forked from Alepha/Alepha
187 lines
6.6 KiB
C++
187 lines
6.6 KiB
C++
static_assert( __cplusplus > 2020'99 );
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#pragma once
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#include <Alepha/Alepha.h>
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#include <tuple>
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#include <utility>
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#include <type_traits>
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#include <Alepha/Concepts.h>
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#include <Alepha/Reflection/detail/config.h>
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#include <Alepha/Reflection/aggregate_initializer_size.h>
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#include <Alepha/Meta/overload.h>
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#include <Alepha/Meta/type_value.h>
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namespace Alepha::Hydrogen::Reflection ::detail:: aggregate_members_m
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{
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inline namespace exports {}
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using Meta::overload;
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/*!
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* Basic methodology here.
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*
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* The number of members in an aggregate is equal to the number of initializer parameters it takes less
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* the number of empty base classes it has. In simple terms, this would be `init_terms< T > - empty_bases< T >`,
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* However, it's not that simple. To do that the easy way, one might need get `std::tuple< InitTerms... >` and then compute
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* which terms were bases. Now that gets a bit complicated, as in C++ one can't just directly get a tuple of initializer
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* arguments (portably) to scoop out the arguments and analyze them one-by-one. One can, however, constrain the arguments
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* one-by-one in templates. Those constraints cannot directly leak out the types they conclude, as that requires side
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* effects. (Yes, template-friend-injection can be used here, but these mechanisms are extremely delicate. They're
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* not really portable. Further, the way that constraints get instantiated for matching is prone to complications.)
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*
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* Instead, a side-stepping approach is required. It's trivial to ask: "Can this object be constructed from these
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* N adaptive types, where the first one is constrained to be a base class of your object's type?" If yes, then
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* this proves a (likely) empty base. One can just recursively iterate through more an more constrained adaptive
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* types until the first non-base type is reached. At this point, there are no more than that many base classes.
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*
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* There may actually be fewer base classes, however. Consider:
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*
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* ```
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* struct SneakyBase {};
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*
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* struct Complicated : SneakyBase
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* {
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* SneakyBase member;
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* };
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* ```
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* In that case, a constrained adaptable argument would see two base types. Here is where a bit of C++ trivia and
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* knowledge comes into play. C++ forbids repetition of a base class's type. Therefore the sequence of base classes
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* cannot have repeats. The solution is to perform a nested exploration of instantiations of `checker` types which
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* has each descent disable casting to `std::is_base_of_v` types which already have been expanded. Thus whatever
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* count this expands to, it will be the correct empty-bases count. Then that count can be subtracted from the
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* initializer list count.
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*
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* Note that this will not work with types that have non-empty bases, but those types cannot be decomposed,
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* anyhow. Such types cannot have C++17 reflection performed on them.
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*
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* For the moment, computing a deep-dive on constrainted adaptable arguments is skipped. It's a lot more
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* complicated than just counting empty bases. As long as the first actual member is not also a base class,
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* this technique will work.
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*/
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// The basic adaptable argument. Because it pretends to be anything, it can be used as a parameter in invoking
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// any initialization method.
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struct argument { template< typename T > constexpr operator T (); };
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// Any empty-base-class argument.
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template< typename Aggregate >
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struct empty_base
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{
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template< typename T >
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requires
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(
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true
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and EmptyType< std::decay_t< T > >
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and not SameAs< std::decay_t< T >, Aggregate >
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and DerivedFrom< Aggregate, std::decay_t< T > >
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)
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constexpr operator T ();
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//template< typename T > constexpr operator T ()= delete;
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};
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template< typename T >
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constexpr bool is_empty_base_v= false;
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template< typename T >
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constexpr bool is_empty_base_v< empty_base< T > >{ true };
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template< typename Tuple, std::size_t baseCount, std::size_t totalCount >
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constexpr void
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check_tuple()
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{
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static_assert( std::tuple_size_v< Tuple > == totalCount );
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}
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template< typename Aggregate, std::size_t bases, std::size_t total >
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constexpr auto
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build_init_tuple()
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{
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static_assert( bases <= total );
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if constexpr( total == 0 ) return std::tuple{};
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else if constexpr( bases > 0 )
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{
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auto result= std::tuple_cat( std::tuple{ empty_base< Aggregate >{} }, build_init_tuple< Aggregate, bases - 1, total - 1 >() );
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check_tuple< decltype( result ), bases, total >();
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return result;
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}
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else
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{
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static_assert( bases == 0 );
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auto result= std::tuple_cat( std::tuple{ argument{} }, build_init_tuple< Aggregate, 0, total - 1 >() );
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check_tuple< decltype( result ), bases, total >();
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return result;
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}
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}
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template< typename T, typename Tuple >
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constexpr bool is_constructible_from_tuple_v= false;
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template< typename T, typename ... TupleArgs >
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constexpr bool is_constructible_from_tuple_v< T, std::tuple< TupleArgs... > >
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{
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ConstructibleFrom< T, TupleArgs... >
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};
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template< Aggregate T, typename InitTuple, std::size_t index= 0 >
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constexpr auto
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build_base_tuple()
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{
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constexpr auto init_size= aggregate_initializer_size_v< T >;
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using DeeperTuple= decltype( build_init_tuple< T, index, init_size >() );
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if constexpr( is_constructible_from_tuple_v< T, DeeperTuple > )
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{
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return build_base_tuple< T, DeeperTuple, index + 1 >();
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}
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else return Meta::type_value< InitTuple >{};
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}
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template< typename ... Args, typename First >
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constexpr std::size_t
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count_empty_bases( Meta::type_value< std::tuple< First, Args... > > )
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{
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if constexpr( is_empty_base_v< First > ) return 1 + count_empty_bases( Meta::type_value< std::tuple< Args... > >{} );
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else return 0;
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}
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constexpr std::size_t
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count_empty_bases( Meta::type_value< std::tuple<> > )
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{
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return 0;
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}
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template< Aggregate T, std::size_t index= 0 >
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constexpr std::size_t
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count_empty_bases()
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{
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return count_empty_bases( build_base_tuple< T, decltype( build_init_tuple< T, 0, aggregate_initializer_size_v< T > > ) >() );
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}
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namespace exports
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{
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template< typename T >
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struct aggregate_empty_bases : std::integral_constant< std::size_t, count_empty_bases< T >() > {};
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template< typename T >
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constexpr std::size_t aggregate_empty_bases_v= aggregate_empty_bases< T >::value;
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template< typename T >
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constexpr std::size_t aggregate_member_count_v= aggregate_initializer_size_v< T > - aggregate_empty_bases_v< T >;
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template< typename T >
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struct aggregate_member_count : std::integral_constant< std::size_t, aggregate_member_count_v< T > > {};
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}
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}
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namespace Alepha::Hydrogen::Reflection::inline exports::inline aggregate_members_m
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{
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using namespace detail::aggregate_members_m::exports;
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}
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