Re: [PFR][review] Andrzej's review

From: Antony Polukhin (antoshkka_at_[hidden])
Date: 2020-10-07 10:30:36

ср, 7 окт. 2020 г. в 10:59, Andrzej Krzemienski via Boost
<boost_at_[hidden]>:
>
> śr., 7 paź 2020 o 09:22 Rainer Deyke via Boost <boost_at_[hidden]>
> napisał(a):
>
> > On 06.10.20 21:50, Andrzej Krzemienski via Boost wrote:
> > > struct Person {
> > >    std::string firstName;
> > >    std::string lastName;
> > >    int age;
> > > };
> > >
> > > struct Employee : Person {
> > >    double salary;
> > > };
> > >
> > > struct Prisoner : Person {
> > >    int cellNumber;
> > > };
> > > ```
> > > This (a) avoids duplication, and (b) I have the slicing do exactly what I
> > > need: convert an Employee to a Person. Now, I am disappointed with what
> > > C++17 did to aggregate initialization. My natural expectation would be
> > that:
> > >
> > > ```
> > > Employee e {"Bilbo", "Baggins", 111, 1000.00};
> > > ```
> > > Flattens the class hierarchy and initializes each of the four members.
> > > Apparently , the C++ committee has a different vision for it. But I have
> > > noticed that if I changed derivation into aggregation (as I indicated
> > > earlier), the "flat" part of FPR would do exactly what I needed.
> >
> > Does that example really work?  My expectation would be that flat
> > reflection would try to break apart the std::strings, and fail with a
> > compile-time error because std::string is not an aggregate.
> >
> > If it does work, then flat reflection is a lot more useful than I had
> > initially thought!
> >
>
> The following compiles and outputs "1 2 3 " on my GCC compiler:
>
> ```
> struct A {
>   int x, y;
> };
>
> struct B {
>   A a;
>   int z;
> };
>
> int main()
> {
>   B b = {{1, 2}, 3};
>   boost::pfr::flat_for_each_field(b, [](auto const& field){ std::cout <<
> field << " "; });
> }
> ```
>
> However, when I substitute strings for ints it no longer compiles. I am not
> sure if this is just a bug in the implementation or an inherent limitation
> of this library.

That's an inherent limitation of flat reflection.
Flat reflection is kind of a legacy - it's the first implemented
"reflection", and it was working only for PODs. Since early versions
of PFR new precise ways of reflection were discovered. Moreover,
precise reflection turned out to be much portable and works on more
compilers.

> I wonder what is your *primary* motivation for removing it:
> A. Because it is not implementable in many cases?
> B. Because there is not enough motivation for it, and people don't understand what it is for?
> C. Because it can be accidentally confused with the "precise" version?

It's rather 'D.' - unlike flat reflection, precise representation work
in much more cases and on more platforms (MSVC fails to do flat
reflection)

-- 
Best regards,
Antony Polukhin