std::meta::unwrap_reference, std::meta::unwrap_ref_decay

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Diagnostics library

Memory management library

Metaprogramming library (C++11)

General utilities library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library (C++17)

Concurrency support library (C++11)

Execution control library (C++26)

Technical specifications

Symbols index

External libraries

 

Metaprogramming library

Type traits (C++11)

Compile-time rational arithmetic (C++11)

Compile-time integer sequences (C++14)

integer_sequence (C++14)

Compile-time reflection (C++26)

 

Reflection library

Checking string literals is_string_literal Types info meta::exception access_context data_member_options member_offset operators Concepts reflection_range Promoting to static storage reflect_constant_string reflect_constant_array define_static_string define_static_array define_static_object Operator representations operator_of symbol_of u8symbol_of Names and locations has_identifier identifier_of u8identifier_of display_string_of u8display_string_of source_location_of Reflection queries ➜ Member accessibility queries is_accessible has_inaccessible_nonstatic_   data_members has_inaccessible_bases has_inaccessible_subobjects

Scope identification current_function current_class current_namespace Reflection member queries members_of bases_of static_data_members_of nonstatic_data_members_of      subobjects_of enumerators_of Reflection layout queries offset_of size_of alignment_of bit_size_of Annotation reflection annotations_of annotations_of_with_type Value extraction extract Reflection substitution can_substitute substitute Expression result reflection reflect_constant reflect_object reflect_function Class definition generation data_member_spec is_data_member_spec define_aggregate

Primary type categories is_void_type is_null_pointer_type is_integral_type is_floating_point_type is_array_type is_pointer_type is_lvalue_reference_type is_rvalue_reference_type is_member_object_pointer_type is_member_function_pointer_type      is_enum_type is_union_type is_class_type is_function_type is_reflection_type Composite type categories is_reference_type is_arithmetic_type is_fundamental_type is_object_type is_scalar_type is_compound_type is_member_pointer_type Type properties ➜ Type relations is_same_type is_base_of_type is_virtual_base_of_type is_convertible_type is_nothrow_convertible_type is_layout_compatible_type is_pointer_interconvertible_   base_of_type is_invocable_type is_invocable_r_type is_nothrow_invocable_type is_nothrow_invocable_r_type

Const-volatile modifications remove_const remove_volatile remove_cv add_const add_volatile add_cv Reference modifications remove_reference add_lvalue_reference add_rvalue_reference Sign modifications make_signed make_unsigned Array modifications remove_extent remove_all_extents Pointer modifications remove_pointer add_pointer Other transformations remove_cvref decay common_type common_reference underlying_type invoke_result unwrap_reference unwrap_ref_decay tuple_size tuple_element is_applicable_type is_nothrow_applicable_type apply_result variant_size variant_alternative type_order

 

Reflection types and queries

Reflection queries type_of object_of constant_of is_public is_protected is_private is_virtual is_pure_virtual is_override is_final is_deleted is_defaulted is_user_provided is_user_declared is_explicit is_noexcept is_bit_field is_enumerator is_annotation is_const is_volatile is_mutable_member is_lvalue_reference_qualified is_rvalue_reference_qualified has_static_storage_duration has_thread_storage_duration has_automatic_storage_duration      has_internal_linkage has_module_linkage has_external_linkage has_linkage has_c_language_linkage is_complete_type is_enumerable_type is_variable is_type is_namespace is_type_alias is_namespace_alias is_function is_conversion_function is_operator_function is_literal_operator is_special_member_function is_destructor

is_constructor is_default_constructor is_copy_constructor is_move_constructor is_assignment is_copy_assignment is_move_assignment is_function_parameter is_explicit_object_parameter has_default_argument is_vararg_function is_template is_function_template is_variable_template is_class_template is_alias_template is_conversion_function_template      is_operator_function_template is_literal_operator_template is_constructor_template is_concept is_value is_object is_structured_binding is_class_member is_namespace_member is_nonstatic_data_member is_static_member is_base has_default_member_initializer has_parent parent_of dealias has_template_arguments template_of template_arguments_of parameters_of variable_of return_type_of Type properties is_const_type is_volatile_type is_trivially_copyable_type is_standard_layout_type is_empty_type

is_polymorphic_type is_abstract_type is_final_type is_aggregate_type is_structural_type is_signed_type is_unsigned_type is_bounded_array_type is_unbounded_array_type is_scoped_enum_type is_constructible_type is_trivially_constructible_type is_nothrow_constructible_type is_default_constructible_type is_trivially_default_constructible_type is_nothrow_default_constructible_type is_copy_constructible_type is_trivially_copy_constructible_type is_nothrow_copy_constructible_type is_move_constructible_type is_trivially_move_constructible_type is_nothrow_move_constructible_type is_assignable_type is_trivially_assignable_type is_nothrow_assignable_type is_copy_assignable_type is_trivially_copy_assignable_type is_nothrow_copy_assignable_type is_move_assignable_type is_trivially_move_assignable_type is_nothrow_move_assignable_type is_swappable_with_type is_swappable_type is_nothrow_swappable_with_type is_nothrow_swappable_type is_destructible_type is_trivially_destructible_type is_nothrow_destructible_type is_implicit_lifetime_type has_virtual_destructor has_unique_object_representations reference_constructs_from_temporary reference_converts_from_temporary Type property queries rank extent

 

Defined in header <meta>
consteval std::meta::info unwrap_reference( std::meta::info r );	(1)	(since C++26)
consteval std::meta::info unwrap_ref_decay( std::meta::info r );	(2)	(since C++26)

See More

Unwraps any std::reference_wrapper by changing std::reference_wrapper<U> to U&.

Let T be a type represented by r.

1) If T is a specialization of std::reference_wrapper, unwraps it; otherwise, T remains the same.

Equivalent to return std::meta::dealias(^^std::unwrap_reference_t<T>);.

2) If the decayed T is a specialization of std::reference_wrapper, unwraps it; otherwise, T is decayed.

Equivalent to return std::meta::dealias(^^std::unwrap_ref_decay_t<T>);.

Parameters

r

-

a reflection value

Return value

A reflection of:

1) U& if T is std::reference_wrapper<U>, T otherwise.

2) U& if std::decay_t<T> is std::reference_wrapper<U>, std::decay_t<T> otherwise.

Exceptions

Throws std::meta::exception if r does not represent a type or type alias.

Notes

std::unwrap_ref_decay performs the same transformation as used by std::make_pair and std::make_tuple.

Example

Run this code

#include <functional>
#include <meta>

static_assert
(
    (unwrap_reference(^^int) == ^^int) &&
    (unwrap_reference(^^const int) == ^^const int) &&
    (unwrap_reference(^^int&) == ^^int&) &&
    (unwrap_reference(^^int&&) == ^^int&&) &&
    (unwrap_reference(^^int*) == ^^int*) &&

    (unwrap_ref_decay(^^int) == ^^int) &&
    (unwrap_ref_decay(^^const int) == ^^int) &&
    (unwrap_ref_decay(^^const int&) == ^^int)
);

int main()
{
    {
        using T = std::reference_wrapper<int>;
        constexpr auto X{unwrap_reference(^^T)};
        static_assert(X == ^^int&);
    }

    {
        using T = std::reference_wrapper<int&>;
        constexpr auto X{unwrap_reference(^^T)};
        static_assert(X == ^^int&);
    }

    {
        using T = std::reference_wrapper<int&&>;
        constexpr auto X{unwrap_ref_decay(^^T)};
        static_assert(X == ^^int&);
    }
}

See also

decay (C++26)	applies type transformations as when passing a function argument by value (function)
reference_wrapper (C++11)	CopyConstructible and CopyAssignable reference wrapper (class template)
make_pair	creates a pair object of type, determined by the argument types (function template)
make_tuple (C++11)	creates a tuple object of the type defined by the argument types (function template)
unwrap_referenceunwrap_ref_decay (C++20)(C++20)	get the reference type wrapped in std::reference_wrapper (class template)