21 Metaprogramming library [meta]

21.4 Reflection [meta.reflection]

21.4.10 Reflection layout queries [meta.reflection.layout]

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;

Returns: bytes * CHAR_BIT + bits.

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consteval member_offset offset_of(info r);

Constant When: r 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.

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.

Returns: {V / CHAR_BIT, V % CHAR_BIT}.

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consteval size_t size_of(info r);

Constant When: 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, N U A)

([class.mem.general]) where W is not ⊥.

If dealias(r) represents a type, then is_complete_type(r) is true.

Returns: If r represents

(6.1)
a non-static data member of type T,
(6.2)
a data member description $(T, N, A, W, N U A)$ , or
(6.3)
dealias(r) represents a type T,

then sizeof(T) if T is not a reference type and size_of(add_pointer(^^T)) otherwise.

Otherwise, size_of(type_of(r)).

[Note 1:

It is possible that while sizeof(char) == size_of(^^char) is true, that sizeof(char&) == size_of(^^char&) is false.

If b represents a direct base class relationship of an empty base class, then size_of(b) > 0 is true.

— end note]

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consteval size_t alignment_of(info r);

If dealias(r) represents a type, then is_complete_type(r) is true.

Returns:

(8.1)
If dealias(r) represents a type T, then the alignment requirement for the layout-associated type ([class.mem.general]) for a non-static data member of type T.
(8.2)
Otherwise, if dealias(r) represents a variable or object, then the alignment requirement of the variable or object.
(8.3)
Otherwise, if r represents a direct base class relationship, then alignment_of(type_of(r)).
(8.4)
Otherwise, if r represents a non-static data member, then the alignment requirement of the subobject associated with the represented entity within any object of type parent_of(r).
(8.5)
Otherwise, r represents a data member description $(T R, N, A, W, N U A)$ ([class.mem.general]).

If A is not ⊥, then the value of A.

Otherwise, alignment_of(^^T).

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consteval size_t bit_size_of(info r);

Constant When: 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.

If dealias(r) represents a type T, there is a point within the evaluation context from which T is not incomplete.

Returns:

(10.1)
If r represents an unnamed bit-field or a non-static data member that is a bit-field with width W, then W.
(10.2)
Otherwise, if r represents a data member description $(T, N, A, W, N U A)$ ([class.mem.general]) and W is not ⊥, then W.
(10.3)
Otherwise, CHAR_BIT * size_of(r).