[freestanding.item]

Unless otherwise specified, the requirements on freestanding items for a freestanding implementation are the same as the corresponding requirements for a hosted implementation, except that not all of the members of those items are required to be present.

Function declarations and function template declarations followed by a comment that include freestanding-deleted are freestanding deleted functions.

On freestanding implementations, it is implementation-defined whether each entity introduced by a freestanding deleted function is a deleted function ([dcl.fct.def.delete]) or whether the requirements are the same as the corresponding requirements for a hosted implementation.

[Note 1:

Deleted definitions reduce the chance of overload resolution silently changing when migrating from a freestanding implementation to a hosted implementation.

— end note]

[Example 1: double abs(double j); / freestanding-deleted — end example]

A declaration in a synopsis is a freestanding item if

(4.1)
it is followed by a comment that includes freestanding,
(4.2)
it is followed by a comment that includes freestanding-deleted, or
(4.3)
the header synopsis begins with a comment that includes freestanding and the declaration is not followed by a comment that includes hosted.
[Note 2:
Declarations followed by hosted in freestanding headers are not freestanding items.

As a result, looking up the name of such functions can vary between hosted and freestanding implementations.
— end note]

[Example 2: / all freestanding namespace std { — end example]

An entity or deduction guide is a freestanding item if its introducing declaration is not followed by a comment that includes hosted, and is:

(5.1)
introduced by a declaration that is a freestanding item,
(5.2)
a member of a freestanding item other than a namespace,
(5.3)
an enumerator of a freestanding item,
(5.4)
a deduction guide of a freestanding item,
(5.5)
an enclosing namespace of a freestanding item,
(5.6)
a friend of a freestanding item,
(5.7)
denoted by a type alias that is a freestanding item, or
(5.8)
denoted by an alias template that is a freestanding item.

A macro is a freestanding item if it is defined in a header synopsis and

(6.1)
the definition is followed by a comment that includes freestanding, or
(6.2)
the header synopsis begins with a comment that includes freestanding and the definition is not followed by a comment that includes hosted.

[Example 3: #define NULL see below / freestanding — end example]

[Note 3:

Freestanding annotations follow some additional exposition conventions that do not impose any additional normative requirements.

Header synopses that begin with a comment containing "all freestanding" contain no hosted items and no freestanding deleted functions.

Header synopses that begin with a comment containing "mostly freestanding" contain at least one hosted item or freestanding deleted function.

Classes and class templates followed by a comment containing "partially freestanding" contain at least one hosted item or freestanding deleted function.

— end note]

[Example 4: template<class T, size_t N> struct array; / partially freestanding template<class T, size_t N> struct array { constexpr reference operator[](size_type n); constexpr const_reference operator[](size_type n) const; constexpr reference at(size_type n); / freestanding-deleted constexpr const_reference at(size_type n) const; / freestanding-deleted }; — end example]