cppdraft_translate/cppdraft/meta/reflection/layout.md

[meta.reflection.layout]

# 21 Metaprogramming library [[meta]](./#meta)

## 21.4 Reflection [[meta.reflection]](meta.reflection#layout)

### 21.4.11 Reflection layout queries [meta.reflection.layout]

[🔗](#lib:member_offset)

`struct member_offset {
  ptrdiff_t bytes;
  ptrdiff_t bits;
  constexpr ptrdiff_t total_bits() const;
  auto operator<=>(const member_offset&) const = default;
};

constexpr ptrdiff_t member_offset::total_bits() const;
`

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/meta.tex#L5663)

*Returns*: bytes * CHAR_BIT + bits[.](#1.sentence-1)

[🔗](#lib:offset_of)

`consteval member_offset offset_of(info r);
`

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/meta.tex#L5674)

Let V be the offset in bits from the beginning of a complete object
of the type represented by parent_of(r) to the subobject associated with the entity represented by r[.](#2.sentence-1)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/meta.tex#L5679)

*Returns*: {V / CHAR_BIT, V % CHAR_BIT}[.](#3.sentence-1)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/meta.tex#L5683)

*Throws*: meta​::​exception unlessr represents a non-static data member,
unnamed bit-field, or
direct base class relationship (D,B) for which either B is not a virtual base class
or D is not an abstract class[.](#4.sentence-1)

[🔗](#lib:size_of)

`consteval size_t size_of(info r);
`

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/meta.tex#L5699)

*Returns*: If r represents

- [(5.1)](#5.1)

a non-static data member of type T,

- [(5.2)](#5.2)

a data member description (T,N,A,W,NUA), or

- [(5.3)](#5.3)

dealias(r) represents a type T,

then sizeof(T) if T is not a reference type
and size_of(​add_pointer(​^^T)) otherwise[.](#5.sentence-1)

Otherwise, size_of(type_of(r))[.](#5.sentence-2)

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

It is possible that while sizeof(char)​ == size_of(^^char) is true,
that sizeof(char&)​ == size_of(​^^char&) is false[.](#5.sentence-3)

If b represents a direct base class relationship of an empty base class,
then size_of(b) > 0 is true[.](#5.sentence-4)

— *end note*]

[6](#6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/meta.tex#L5719)

*Throws*: meta​::​exception unless
all of the following conditions are met:

- [(6.1)](#6.1)

  dealias(r) is a reflection of a
type,
object,
value,
variable of non-reference type,
non-static data member that is not a bit-field,
direct base class relationship, or
data member description (T,N,A,W,NUA) ([[class.mem.general]](class.mem.general "11.4.1 General"))
where W is not ⊥[.](#6.1.sentence-1)

- [(6.2)](#6.2)

  If dealias(r) represents a type,
then is_complete_type(r) is true[.](#6.2.sentence-1)

[🔗](#lib:alignment_of)

`consteval size_t alignment_of(info r);
`

[7](#7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/meta.tex#L5746)

*Returns*: 

- [(7.1)](#7.1)

  If dealias(r) represents a type T,
 then alignment_of(add_pointer(r)) if T is a reference type
 and the alignment requirement of T otherwise[.](#7.1.sentence-1)

- [(7.2)](#7.2)

  Otherwise, if dealias(r) represents a variable or object,
 then the alignment requirement of the variable or object[.](#7.2.sentence-1)

- [(7.3)](#7.3)

  Otherwise, if r represents a direct base class relationship,
 then alignment_of(type_of(r))[.](#7.3.sentence-1)

- [(7.4)](#7.4)

  Otherwise, if r represents a non-static data member M of a class C,
 then the alignment of the direct member subobject
 corresponding to M of a complete object of type C[.](#7.4.sentence-1)

- [(7.5)](#7.5)

  Otherwise, r represents a data member description (T,N,A,W,NUA) ([[class.mem.general]](class.mem.general "11.4.1 General"))[.](#7.5.sentence-1)
  If A is not ⊥,
 then the value A[.](#7.5.sentence-2)
  Otherwise, alignment_of(^^T)[.](#7.5.sentence-3)

[8](#8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/meta.tex#L5771)

*Throws*: meta​::​exception unless
all of the following conditions are met:

- [(8.1)](#8.1)

  dealias(r) is a reflection of a
type,
object,
variable of non-reference type,
non-static data member that is not a bit-field,
direct base class relationship, or
data member description[.](#8.1.sentence-1)

- [(8.2)](#8.2)

  If dealias(r) represents a type,
then is_complete_type(r) is true[.](#8.2.sentence-1)

[🔗](#lib:bit_size_of)

`consteval size_t bit_size_of(info r);
`

[9](#9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/meta.tex#L5796)

*Returns*: 

- [(9.1)](#9.1)

  If r represents
 an unnamed bit-field
 or a non-static data member that is a bit-field
 with width W,
 then W[.](#9.1.sentence-1)

- [(9.2)](#9.2)

  Otherwise, if r represents a data member description (T,N,A,W,NUA) ([[class.mem.general]](class.mem.general "11.4.1 General"))
 and W is not ⊥,
 then W[.](#9.2.sentence-1)

- [(9.3)](#9.3)

  Otherwise, CHAR_BIT * size_of(r)[.](#9.3.sentence-1)

[10](#10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/meta.tex#L5814)

*Throws*: meta​::​exception unless
all of the following conditions are met:

- [(10.1)](#10.1)

  dealias(r) is a reflection of a
type,
object,
value,
variable of non-reference type,
non-static data member,
unnamed bit-field,
direct base class relationship, or
data member description[.](#10.1.sentence-1)

- [(10.2)](#10.2)

  If dealias(r) represents a type T,
there is a point within the evaluation context from which T is not incomplete[.](#10.2.sentence-1)