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C++
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Feature test macros (C++20)
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Technical specifications
Symbols index
External libraries
[edit] Metaprogramming library
Type traits
Type categories
is_void(C++11)
is_null_pointer(C++14)
is_array(C++11)
is_pointer(C++11)
is_enum(C++11)
is_union(C++11)
is_class(C++11)
is_function(C++11)
is_object(C++11)
is_scalar(C++11)
is_compound(C++11)
is_integral(C++11)
is_floating_point(C++11)
is_fundamental(C++11)
is_arithmetic(C++11)
is_reference(C++11)
is_lvalue_reference(C++11)
is_rvalue_reference(C++11)
is_member_pointer(C++11)
is_member_object_pointer(C++11)
is_member_function_pointer(C++11)
Type properties
is_const(C++11)
is_volatile(C++11)
is_empty(C++11)
is_polymorphic(C++11)
is_final(C++14)
is_abstract(C++11)
is_aggregate(C++17)
is_implicit_lifetime(C++23)
is_trivial(C++11)
is_trivially_copyable(C++11)
is_standard_layout(C++11)
is_literal_type(C++11)(until C++20)
is_pod(C++11)(deprecated in C++20)
is_signed(C++11)
is_unsigned(C++11)
is_bounded_array(C++20)
is_unbounded_array(C++20)
is_scoped_enum(C++23)
has_unique_object_representations(C++17)
Type trait constants
integral_constantbool_constanttrue_typefalse_type(C++11)(C++17)(C++11)(C++11)
Metafunctions
conjunction(C++17)
disjunction(C++17)
negation(C++17)
Supported operations
is_constructibleis_trivially_constructibleis_nothrow_constructible(C++11)(C++11)(C++11)
is_default_constructibleis_trivially_default_constructibleis_nothrow_default_constructible(C++11)(C++11)(C++11)
is_copy_constructibleis_trivially_copy_constructibleis_nothrow_copy_constructible(C++11)(C++11)(C++11)
is_move_constructibleis_trivially_move_constructibleis_nothrow_move_constructible(C++11)(C++11)(C++11)
is_assignableis_trivially_assignableis_nothrow_assignable(C++11)(C++11)(C++11)
is_copy_assignableis_trivially_copy_assignableis_nothrow_copy_assignable(C++11)(C++11)(C++11)
is_move_assignableis_trivially_move_assignableis_nothrow_move_assignable(C++11)(C++11)(C++11)
is_destructibleis_trivially_destructibleis_nothrow_destructible(C++11)(C++11)(C++11)
has_virtual_destructor(C++11)
is_swappable_withis_swappableis_nothrow_swappable_withis_nothrow_swappable(C++17)(C++17)(C++17)(C++17)
Relationships and property queries
is_same(C++11)
is_base_of(C++11)
is_convertibleis_nothrow_convertible(C++11)(C++20)
is_layout_compatible(C++20)
is_pointer_interconvertible_base_of(C++20)
is_pointer_interconvertible_with_class(C++20)
is_corresponding_member(C++20)
reference_constructs_from_temporary(C++23)
alignment_of(C++11)
rank(C++11)
extent(C++11)
is_invocableis_invocable_ris_nothrow_invocableis_nothrow_invocable_r(C++17)(C++17)(C++17)(C++17)
reference_converts_from_temporary(C++23)
Type modifications
remove_cvremove_constremove_volatile(C++11)(C++11)(C++11)
add_cvadd_constadd_volatile(C++11)(C++11)(C++11)
make_signed(C++11)
make_unsigned(C++11)
remove_reference(C++11)
add_lvalue_referenceadd_rvalue_reference(C++11)(C++11)
remove_pointer(C++11)
add_pointer(C++11)
remove_extent(C++11)
remove_all_extents(C++11)
Type transformations
aligned_storage(C++11)(deprecated in C++23)
aligned_union(C++11)(deprecated in C++23)
decay(C++11)
remove_cvref(C++20)
enable_if(C++11)
void_t(C++17)
conditional(C++11)
common_type(C++11)
common_reference(C++20)
underlying_type(C++11)
result_ofinvoke_result(C++11)(until C++20)(C++17)
type_identity(C++20)
Compile-time rational arithmetic
Compile-time integer sequences
integer_sequence(C++14)
[edit]
Defined in header
template
struct is_empty;
(since C++11)
If T is an empty type (that is, a non-union class type with no non-static data members other than bit-fields of size 0, no virtual functions, no virtual base classes, and no non-empty base classes), provides the member constant value equal to true. For any other type, value is false. If T is a non-union class type, T shall be a complete type; otherwise, the behavior is undefined. The behavior of a program that adds specializations for is_empty or is_empty_v (since C++17) is undefined. Contents 1 Template parameters 2 Helper variable template 3 Inherited from std::integral_constant 3.1 Member constants 3.2 Member functions 3.3 Member types 3.4 Notes 3.5 Example 3.6 See also [edit] Template parameters T - a type to check [edit] Helper variable template template inline constexpr bool is_empty_v = is_empty::value; (since C++17) [edit] Inherited from std::integral_constant Member constants value[static] true if T is an empty class type , false otherwise (public static member constant) Member functions operator bool converts the object to bool, returns value (public member function) operator()(C++14) returns value (public member function) Member types Type Definition value_type bool type std::integral_constant [edit] NotesInheriting from empty base classes usually does not increase the size of a class due to empty base optimization. std::is_empty and all other type traits are empty classes. [edit] Example Run this code #include #include struct A {}; struct B { int m; }; struct C { static int m; }; struct D { virtual ~D(); }; union E {}; struct F { [[no_unique_address]] E e; }; struct G { int:0; // C++ standard allow "as a special case, 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." }; int main() { std::cout |
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