Abstract: This paper proposes views::maybe a range adaptor that produces a view with cardinality 0 or 1 which adapts nullable types such as std::optional and pointer to object types.

1 Changes

1.1 Changes since R5

• Fix reversed before/after table entry
• Update to match C++20 style [N4849] and changes in Ranges since [P0896R3]
• size is now size_t, like other ranges are also
• add synopsis for adding to <ranges> header
• Wording clean up, formatting, typesetting
• Add implementation notes and references

1.2 Changes since R4

• Use std::unwrap_reference
• Remove conditional noexceptness
• Adopted the great concept renaming

1.3 Changes since R3

• Always Capture
• Support reference_wrapper

1.4 Changes since R2

• Reflects current code as reviewed
• Nullable concept specification

Remove Readable as part of the specification, use the useful requirements from Readable

• Wording for views::maybe as proposed
• Appendix A: wording for a view_maybe that always captures

1.5 Changes since R1

• Refer to views::all Behavior of capture vs refer is similar to how views::all works over the expression it is given

• Use wording ‘range adaptor object’ Match current working paper language

1.6 Changes since R0

• Remove customization point objects

Removed views::maybe_has_value and views::maybe_value, instead requiring that the nullable type be dereferenceable and contextually convertible to bool.

• Concept Nullable, for exposition

Concept Nullable, which is Readable and contextually convertible to bool

• Capture rvalues by decay copy

Hold a copy when constructing a view over a nullable rvalue.

• Remove maybe_view as a specified type

Introduced two exposition types, one safely holding a copy, the other referring to the nullable

2 Before / After Table

{
    auto&& opt = possible_value();
    if (opt) {
        // a few dozen lines ...
        use(*opt); // is *opt OK ?
    }
}

for (auto&& opt : views::maybe(possible_value())) {

    // a few dozen lines ...
    use(opt); // opt is OK
}

std::optional o{7};
if (o) {
    *o = 9;
    std::cout << "o=" << *o << " prints 9\n";
}
std::cout << "o=" << *o << " prints 9\n";

std::optional o{7};
for (auto&& i : views::maybe(std::ref(o))) {
    i = 9;
    std::cout << "i=" << i << " prints 9\n";
}
std::cout << "o=" << *o << " prints 9\n";

std::vector<int> v{2, 3, 4, 5, 6, 7, 8, 9, 1};
auto test = [](int i) -> std::optional<int> {
    switch (i) {
      case 1:
      case 3:
      case 7:
      case 9:
    return i;
      default:
    return {};
    }
};

auto&& r = v |
    ranges::views::transform(test) |
    ranges::views::filter([](auto x){return bool(x);}) |
    ranges::views::transform([](auto x){return *x;}) |
    ranges::views::transform(
        [](int i) {
            std::cout << i;
            return i;
        });

std::vector<int> v{2, 3, 4, 5, 6, 7, 8, 9, 1};
auto test = [](int i) -> std::optional<int> {
    switch (i) {
      case 1:
      case 3:
      case 7:
      case 9:
    return i;
      default:
    return {};
    }
};

auto&& r = v |
    ranges::views::transform(test) |
    ranges::views::transform(views::maybe) |
    ranges::views::join |
    ranges::views::transform(
        [](int i) {
            std::cout << i;
            return i;
        });

3 Motivation

In writing range transformation it is useful to be able to lift a nullable value into a view that is either empty or contains the value held by the nullable. The adapter views::single fills a similar purpose for non-nullable values, lifting a single value into a view, and views::empty provides a range of no values of a given type. A views::maybe adaptor also allows nullable values to be treated as ranges when it is otherwise undesirable to make them containers, for example std::optional.

std::vector<std::optional<int>> v{
  std::optional<int>{42},
  std::optional<int>{},
  std::optional<int>{6 * 9}};

auto r = views::join(views::transform(v, views::maybe));

for (auto i : r) {
    std::cout << i; // prints 42 and 54
}

In addition to range transformation pipelines, views::maybe can be used in range based for loops, allowing the nullable value to not be dereferenced within the body. This is of small value in small examples in contrast to testing the nullable in an if statement, but with longer bodies the dereference is often far away from the test. Often the first line in the body of the if is naming the dereferenced nullable, and lifting the dereference into the for loop eliminates some boilerplate code, the same way that range based for loops do.

{
    auto&& opt = possible_value();
    if (opt) {
        // a few dozen lines ...
        use(*opt); // is *opt OK ?
    }
}

for (auto&& opt : views::maybe(possible_value())) {
    // a few dozen lines ...
    use(opt); // opt is OK
}

The view can be on a std::reference_wrapper, allowing the underlying nullable to be modified:

std::optional o{7};
for (auto&& i : views::maybe(std::ref(o))) {
    i = 9;
    std::cout << "i=" << i << " prints 9\n";
}
std::cout << "o=" << *o << " prints 9\n";

Of course, if the nullable is empty, there is nothing in the view to modify.

auto oe = std::optional<int>{};
for (int i : views::maybe(std::ref(oe)))
    std::cout << "i=" << i << '\n'; // does not print

Converting an optional type into a view can make APIs that return optional types, such a lookup operations, easier to work with in range pipelines.

std::unordered_set<int> set{1, 3, 7, 9};

auto flt = [=](int i) -> std::optional<int> {
    if (set.contains(i))
        return i;
    else
        return {};
};

for (auto i : ranges::iota_view{1, 10} | ranges::views::transform(flt)) {
    for (auto j : views::maybe(i)) {
        for (auto k : ranges::iota_view(0, j))
            std::cout << '\a';
        std::cout << '\n';
    }
}

// Produce 1 ring, 3 rings, 7 rings, and 9 rings

4 Proposal

Add a range adaptor object views::maybe, returning a view over a nullable object, capturing by value temporary nullables. A nullable object is one that is both contextually convertible to bool and for which the type produced by dereferencing is an equality preserving object. Non void pointers, std::optional, and the proposed expected [P0323R9] types all model nullable. Function pointers do not, as functions are not objects. Iterators do not generally model nullable, as they are not required to be contextually convertible to bool.

5 Design

The basis of the design is to hybridize views::single and views::empty. If the underlying object claims to hold a value, as determined by checking if the object when converted to bool is true, begin and end of the view are equivalent to the address of the held value within the underlying object and one past the underlying object. If the underlying object does not have a value, begin and end return nullptr.

6 Implementation

A publically available implementation at https://github.com/steve-downey/view_maybe based on the Ranges implementation in cmcstl2 at https://github.com/CaseyCarter/cmcstl2 . There are no particular implementation difficulties or tricks. The declarations are essentially what is quoted in the Wording section and the implementations are described as Effects.

Compiler Explorer Link to Before/After Examples

7 LEWG Attention

Call LEWG’s attention to the use of ptrdiff_t as the return type of size (which is consistent with single_view). The author has a weak preference for a signed type here, but a strong preference for consistency with other Range types. single_view now uses size_t.

Call LEWG’s attention to removing the conditional noexcept constructors, consistent with single_view.

8 Wording

8.1 Synopsis

Modify 24.2 Header synopsis

  // [range.maybe], maybe view
  template<copy_constructible T>
    requires see below
  class maybe_view;

  namespace views { inline constexpr unspecified maybe = unspecified; }

8.2 Maybe View [range.maybe]

8.2.1 Overview

1 maybe_view produces a view over a nullable that is either empty if the nullable is empty, or provides access to the contents of the nullable object.

2 The name views::maybe denotes a customization point object ([customization.point.object]). For some subexpression E, the expression views::maybe(E) is expression-equivalent to:

• maybe_view{E}, the view specified below, if the expression is well formed, where decay-copy(E) is moved into the maybe_view
• otherwise views::maybe(E) is ill-formed.

[Note: Whenever views::maybe(E) is a valid expression, it is a prvalue whose type models view. — end note ]

3 [ Example:

  optional o{4};
  maybe_view m{o};
  for (int i : m)
    cout << i;        // prints 4

— end example ]

8.2.2 Concept nullable

1 Types that:

• are contextually convertible to bool
• are dereferenceable
• have const references which are dereferenceable
• the iter_reference_t of the type and the iter_reference_t of the const type, will :
  • satisfy is_lvalue_reference
  • satisfy is_object when the reference is removed
  • for const pointers to the referred to types, satisfy convertible_to
• or are a reference_wrapper around a type that satifies nullable

model the exposition only nullable concept

2 Given a value i of type I, I models nullable only if the expression *i is equality-preserving. [ Note: The expression *i is required to be valid via the exposition-only nullable concept). — end note ]

3 For convienence, the exposition-only is-reference-wrapper-v is used below.

// For Exposition
    template <typename T>
    struct is_reference_wrapper : false_type {};

    template <typename T>
    struct is_reference_wrapper<reference_wrapper<T>> : true_type {};

    template <typename T>
    inline constexpr bool is_reference_wrapper_v
        = is_reference_wrapper<T>::value;

// For Exposition
template <class Ref, class ConstRef>
concept readable_references =
    is_lvalue_reference_v<Ref> &&
    is_object_v<remove_reference_t<Ref>> &&
    is_lvalue_reference_v<ConstRef> &&
    is_object_v<remove_reference_t<ConstRef>> &&
    convertible_to<add_pointer_t<ConstRef>,
                   const remove_reference_t<Ref>*>;

template <class T>
concept nullable =
    is_object_v<T> &&
    requires(T& t, const T& ct) {
        bool(ct); // Contextually bool
        *t; // T& is deferenceable
        *ct; // const T& is deferenceable
    }
    && readable_references<iter_reference_t<T>,        // Ref
                           iter_reference_t<const T>>; // ConstRef

template <class T>
concept wrapped_nullable =
    is-reference-wrapper-v<T>
    && nullable<typename T::type>;

8.2.3 Class template maybe_view

namespace std::ranges {
  template <copy_constructible Maybe>
    requires (nullable<Maybe> || wrapped-nullable<Maybe>)
  class maybe_view : public view_interface<maybe_view<Maybe>> {
  private:
    using T = /* see below */
    semiregular-box<Maybe> value_; // exposition only (see 24.7.2 [range.semi.wrap])

  public:
    constexpr maybe_view() = default;
    constexpr explicit maybe_view(Maybe const& maybe);

    constexpr explicit maybe_view(Maybe&& maybe);

    template<class... Args>
    requires constructible_from<Maybe, Args...>
    constexpr maybe_view(in_place_t, Args&&... args);

    constexpr T*       begin() noexcept;
    constexpr const T* begin() const noexcept;
    constexpr T*       end() noexcept;
    constexpr const T* end() const noexcept;

    constexpr size_t size() const noexcept;

    constexpr T* data() noexcept;
    constexpr const T* data() const noexcept;
  };
}

// For Exposition
using T = remove_reference_t<iter_reference_t<typename unwrap_reference_t<Maybe>>>;

constexpr explicit maybe_view(Maybe const& maybe);

1 Effects: Initializes value_ with maybe.

constexpr explicit maybe_view(Maybe&& maybe);

2 Effects: Initializes value_ with move(maybe).

template<class... Args>
constexpr maybe_view(in_place_t, Args&&... args);

3 Effects: Initializes value_ as if by value_{in_place, forward<Args>(args)...}.

constexpr T* begin() noexcept;
constexpr const T* begin() const noexcept;

4 Effects: Equivalent to: return data();.

constexpr T* end() noexcept;
constexpr const T* end() const noexcept;

5 Effects: Equivalent to: return data() + size();.

static constexpr ptrdiff_t size() noexcept;

6 Effects: Equivalent to:

        if constexpr (is-reference-wrapper-v<Maybe>) {
            return bool(value_.get().get());
        } else {
            return bool(value_.get());
        }

🔗

constexpr T* data() noexcept;

7 Effects: Equivalent to:

        Maybe& m = value_.get();
        if constexpr (is-reference-wrapper-v<Maybe>) {
            return m.get() ? addressof(*(m.get())) : nullptr;
        } else {
            return m ? addressof(*m) : nullptr;
        }

constexpr const T* data() const noexcept;

8 Effects: Equivalent to:

        const Maybe& m = value_.get();
        if constexpr (is-reference-wrapper-v<Maybe>) {
            return m.get() ? addressof(*(m.get())) : nullptr;
        } else {
            return m ? addressof(*m) : nullptr;
        }

9 Impact on the standard

A pure library extension, affecting no other parts of the library or language.

10 References