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Partially on the way to complete 17 reflection.

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I can't figure out why my computation on empty bases is 1 higher
than it should be -- so I'm punting, for now.  I'll revisit it
when I have more time.
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ADAM David Alan Martin
2021-10-26 05:04:25 -04:00
parent 304640d76b
commit 7b7e8a1b67
6 changed files with 478 additions and 0 deletions
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Reflection/tuplizeAggregate.h Normal file
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static_assert( __cplusplus > 201700, "C++17 Required" );
#pragma once
#include <Alepha/Alepha.h>
#include <boost/preprocessor.hpp>
#include <Alepha/Reflection/detail/config.h>
#include <Alepha/Reflection/aggregate_members.h>
namespace Alepha::Hydrogen::Reflection
{
inline namespace exports { inline namespace tuplize_aggregate {} }
namespace detail::tuplize_aggregate
{
inline namespace exports
{
/*!
* Deconstruct an aggregate object into a tie-based tuple pointing at its members.
*
* C++17's primary new reflection-oriented introduction is Structured Binding Declarations.
* What these let one do is to introduce a set of named variables that bind to each member
* of a (raw aggregate) `struct` in turn. This leads to some very interesting forms of
* "reflection" about what a user defined type is made of. Combined with the `std::is_aggregate`
* trait function and a way to determine the number of member objects, this provides a
* powerful new way to inspect any type.
*
* Structured Binding Declarations can also be used with arrays or types which implement a subset
* of the `std::tuple` interface. Those cases are not as interesting. We've always had the
* ability to inspect arrays via templates -- simple deduction operations work for that. And
* C++11's `std::tuple`s are already inspectable by their nature and types which implement a tuple-like
* interface are also easily inspected by pre-C++17 means.
*
* The most important thing C++17 Structured Binding brings to the language is the ability to
* (at compiletime) programmatically inspect any structure's members -- to learn their types,
* and with a bit of special effort, to learn their offsets. The names of those members are
* hidden, but their types are available, as is a way to work with all of them at once. Any
* Structured Binding is sufficient to do this -- one need only give a new name for each member of
* the type. `auto &[ a, b, c, d ]= someStruct;` is all that is needed and one has already performed
* an interesting feat of rudimentary reflection on the type `someStruct`. By loading those values
* into a tuple (by reference), by code such as `std::tie( a, b, c, d )`, a programmer can provide
* an anonymized, distilled reflection of the contents of that `struct`. This said, a library function
* which can decompose any `struct` into such a tie is very useful. `tuplizeAggregate` is exactly this.
*
* This function contains a pre-built set of such decompositions for structs of various sizes. C++17
* does not permit arbitrarily sized Structured Bindings, and so a limit had to be placed. The limit
* is fairly generous, however. If an aggregate size which is greater than the pre-build maximum is
* provided, then the compile will fail on a `static_assert` indicating this.
*
* Unfortunately, as a declaration syntax, the number of members in a `struct`'s body cannot be inferred
* through SFINAE by this means. Normally the user must explicitly provide the number of member
* variables. However, combined with a pair of C++11 features (based upon variadic templates and
* aggregate initialization syntax) we can infer the number of memmber values via a set of helper
* templates (which can also be called directly.)
*
* This kind of reflection into an aggregate type can prove very useful. Code generators for
* serialization, conversion tools, universal utility functions, and much more can all be built in
* C++17, today, using this kind of reflection! There's no need to wait for C++23 or beyond when
* static reflection is added to the language. A great deal of desired reflection use cases can be
* attained today. One just need write some code generators in terms of `std::tuple` and `std::tie`,
* then make any overloads (perhaps using ADL hooking tricks) which call `Alepha::Reflection::tuplizeAggregate`
* and pass that result to the general tuple form. For serializers and such, other techniques such as
* `boost::core::demangle( typeid( instance ).name() )` can be used to get nice names for types when
* implementing universal serializers. In fact, this can be used as a crutch for serializing more
* complicated user types (with private data and such). Those types can produce an aggregate "view"
* of what they must serialize or deserialize, and then they can hand that view off to such code
* generators. And, of cousre, this need not apply just to serialization.
*
* @param agg Aggregate instance to decompose into a `std::tie` based `std::tuple`.
* @tparam aggregate_size The number of members in the aggregate argument `agg`'s definition.
* @tparam Aggregate The type of the aggregate to decompose.
*/
// TODO: Make `aggregate_size` deduced via `Reflection::aggregate_ctor...` means.
template< std::size_t aggregate_size, typename Aggregate, typename= std::enable_if_t< not std::is_rvalue_reference_v< Aggregate > > >
constexpr decltype( auto )
tuplizeAggregate( Aggregate &&agg )
{
static_assert( std::is_aggregate_v< std::decay_t< Aggregate > >, "`tuplizeAggregate` only can be used on aggregates" );
// TODO: Generate these cases via boost preprocessor, to cut down on repetition...
if constexpr( aggregate_size == 0 ) return std::tuple{};
else if constexpr( aggregate_size == 1 )
{
auto &[ a0 ]= agg;
return std::tie( a0 );
}
else if constexpr( aggregate_size == 2 )
{
auto &[ a0, a1 ]= agg;
return std::tie( a0, a1 );
}
else if constexpr( aggregate_size == 3 )
{
auto &[ a0, a1, a2 ]= agg;
return std::tie( a0, a1, a2 );
}
else if constexpr( aggregate_size == 4 )
{
auto &[ a0, a1, a2, a3 ]= agg;
return std::tie( a0, a1, a2, a3 );
}
else if constexpr( aggregate_size == 5 )
{
auto &[ a0, a1, a2, a3, a4 ]= agg;
return std::tie( a0, a1, a2, a3, a4 );
}
else if constexpr( aggregate_size == 6 )
{
auto &[ a0, a1, a2, a3, a4, a5 ]= agg;
return std::tie( a0, a1, a2, a3, a4, a5 );
}
else if constexpr( aggregate_size == 7 )
{
auto &[ a0, a1, a2, a3, a4, a5, a6 ]= agg;
return std::tie( a0, a1, a2, a3, a4, a5, a6 );
}
else if constexpr( aggregate_size == 8 )
{
auto &[ a0, a1, a2, a3, a4, a5, a6, a7 ]= agg;
return std::tie( a0, a1, a2, a3, a4, a5, a6, a7 );
}
else if constexpr( aggregate_size == 9 )
{
auto &[ a0, a1, a2, a3, a4, a5, a6, a7, a8 ]= agg;
return std::tie( a0, a1, a2, a3, a4, a5, a6, a7, a8 );
}
else if constexpr( aggregate_size == 10 )
{
auto &[ a0, a1, a2, a3, a4, a5, a6, a7, a8, a9 ]= agg;
return std::tie( a0, a1, a2, a3, a4, a5, a6, a7, a8, a9 );
}
else if constexpr( aggregate_size == 11 )
{
auto &[ a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10 ]= agg;
return std::tie( a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10 );
}
else if constexpr( aggregate_size == 12 )
{
auto &[ a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11 ]= agg;
return std::tie( a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11 );
}
else if constexpr( aggregate_size == 13 )
{
auto &[ a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12 ]= agg;
return std::tie( a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12 );
}
else if constexpr( aggregate_size == 14 )
{
auto &[ a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13 ]= agg;
return std::tie( a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13 );
}
else if constexpr( aggregate_size == 15 )
{
auto &[ a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14 ]= agg;
return std::tie( a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14 );
}
else if constexpr( aggregate_size == 16 )
{
auto &[ a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15 ]= agg;
return std::tie( a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15 );
}
// Impossible, in this case -- we would have taken the original 0 branch were this so.
else static_assert( aggregate_size == 0, "The specified aggregate has more members than `tuplizeAggregate` can handle" );
}
// This overload deduces the aggregate size using the initializer inspection utilities.
template< typename Aggregate >
constexpr decltype( auto )
tuplizeAggregate( Aggregate &&agg )
{
return tuplizeAggregate< aggregate_member_count_v< std::decay_t< Aggregate > > >( std::forward< Aggregate >( agg ) );
}
}
}
namespace exports::tuplize_aggregate
{
using namespace detail::tuplize_aggregate::exports;
}
}