Freestanding Library: Partial Classes

Changes from previous revisions

Changes from R3

• Added fill_n and swap_ranges from <algorithm>
• optional now specified in terms of **this instead of .value()
• variant now specified in terms of get_if instead of get
• string_view find / search operations now specified in terms of data_ instead of at()
• Added missing parenthesis on size() call in string_view::ends_with
• Made feature test macros // freestanding
• More deliberate with use of "synopsis" vs. "header synopsis"
• freestanding-deleted and freestanding-partial are now freestanding items with altered requirements
• Added notes stating overload resolution consequences of // hosted vs. // freestanding-deleted
• More explicitly making the members of freestanding items also be freestanding items
• Deleted note discussing implication of having the same requirements, as those are covered more explicitly now

Changes from R2

• // freestanding-delete to // freestanding-deleted
• Adding bad_optional_access per LWG's request for consistency
• Rewording string_view::starts_with and string_view::ends_with in terms of freestanding functions
• Avoiding word "item" in instructions to the editor
• Made example in wording use // freestanding-deleted
• Added // mostly freestanding header marker

Changes from R1

• Ported paper to HTML
• Only annotating the deleted parts of partial classes, and not the included parts
• Mentioning forward_like in relation to variant's value categories
• New prose for [freestanding.item]

Changes from R0

• Add wording for feature test macros
• Mention monadic optional and string_view::contains

Introduction

Motivation and Scope

All of the added classes are fundamentally compatible with freestanding, except for a few methods that throw (e.g. array::at). We explicitly =delete these undesirable methods.

The main driving factor for these additions is the immense usefulness of these types in practice.

Scope

About <bitset>

Implementation experience

The Existing Standard Library

In Practice

Since we aren’t changing the semantics of any of the classes (except deleted non-critical methods), it is fair to say that all of the (implementer and user) experience gathered as part of hosted applies the same to freestanding.

The only question is, whether these classes are compatible with freestanding. To which the answer is yes! For example, the [Embedded Template Library] offers direct mappings of the std types. Even in kernel-level libraries, like Serenity’s [AK] use a form of these utilities.

Design decisions

Deleting behavior

Our decision to delete methods we can’t mark as freestanding was made to keep overload resolution the same on freestanding as hosted.

An additional benefit here is, that users of these classes, who might expect to use a throwing method, which was not provided by the implementation, will get a more meaningful error than the method simply missing. This also means we can keep options open for reintroducing the deleted functions into freestanding. (e.g. operator<<(ostream, string_view), should <ostream> be added).

[conventions] changes

The predecessor to this paper used //freestanding, partial to mean a class (template) is only required to be partially implemented, in conjunction with //freestanding, omit meaning a declaration is not in freestanding.

In this paper, we mark not fully freestanding classes templates as // freestanding-partial, and use P2338's // freestanding-deleted to mark which pieces of the class should be omitted. We no longer annotate all the class members, favoring terseness over explicitness.

On std::visit

In this paper, we mark std::visit as freestanding, even though it is theoretically throwing. However, the conditions for std::visit to throw are as follows:

It is possible for a variant to hold no value if an exception is thrown during a type-changing assignment or emplacement.

This means a variant will only throw on visit if a user type throws (library types don’t throw on freestanding). In this case, std::visit throwing isn’t a problem, since the user’s code is already using, and (hopefully) handling exceptions.

This however has the unfortunate side-effect that we need to keep bad_variant_access freestanding.

Notes on variant and value categories

By getting rid of std::get, we force users to use std::get_if. Since std::get_if returns a pointer, one can only access the value of a variant by dereferencing said pointer, obtaining an lvalue, discarding the value category of the held object. This is unlikely to have an impact on application code, but might impact highly generic library code.

std::forward_like can help in these cases. The value category of the variant can be transferred to the dereferenced pointer returned from set::get_if.

Justification for deletions

Monadic optional and string_view::contains

Minimal <algorithm> inclusions

Wording

Change in [freestanding.item]

2

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 the namespaces are required to be present.

[Note 1:

This implies that freestanding item enumerations have the same enumerators on freestanding implementations and hosted implementations.

Furthermore, class types have the same members and class templates have the same deduction guides on freestanding implementations and hosted implementations.

— end note]

6?

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 function definition introduced by a freestanding deleted function is a freestanding item or a deleted function ([dcl.fct.def.delete]) or whether the requirements are the same as the corresponding requirements for a hosted implementation..

[ Note: Deleted definitions ensure that overload resolution does not silently change when migrating a library from a freestanding implementation to a hosted implementation. -end note]

[ Example:

double abs(double j); // freestanding-deleted

-end example]

?

A class type declaration or class template declaration in a synopsis that is followed by a comment that includes freestanding-partial is a freestanding item, except that it contains at least one freestanding deleted function.

[ Example:

template <class T, size_t N> struct array; //freestanding-partial

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]

3

A declaration in a header synopsis is a freestanding item if

• (3.1)
  it is followed by a comment that includes freestanding, or
• (3.?)
  it is followed by a comment that includes freestanding-deleted,
• (3.?)
  it is followed by a comment that includes freestanding-partial,
• (3.2)
  the header synopsis begins with a comment that includes all freestanding., or
• (3.?)
  the header synopsis begins with a comment that includes mostly freestanding and the declaration is not followed by a comment that includes hosted.
  [ Note: Declarations followed by hosted in mostly freestanding headers are not freestanding items. Overload resolution with such functions could vary between hosted and freestanding implementations. -end note]

4

An entity, deduction guide, or typedef-name is a freestanding item if it is:

• (4.1)
  introduced by a declaration that is a freestanding item,
• (4.?)
  a member of a freestanding item,
• (4.?)
  an enumerator of a freestanding item,
• (4.?)
  a deduction guide of a freestanding item,
• (4.2)
  an enclosing namespace of a freestanding item,
• (4.3)
  a friend of a freestanding item,
• (4.4)
  denoted by a typedef-name that is a freestanding item, or
• (4.5)
  denoted by an alias template that is a freestanding item.

5

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

• (5.1)
  the definition is followed by a comment that includes freestanding, or
• (5.2)
  the header synopsis begins with a comment that includes all freestanding., or
• (5.?)
  the header synopsis begins with a comment that includes mostly freestanding and the definition is not followed by a comment that includes hosted.

Changes in [compliance]

Changes in [version.syn]

#define __cpp_lib_freestanding_algorithm 20XXXXL   // freestanding, also in <algorithm>
#define __cpp_lib_freestanding_array 20XXXXL       // freestanding, also in <array>
#define __cpp_lib_freestanding_optional 20XXXXL    // freestanding, also in <optional>
#define __cpp_lib_freestanding_string_view 20XXXXL // freestanding, also in <string_view>
#define __cpp_lib_freestanding_variant 20XXXXL     // freestanding, also in <variant>

Changes in [optional.syn]

Please insert a // mostly freestanding comment at the beginning of the [optional.syn] synopsis.

• optional

Changes in [optional.optional.general]

Instructions to the editor:

Please append a // freestanding-deleted comment to every overload of value.

Changes in [optional.monadic]

template<class F> constexpr auto and_then(F&& f) &; template<class F> constexpr auto and_then(F&& f) const &;

1

Let U be invoke_result_t<F, decltype (value()**this)>.

2

Mandates: remove_cvref_t<U> is a specialization of optional.

3

Effects: Equivalent to: if (*this) { return invoke(std::forward<F>(f), value()**this); } else { return remove_cvref_t<U>(); }

template<class F> constexpr auto and_then(F&& f) &&; template<class F> constexpr auto and_then(F&& f) const &&;

4

Let U be invoke_result_t<F, decltype(std::move(value()**this))>.

5

Mandates: remove_cvref_t<U> is a specialization of optional.

6

Effects: Equivalent to: if (*this) { return invoke(std::forward<F>(f), std::move(value()**this)); } else { return remove_cvref_t<U>(); }

template<class F> constexpr auto transform(F&& f) &; template<class F> constexpr auto transform(F&& f) const &;

7

Let U be remove_cv_t<invoke_result_t<F, decltype(value()**this)>>.

8

Mandates: U is a non-array object type other than in_place_t or nullopt_t.

The declaration U u(invoke(std::forward<F>(f), value()**this)); is well-formed for some invented variable u.

[Note 1:

There is no requirement that U is movable ([dcl.init.general]).

— end note]

9

Returns: If *this contains a value, an optional<U> object whose contained value is direct-non-list-initialized with invoke(std::forward<F>(f), value()**this); otherwise, optional<U>().

template<class F> constexpr auto transform(F&& f) &&; template<class F> constexpr auto transform(F&& f) const &&;

10

Let U be remove_cv_t<invoke_result_t<F, decltype(std::move(value()**this))>>.

11

Mandates: U is a non-array object type other than in_place_t or nullopt_t.

The declaration U u(invoke(std::forward<F>(f), std::move(value()**this))); is well-formed for some invented variable u.

[Note 2:

There is no requirement that U is movable ([dcl.init.general]).

— end note]

12

Returns: If *this contains a value, an optional<U> object whose contained value is direct-non-list-initialized with invoke(std::forward<F>(f), std::move(value()**this)); otherwise, optional<U>().

Changes in [variant.syn]

Please insert a // mostly freestanding comment at the beginning of the [variant.syn] synopsis.

Please append a // freestanding-deleted comment to every get overload in the synopsis.

Changes in [variant.ctor]

constexpr variant(const variant& w);

7

Effects: If w holds a value, initializes the variant to hold the same alternative as w and direct-initializes the contained value with get<j>(w)*get_if<j>(addressof(w)), where j is w.index().

Otherwise, initializes the variant to not hold a value.

8

Throws: Any exception thrown by direct-initializing any ${\text{T}}_i$ for all i.

9

Remarks: This constructor is defined as deleted unless is_copy_constructible_v<${\text{T}}_i$> is true for all i.

If is_trivially_copy_constructible_v<${\text{T}}_i$> is true for all i, this constructor is trivial.

constexpr variant(variant&& w) noexcept(see below);

10

Constraints: is_move_constructible_v<${\text{T}}_i$> is true for all i.

11

Effects: If w holds a value, initializes the variant to hold the same alternative as w and direct-initializes the contained value with get<j>(std::move(w))std::move(*get_if<j>(addressof(w))), where j is w.index().

Otherwise, initializes the variant to not hold a value.

Changes in [variant.assign]

constexpr variant& operator=(const variant& rhs);

1

Let j be rhs.index().

2

Effects:

• (2.1)
  If neither *this nor rhs holds a value, there is no effect.
• (2.2)
  Otherwise, if *this holds a value but rhs does not, destroys the value contained in *this and sets *this to not hold a value.
• (2.3)
  Otherwise, if index() == j, assigns the value contained in rhs to the value contained in *this.
• (2.4)
  Otherwise, if either is_nothrow_copy_constructible_v<${\text{T}}_j$> is true or is_nothrow_move_constructible_v<${\text{T}}_j$> is false, equivalent to emplace<j>(get<j>(rhs)*get_if<j>(addressof(rhs))).
• (2.5)
  Otherwise, equivalent to operator=(variant(rhs)).

3

Postconditions: index() == rhs.index().

4

Returns: *this.

5

Remarks: This operator is defined as deleted unless is_copy_constructible_v<${\text{T}}_i$> && is_copy_assignable_v<${\text{T}}_i$> is true for all i.

If is_trivially_copy_constructible_v<${\text{T}}_i$> && is_trivially_copy_assignable_v<${\text{T}}_i$> && is_trivially_destructible_v<${\text{T}}_i$> is true for all i, this assignment operator is trivial.

🔗

constexpr variant& operator=(variant&& rhs) noexcept(see below);

6

Let j be rhs.index().

7

Constraints: is_move_constructible_v<${\text{T}}_i$> && is_move_assignable_v<${\text{T}}_i$> is true for all i.

8

Effects:

• (8.1)
  If neither *this nor rhs holds a value, there is no effect.
• (8.2)
  Otherwise, if *this holds a value but rhs does not, destroys the value contained in *this and sets *this to not hold a value.
• (8.3)
  Otherwise, if index() == j, assigns get<j>(std::move(rhs))std::move(*get_if<j>(addressof(rhs))) to the value contained in *this.
• (8.4)
  Otherwise, equivalent to emplace<j>(get<j>(std::move(rhs))std::move(*get_if<j>(addressof(rhs)))).

Changes in [variant.swap]

🔗

constexpr void swap(variant& rhs) noexcept(see below);

1

Mandates: is_move_constructible_v<${\text{T}}_i$> is true for all i.

2

Preconditions: Each ${\text{T}}_i$ meets the Cpp17Swappable requirements ([swappable.requirements]).

3

Effects:

• (3.1)
  If valueless_by_exception() && rhs.valueless_by_exception() no effect.
• (3.2)
  Otherwise, if index() == rhs.index(), calls swap(get<i>(*this)*get_if<i>(this), get<i>(rhs)*get_if<i>(addressof(rhs))) where i is index().
• (3.3)
  Otherwise, exchanges values of rhs and *this.

4

Throws: If index() == rhs.index(), any exception thrown by swap(get<i>(*this)*get_if<i>(this), get<i>(rhs)*get_if<i>(addressof(rhs))) with i being index().

Otherwise, any exception thrown by the move constructor of ${\text{T}}_i$ or ${\text{T}}_j$ with i being index() and j being rhs.index().

5

Remarks: If an exception is thrown during the call to function swap(get<i>(*this)*get_if<i>(this), get<i>(rhs)*get_if<i>(addressof(rhs))), the states of the contained values of *this and of rhs are determined by the exception safety guarantee of swap for lvalues of ${\text{T}}_i$ with i being index().

If an exception is thrown during the exchange of the values of *this and rhs, the states of the values of *this and of rhs are determined by the exception safety guarantee of variant's move constructor.

The exception specification is equivalent to the logical and of is_nothrow_move_constructible_v<${\text{T}}_i$> && is_nothrow_swappable_v<${\text{T}}_i$> for all i.

Changes in [variant.relops]

Replace every instance of get<i>(v) in [variant.relops] with *get_if<i>(addressof(v)).

Replace every instance of get<i>(w) in [variant.relops] with *get_if<i>(addressof(w)).

Changes in [variant.visit]

4

Let m be a pack of n values of type size_t.

Such a pack is valid if
$0\le m_i<{\text{variant\_size\_v<remove\_reference\_t<V}}_i\text{>>}$ for all $0\le i<n$.

For each valid pack m, let e(m) denote the expression: INVOKE(std::forward<Visitor>(vis), get<m>(std::forward<V>(vars))...) // see [func.require] INVOKE(std::forward<Visitor>(vis), forward_like<V>(*get_if<m>(addressof(vars)))...) // see [func.require] for the first form and INVOKE<R>(std::forward<Visitor>(vis), get<m>(std::forward<V>(vars))...) // see [func.require] INVOKE<R>(std::forward<Visitor>(vis), forward_like<V>(*get_if<m>(addressof(vars)))...) // see [func.require] for the second form.

Changes in [string.view.synop]

Please insert a // mostly freestanding comment at the beginning of the [string.view.synop] synopsis.

• operator<<

• basic_string_view

Changes in [string.view.template.general]

• at
• copy
• substr
• The following overloads of compare:
  • compare(size_type pos1, size_type n1, basic_string_view s)
  • compare(size_type pos1, size_type n1, basic_string_view s, size_type pos2, size_type n2)
  • compare(size_type pos1, size_type n1, const charT* s)
  • compare(size_type pos1, size_type n1, const charT* s, size_type n2)

Note that the compare(basic_string_view str) const and compare(const charT* s) const overloads are intentionally not freestanding-deleted.

Changes in [string.view.ops]

constexpr bool starts_with(basic_string_view x) const noexcept;

?

Let rlen be the smaller of size() and x.size().

?

Effects: Equivalent to: return substr(0, x.size()) == x; return basic_string_view(data(), rlen) == x;

constexpr bool ends_with(basic_string_view x) const noexcept;

?

Let rlen be the smaller of size() and x.size().

?

Effects: Equivalent to: return size() >= x.size() && compare(size() - x.size(), npos, x) == 0 return basic_string_view(data() + (size() - rlen), rlen) == x;

Changes in [string.view.find]

Replace every instance of at(xpos + I) in [string.view.find] with data_[xpos + I].

Replace every instance of at(xpos) in [string.view.find] with data_[xpos].

Replace every instance of str.at(I) in [string.view.find] with str.data_[I].

Changes in [array.syn]

Please insert a // mostly freestanding comment at the beginning of the [array.syn] synopsis.

Please append a // freestanding-partial comment to array

Changes in [array.overview]

Changes in [algorithm.syn]

Please append a // freestanding comment to the following functions:

• fill_n(OutputIterator first, Size n, const T& value)
• swap_ranges(ForwardIterator1 first1, ForwardIterator1 last1, ForwardIterator2 first2)

References