std::aligned_union

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Metaprogramming library

Type traits

Type categories

is_void

(C++11)

is_null_pointer

(C++11)(DR*)

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_reference (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_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

(C++11)

(C++11)

(C++11)

(C++11)

(C++14)

(C++11)

(C++17)

(C++23)

(C++11)(deprecated in C++26)

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)

(C++11)

(C++11)

(C++20)

(C++20)

(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_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)
reference_converts_from_temporary (C++23)

is_base_of (C++11)
is_virtual_base_of (C++26)
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)

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)
result_ofinvoke_result (C++11)(until C++20*)(C++17)

conditional (C++11)
common_type (C++11)
common_reference (C++20)
underlying_type (C++11)
type_identity (C++20)
enable_if (C++11)
void_t (C++17)

Compile-time rational arithmetic

Compile-time integer sequences

integer_sequence

(C++14)

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Defined in header`<type_traits>`
template<std::size_t Len,class...Types> struct aligned_union;		(since C++11) (deprecated in C++23)

Provides the nested typetype, which is atrivial standard-layout type of a size and alignment suitable for use as uninitialized storage for an object of any of the types listed inTypes. The size of the storage is at leastLen.std::aligned_union also determines the strictest (largest) alignment requirement among allTypes and makes it available as the constantalignment_value.

Ifsizeof...(Types)==0 or if any of the types inTypes is not a complete object type, the behavior is undefined.

It is implementation-defined whether anyextended alignment is supported.

If the program adds specializations forstd::aligned_union, the behavior is undefined.

[edit]Member types

Name	Definition
`type`	a trivial and standard-layout type suitable for storage of any type from`Types`

[edit]Helper types

template<std::size_t Len,class...Types> using aligned_union_t=typename aligned_union<Len,Types...>::type;		(since C++14) (deprecated in C++23)

[edit]Member constants

alignment_value

[static]

the strictest alignment requirement of allTypes
(public static member constant)

[edit]Possible implementation

#include <algorithm> template<std::size_t Len,class...Types>struct aligned_union{staticconstexprstd::size_t alignment_value=std::max({alignof(Types)...}); struct type{        alignas(alignment_value)char _s[std::max({Len, sizeof(Types)...})];};};

[edit]Example

Run this code

#include <iostream>#include <string>#include <type_traits> int main(){std::cout<< sizeof(std::aligned_union_t<0,char>)<<' '// 1<< sizeof(std::aligned_union_t<2,char>)<<' '// 2<< sizeof(std::aligned_union_t<2,char[3]>)<<' '// 3 (!)<< sizeof(std::aligned_union_t<3,char[4]>)<<' '// 4<< sizeof(std::aligned_union_t<1,char,int,double>)<<' '// 8<< sizeof(std::aligned_union_t<12,char,int,double>)<<'\n';// 16 (!) using var_t= std::aligned_union<16,int,std::string>; std::cout<<"var_t::alignment_value = "<< var_t::alignment_value<<'\n'<<"sizeof(var_t::type) = "<< sizeof(var_t::type)<<'\n';     var_t::type aligned_storage;int* int_ptr= new(&aligned_storage)int(42);// placement newstd::cout<<"*int_ptr = "<<*int_ptr<<'\n'; std::string* string_ptr= new(&aligned_storage)std::string("bar");std::cout<<"*string_ptr = "<<*string_ptr<<'\n';*string_ptr="baz";std::cout<<"*string_ptr = "<<*string_ptr<<'\n';    string_ptr->~basic_string();}

Possible output:

1 2 3 4 8 16var_t::alignment_value = 8sizeof(var_t::type) = 32*int_ptr = 42*string_ptr = bar*string_ptr = baz

[edit]Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2979	C++11	complete type was not required	requires complete types

[edit]See also

alignment_of (C++11)	obtains the type's alignment requirements (class template)[edit]
aligned_storage (since C++11)(deprecated in C++23)	defines the type suitable for use as uninitialized storage for types of given size (class template)[edit]