20 Memory management library [mem]

20.3 Pointer tagging [ptrtag]

20.3.2 Class template pointer_tag_pair [ptrtag.pair]

20.3.2.4 Tagged pointer operations [ptrtag.pair.tagops]

using tagged_pointer_type = see below;

tagged_pointer_type denotes cv void*, for cv such that pointer denotes cv U* for some type U.

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tagged_pointer_type tagged_pointer() const noexcept;

Returns: An unspecified value tp of tagged_pointer_type type such that for any specialization DP of pointer_tag_pair and alignment value A for which

(2.1)
pointer_bits_available(A) >= DP::bits_requested is true,
(2.2)
is_sufficiently_aligned<A>(ptr) is true, and
(2.3)
tag-bit-width(tag) <= DP::bits_requested is true,

DP::from_tagged(tp) produces an object dp such that reinterpret_cast<pointer_type>(dp.pointer()) is equal to ptr and static_cast<TagT>(dp.tag()) is equal to tag.

[Note 1:

This function returns an invalid pointer value ([basic.compound]) with implementation-defined behavior.

— end note]

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static pointer_tag_pair from_tagged(tagged_pointer_type p) noexcept;

Returns: An object dp of type pointer_tag_type, such that

(3.1)
reinterpret_cast<pointer_type>(sp.pointer()) is equal to dp.pointer() and
static_cast<TagT>(sp.tag()) is equal to dp.tag(), if p is equal to sp.tagged_pointer() for some object sp of a type that is a specialization of pointer_tag_type, such that for some alignment value A
- (3.1.1)
  pointer_bits_available(A) >= bits_requested is true,
- (3.1.2)
  is_sufficiently_aligned<A>(sp.pointer()) is true, and
- (3.1.3)
  tag-bit-width(sp.tag()) <= bits_requested is true,
(3.2)
otherwise, the values of dp.pointer() and dp.tag() are unspecified.