From: Gavin Lambert (boost_at_[hidden])
Date: 2020-10-05 00:29:54
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.)