std::meta::is_empty_type

 

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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

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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

 

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Defined in header <meta>
consteval bool is_empty_type( std::meta::info r );		(since C++26)

Returns true if r represents a class (but not union) type and has no non-static data members. Otherwise returns false.

Parameters

r

-

a reflection value

Return value

true if r represents an empty class type; otherwise false.

Exceptions

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

Example

Run this code

#include <meta>
#include <print>

struct A {};
static_assert(is_empty_type(^^A));

struct B { int m; };
static_assert(!is_empty_type(^^B));

struct C { static int m; };
static_assert(is_empty_type(^^C));

struct D { virtual ~D(); };
static_assert(!is_empty_type(^^D));

union E {};
static_assert(!is_empty_type(^^E));

struct F
{
    int: 0;
    // C++ standard allows "an unnamed bit-field with a width of zero
    // specifies alignment of the next bit-field at an allocation unit boundary.
    // Only when declaring an unnamed bit-field may the width be zero."
};
static_assert(is_empty_type(^^F)); // holds only unnamed bit-fields of zero width

struct G { [[no_unique_address]] E e; };

int main()
{
    std::println("G: {}", is_empty_type(^^G)); // the result is ABI-dependent
}

Possible output:

G: true

See also

is_empty (C++11)	checks if a type is a class (but not union) type and has no non-static data members (class template)
is_class_type (C++26)	checks if reflected type is a non-union class type (function)
is_union_type (C++26)	checks if reflected type is a union type (function)