, and I also do not see a
> use case for flat reflection. I do however see a use case like this I have
> three similar records:
>
> ```
> struct Person {
>   std::string firstName;
>   std::string lastName;
>   int age;
> };
>
> struct Employee : Person {
>   double salary;
> };
>
> struct Prisoner: Person {
>   int cellNumber;
> };
> ```
>
> And I would like to get index-based access to all of these. I understand
> that PFR will not be able to provide it magically, but if there was a way
> to define a tuple_size and tuple_get for them with as few declarations as
> possible, that might have been of help.
>
> Regards,
> &rzej;
>

Having said that, I now actually see how the "flat" part can address my use
case. I need to define my types like this:

```
struct Person {
  std::string firstName;
  std::string lastName;
  int age;
};

struct Employee {
  Person person;
  double salary;
};

struct Prisoner {
  Person person
  int cellNumber;
};
```

This library is great (at what it is doing). It just does not demonstrate
it clear enough in the documentation.

I guess the key to understanding and appreciating it is to understand the
flow:
1. First, I have a library (Spirit, "Clever SQL") that requires structures
with index-based access.
2. Second, I define aggregates *only* for the purpose of working with this
library.
3. PFR helps me with plugging these aggregates into the library.

IOW, you appreciate the PFR library when you create aggregates as an
intermediate representation of data from libraries like Boost.Spirit.

Regards,
&rzej;