Reflection in C++26: Metafunctions
Reflection offers many metafunctions that run at compile time.
The metafunctions are declared as consteval
. consteval
creates a so-called immediate function. Each invocation of an immediate function creates a compile-time constant. To say it more directly. A consteval
(immediate) function is executed at compile-time.
Read more about consteval
and constinit
in my previous post: Two new Keywords in C++20: consteval and constinit.
What we Know so far:
constexpr auto r = ^int; typename[:r:] x = 42; // Same as: int x = 42; typename[:^char:] c = '*'; // Same as: char c = '*';
The reflection operator (^
) produces out of the grammatical element (C++ element) a reflection value. The type of the reflection value std::meta::info.
The reflection value (the reflection) can be an argument of the metafunctions or the splicers ([: refl :]
). The splicers create grammatical elements.
So reflection value can be defined as follows:
namespace std { namespace meta { using info = decltype(^::); } }
So, what is missing? Of course, the metafunctions. I will present you all of them for two reasons:
- There are many metafunctions.
- There is no official listing of all the meta functions (October 2024)
Metafunctions
namespace std::meta { using info = decltype(^::); template <typename R> concept reflection_range = /* see above */; // name and location consteval auto identifier_of(info r) -> string_view; consteval auto u8identifier_of(info r) -> u8string_view; consteval auto display_string_of(info r) -> string_view; consteval auto u8display_string_of(info r) -> u8string_view; consteval auto source_location_of(info r) -> source_location; // type queries consteval auto type_of(info r) -> info; consteval auto parent_of(info r) -> info; consteval auto dealias(info r) -> info; // object and value queries consteval auto object_of(info r) -> info; consteval auto value_of(info r) -> info; // template queries consteval auto template_of(info r) -> info; consteval auto template_arguments_of(info r) -> vector<info>; // member queries consteval auto members_of(info type_class) -> vector<info>; consteval auto bases_of(info type_class) -> vector<info>; consteval auto static_data_members_of(info type_class) -> vector<info>; consteval auto nonstatic_data_members_of(info type_class) -> vector<info>; consteval auto subobjects_of(info type_class) -> vector<info>; consteval auto enumerators_of(info type_enum) -> vector<info>; // member access struct access_context { static consteval access_context current() noexcept; consteval access_context() noexcept; }; consteval auto is_accessible( info r, acess_context from = access_context::current()); consteval auto accessible_members_of( info target, access_context from = access_context::current()) -> vector<info>; consteval auto accessible_bases_of(info target, info target, access_context from = access_context::current()) -> vector<info>; consteval auto accessible_nonstatic_data_members_of( info target, access_context from = access_context::current()) -> vector<info>; consteval auto accessible_static_data_members_of( info target, access_context from = access_context::current()) -> vector<info>; consteval auto accessible_subobjects_of( info target, access_context from = access_context::current()) -> vector<info>; // substitute template <reflection_range R = initializer_list<info>> consteval auto can_substitute(info templ, R&& args) -> bool; template <reflection_range R = initializer_list<info>> consteval auto substitute(info templ, R&& args) -> info; // reflect_invoke template <reflection_range R = initializer_list<info>> consteval auto reflect_invoke(info target, R&& args) -> info; template <reflection_range R1 = initializer_list<info>, reflection_range R2 = initializer_list<info>> consteval auto reflect_invoke(info target, R1&& tmpl_args, R2&& args) -> info; // reflect expression results template <typename T> consteval auto reflect_value(T value) -> info; template <typename T> consteval auto reflect_object(T& value) -> info; template <typename T> consteval auto reflect_function(T& value) -> info; // extract template <typename T> consteval auto extract(info) -> T; // other type predicates (see the wording) consteval auto is_public(info r) -> bool; consteval auto is_protected(info r) -> bool; consteval auto is_private(info r) -> bool; consteval auto is_virtual(info r) -> bool; consteval auto is_pure_virtual(info entity) -> bool; consteval auto is_override(info entity) -> bool; consteval auto is_final(info r) -> bool; consteval auto is_deleted(info entity) -> bool; consteval auto is_defaulted(info entity) -> bool; consteval auto is_explicit(info entity) -> bool; consteval auto is_noexcept(info entity) -> bool; consteval auto is_bit_field(info entity) -> bool; consteval auto is_enumerator(info entity) -> bool; consteval auto is_const(info r) -> bool; consteval auto is_volatile(info r) -> bool; consteval auto is_lvalue_reference_qualified(info r) -> bool; consteval auto is_rvalue_reference_qualified(info r) -> bool; consteval auto has_static_storage_duration(info r) -> bool; consteval auto has_thread_storage_duration(info r) -> bool; consteval auto has_automatic_storage_duration(info r) -> bool; consteval auto has_internal_linkage(info r) -> bool; consteval auto has_module_linkage(info r) -> bool; consteval auto has_external_linkage(info r) -> bool; consteval auto has_linkage(info r) -> bool; consteval auto is_class_member(info entity) -> bool; consteval auto is_namespace_member(info entity) -> bool; consteval auto is_nonstatic_data_member(info entity) -> bool; consteval auto is_static_member(info entity) -> bool; consteval auto is_base(info entity) -> bool; consteval auto is_data_member_spec(info r) -> bool; consteval auto is_namespace(info entity) -> bool; consteval auto is_function(info entity) -> bool; consteval auto is_variable(info entity) -> bool; consteval auto is_type(info entity) -> bool; consteval auto is_type_alias(info entity) -> bool; consteval auto is_namespace_alias(info entity) -> bool; consteval auto is_complete_type(info entity) -> bool; consteval auto is_template(info entity) -> bool; consteval auto is_function_template(info entity) -> bool; consteval auto is_variable_template(info entity) -> bool; consteval auto is_class_template(info entity) -> bool; consteval auto is_alias_template(info entity) -> bool; consteval auto is_conversion_function_template(info entity) -> bool; consteval auto is_operator_function_template(info entity) -> bool; consteval auto is_literal_operator_template(info entity) -> bool; consteval auto is_constructor_template(info entity) -> bool; consteval auto is_concept(info entity) -> bool; consteval auto is_structured_binding(info entity) -> bool; consteval auto is_value(info entity) -> bool; consteval auto is_object(info entity) -> bool; consteval auto has_template_arguments(info r) -> bool; consteval auto has_default_member_initializer(info r) -> bool; consteval auto is_special_member(info r) -> bool; consteval auto is_conversion_function(info r) -> bool; consteval auto is_operator_function(info r) -> bool; consteval auto is_literal_operator(info r) -> bool; consteval auto is_constructor(info r) -> bool; consteval auto is_default_constructor(info r) -> bool; consteval auto is_copy_constructor(info r) -> bool; consteval auto is_move_constructor(info r) -> bool; consteval auto is_assignment(info r) -> bool; consteval auto is_copy_assignment(info r) -> bool; consteval auto is_move_assignment(info r) -> bool; consteval auto is_destructor(info r) -> bool; consteval auto is_user_provided(info r) -> bool; // define_class struct data_member_options_t; consteval auto data_member_spec(info type_class, data_member_options_t options = {}) -> info; template <reflection_range R = initializer_list<info>> consteval auto define_class(info type_class, R&&) -> info; // define_static_string consteval auto define_static_string(string_view str) -> const char *; consteval auto define_static_string(u8string_view str) -> const char8_t *; // data layout struct member_offsets { size_t bytes; size_t bits; constexpr auto total_bits() const -> . double quote. double quote. double quote. double quote. size_t; auto operator<=>(member_offsets const&) const = default; }; consteval auto offset_of(info entity) -> member_offsets; consteval auto size_of(info entity) -> size_t; consteval auto alignment_of(info entity) -> size_t; consteval auto bit_size_of(info entity) -> size_t; }
First of all, the metafunctions may remind you of the type traits metafunctions. There’s a big difference. The type traits metafunctions are declared as constexpr
, but the reflection metafunctions as consteval
. This means the type traits metafunctions can run at compile time, but the reflection metafunctions must run at compile time. Consequently, the arguments must be constant expressions.
Now, let’s have a closer look
The reflection library has metafunctions for
- Returns the name or location
- Returns the reflection value from a type, object and value, template, and member
- Returns the member access
- Returns the reflection on the template arguments
- Returns the result of a call expression
- Returns the result of the evaluated expression
- Extract in the value similar to the splicers
Furthermore, the reflection library has more than 50 compile time predicates. A compile time predicate is a callable which returns bool
at compile time.
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Additionally, there are metafunctions to define a class.
What’s next?
In my next post, I will apply the metafunctions.
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