[depr.meta.types]

# Annex D (normative) Compatibility features [[depr]](./#depr)

## D.13 Deprecated type traits [depr.meta.types]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/future.tex#L283)

The header [<type_traits>](meta.type.synop#header:%3ctype_traits%3e "21.3.3 Header <type_­traits> synopsis [meta.type.synop]") has the following addition:namespace std {template<class T> struct is_trivial; template<class T> constexpr bool is_trivial_v = is_trivial<T>::value; template<class T> struct is_pod; template<class T> constexpr bool is_pod_v = is_pod<T>::value; template<size_t Len, size_t Align = *default-alignment*> // *see below*struct aligned_storage; template<size_t Len, size_t Align = *default-alignment*> // *see below*using [aligned_storage_t](#lib:aligned_storage_t "D.13 Deprecated type traits [depr.meta.types]") = typename aligned_storage<Len, Align>::type; template<size_t Len, class... Types>struct aligned_union; template<size_t Len, class... Types>using [aligned_union_t](#lib:aligned_union_t "D.13 Deprecated type traits [depr.meta.types]") = typename aligned_union<Len, Types...>::type;}

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/future.tex#L303)

The behavior of a program that adds specializations for
any of the templates defined in this subclause is undefined,
unless explicitly permitted by the specification of the corresponding template[.](#2.sentence-1)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/future.tex#L308)

A [*trivial class*](#def:class,trivial "D.13 Deprecated type traits [depr.meta.types]") is a class that is trivially copyable and
has one or more eligible default constructors, all of which are trivial[.](#3.sentence-1)

[*Note [1](#note-1)*:

In particular,
a trivial class does not have virtual functions or virtual base classes[.](#3.sentence-2)

â *end note*]

A [*trivial type*](#def:type,trivial "D.13 Deprecated type traits [depr.meta.types]") is a scalar type, a trivial class,
an array of such a type, or a cv-qualified version of one of these types[.](#3.sentence-3)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/future.tex#L319)

A [*POD class*](#def:POD_class) is a class that is both a trivial class and a
standard-layout class, and has no non-static data members of type non-POD class
(or array thereof)[.](#4.sentence-1)

A [*POD type*](#def:POD_type) is a scalar type, a POD class, an array
of such a type, or a cv-qualified version of one of these types[.](#4.sentence-2)

[ð](#lib:is_trivial)

`template<class T> struct is_trivial;
`

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/future.tex#L332)

*Preconditions*: remove_all_extents_t<T> shall be a complete type or cv void[.](#5.sentence-1)

[6](#6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/future.tex#L336)

*Remarks*: is_trivial<T> is a *Cpp17UnaryTypeTrait* ([[meta.rqmts]](meta.rqmts "21.3.2 Requirements"))
with a base characteristic of true_type if T is a trivial type,
and false_type otherwise[.](#6.sentence-1)

[7](#7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/future.tex#L343)

[*Note [2](#note-2)*:

It is unspecified
whether a closure type ([[expr.prim.lambda.closure]](expr.prim.lambda.closure "7.5.6.2 Closure types")) is a trivial type[.](#7.sentence-1)

â *end note*]

[ð](#lib:is_pod)

`template<class T> struct is_pod;
`

[8](#8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/future.tex#L356)

*Preconditions*: remove_all_extents_t<T> shall be a complete type or cv void[.](#8.sentence-1)

[9](#9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/future.tex#L360)

*Remarks*: is_pod<T> is a [*Cpp17UnaryTypeTrait*](meta.rqmts#:Cpp17UnaryTypeTrait "21.3.2 Requirements [meta.rqmts]") ([[meta.rqmts]](meta.rqmts "21.3.2 Requirements"))
with a base characteristic of true_type if T is a POD type,
and false_type otherwise[.](#9.sentence-1)

[10](#10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/future.tex#L367)

[*Note [3](#note-3)*:

It is unspecified whether a closure type ([[expr.prim.lambda.closure]](expr.prim.lambda.closure "7.5.6.2 Closure types")) is a POD type[.](#10.sentence-1)

â *end note*]

[ð](#lib:aligned_storage)

`template<size_t Len, size_t Align = default-alignment>
  struct aligned_storage;
`

[11](#11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/future.tex#L380)

The value of *default-alignment* is the most
stringent alignment requirement for any object type whose size
is no greater than Len ([[basic.types]](basic.types "6.9 Types"))[.](#11.sentence-1)

[12](#12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/future.tex#L385)

*Mandates*: Len is not zero[.](#12.sentence-1)

Align is equal to alignof(T) for some type T or
to *default-alignment*[.](#12.sentence-2)

[13](#13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/future.tex#L391)

The member typedef type denotes a trivial standard-layout type
suitable for use as uninitialized storage for any object
whose size is at most Len and
whose alignment is a divisor of Align[.](#13.sentence-1)

[14](#14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/future.tex#L397)

[*Note [4](#note-4)*:

Uses of aligned_storage<Len, Align>​::​type can be replaced
by an array std​::​byte[Len] declared with alignas(Align)[.](#14.sentence-1)

â *end note*]

[15](#15)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/future.tex#L403)

[*Note [5](#note-5)*:

A typical implementation would define aligned_storage as:template<size_t Len, size_t Alignment>struct aligned_storage {typedef struct {alignas(Alignment) unsigned char __data[Len]; } type;};

â *end note*]

[ð](#lib:aligned_union)

`template<size_t Len, class... Types>
  struct aligned_union;
`

[16](#16)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/future.tex#L425)

*Mandates*: At least one type is provided[.](#16.sentence-1)

Each type in the template parameter pack Types is a complete object type[.](#16.sentence-2)

[17](#17)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/future.tex#L431)

The member typedef type denotes a trivial standard-layout type
suitable for use as uninitialized storage for any object
whose type is listed in Types;
its size shall be at least Len[.](#17.sentence-1)

The static member alignment_value is an integral constant of type size_t whose value is the strictest alignment of all types listed in Types[.](#17.sentence-2)
