Document Number:N3848
Date:
Editor: Jeffrey Yasskin
Google, Inc.
jyasskin@google.com

Working Draft, Technical Specification on C++ Extensions for Library Fundamentals

Note: this is an early draft. It’s known to be incomplet and incorrekt, and it has lots of bad formatting.

1 General [general]

1.1 Scope [general.scope]

This technical specification describes extensions to the C++ Standard Library (1.2). These extensions are classes and functions that are likely to be used widely within a program and/or on the interface boundaries between libraries written by different organizations.

This technical specification is non-normative. Some of the library components in this technical specification may be considered for standardization in a future version of C++, but they are not currently part of any C++ standard. Some of the components in this technical specification may never be standardized, and others may be standardized in a substantially changed form.

The goal of this technical specification it to build more widespread existing practice for an expanded C++ standard library. It gives advice on extensions to those vendors who wish to provide them.

1.2 Normative references [general.references]

The following referenced document is indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

  • ISO/IEC 14882:2011, Programming Languages — C++

ISO/IEC 14882:2011 is herein called the C++ Standard. References to clauses within the C++ Standard are written as "C++11 §3.2". The library described in ISO/IEC 14882:2011 clauses 17–30 is herein called the C++ Standard Library.

Unless otherwise specified, the whole of the C++ Standard's Library introduction (C++11 §17) is included into this Technical Specification by reference.

1.3 Namespaces and headers [general.namespaces]

Since the extensions described in this technical specification are experimental and not part of the C++ standard library, they should not be declared directly within namespace std. Unless otherwise specifed, all components described in this technical specification are declared in namespace std::experimental::fundamentals_v1 or a subnamespace thereof. Each header described in this technical specification shall import the contents of std::experimental::fundamentals_v1 into std::experimental as if by

namespace std {
  namespace experimental {
    inline namespace fundamentals_v1 {}
  }
}

Unless otherwise specified, references to other entities described in this technical specification are assumed to be qualified with std::experimental::fundamentals_v1::, and references to entities described in the standard are assumed to be qualified with std::.

Extensions that are expected to eventually be added to an existing header <meow> are provided inside the <experimental/meow> header, which shall include the standard contents of <meow> as if by

#include <meow>

[ Note: This implies that this technical specification cannot remove an existing overload of a function. — end note ]

New headers are also provided in the <experimental/> directory, but without such an #include.

1.4 Terms and definitions [general.defns]

For the purposes of this document, the terms and definitions given in the C++ Standard and the folowing apply.

1.5 Future plans (Informative) [general.plans]

This section describes tentative plans for future versions of this technical specification and plans for moving content into future versions of the C++ Standard.

The C++ committee intends to release a new version of this technical specification approximately every year, containing the library extensions we hope to add to a near-future version of the C++ Standard. Future versions will define their contents in std::experimental::fundamentals_v2, std::experimental::fundamentals_v3, etc., with the most recent implemented version inlined into std::experimental.

When an extension defined in this or a future version of this technical specification represents enough existing practice, it will be moved into the next version of the C++ Standard by removing the experimental::fundamentals_vN segment of its namespace and by removing the experimental/ prefix from its header's path.

2 Optional objects [optional]

2.1 In general [optional.general]

This subclause describes class template optional that represents optional objects. An optional object for object types is an object that contains the storage for another object and manages the lifetime of this contained object, if any. The contained object may be initialized after the optional object has been initialized, and may be destroyed before the optional object has been destroyed. The initialization state of the contained object is tracked by the optional object.

2.2 Header <optional> synopsis [optional.synop]

namespace std {
namespace experimental {
inline namespace fundamentals_v1 {
  // 2.4, optional for object types
  template <class T> class optional;

  // 2.5, In-place construction
  struct in_place_t{};
  constexpr in_place_t in_place{};

  // 2.6, Disengaged state indicator
  struct nullopt_t{see below};
  constexpr nullopt_t nullopt(unspecified);

  // 2.7, Class bad_optional_access
  class bad_optional_access;

  // 2.8, Relational operators
  template <class T>
    constexpr bool operator==(const optional<T>&, const optional<T>&);
  template <class T>
    constexpr bool operator<(const optional<T>&, const optional<T>&);

  // 2.9, Comparison with nullopt
  template <class T> constexpr bool operator==(const optional<T>&, nullopt_t) noexcept;
  template <class T> constexpr bool operator==(nullopt_t, const optional<T>&) noexcept;
  template <class T> constexpr bool operator<(const optional<T>&, nullopt_t) noexcept;
  template <class T> constexpr bool operator<(nullopt_t, const optional<T>&) noexcept;

  // 2.10, Comparison with T
  template <class T> constexpr bool operator==(const optional<T>&, const T&);
  template <class T> constexpr bool operator==(const T&, const optional<T>&);
  template <class T> constexpr bool operator<(const optional<T>&, const T&);
  template <class T> constexpr bool operator<(const T&, const optional<T>&);

  // 2.11, Specialized algorithms
  template <class T> void swap(optional<T>&, optional<T>&) noexcept(see below);
  template <class T> constexpr optional<see below> make_optional(T&&);

} // namespace fundamentals_v1
} // namespace experimental

  // 2.12, Hash support
  template <class T> struct hash;
  template <class T> struct hash<experimental::optional<T>>;
} // namespace std

A program that necessitates the instantiation of template optional for a reference type, or for possibly cv-qualified types in_place_t or nullopt_t is ill-formed.

2.3 Definitions [optional.defs]

An instance of optional<T> is said to be disengaged if it has been default constructed, constructed with or assigned with a value of type nullopt_t, constructed with or assigned with a disengaged optional object of type optional<T>.

An instance of optional<T> is said to be engaged if it is not disengaged.

2.4 optional for object types [optional.object]

template <class T>
class optional
{
public:
  typedef T value_type;

  // 2.4.1, Constructors
  constexpr optional() noexcept;
  constexpr optional(nullopt_t) noexcept;
  optional(const optional&);
  optional(optional&&) noexcept(see below);
  constexpr optional(const T&);
  constexpr optional(T&&);
  template <class... Args> constexpr explicit optional(in_place_t, Args&&...);
  template <class U, class... Args>
    constexpr explicit optional(in_place_t, initializer_list<U>, Args&&...);

  // 2.4.2, Destructor
  ~optional();

  // 2.4.3, Assignment
  optional& operator=(nullopt_t) noexcept;
  optional& operator=(const optional&);
  optional& operator=(optional&&) noexcept(see below);
  template <class U> optional& operator=(U&&);
  template <class... Args> void emplace(Args&&...);
  template <class U, class... Args>
    void emplace(initializer_list<U>, Args&&...);

  // 2.4.4, Swap
  void swap(optional&) noexcept(see below);

  // 2.4.5, Observers
  constexpr T const* operator ->() const;
  T* operator ->();
  constexpr T const& operator *() const;
  T& operator *();
  constexpr explicit operator bool() const noexcept;
  constexpr T const& value() const;
  T& value();
  template <class U> constexpr T value_or(U&&) const&;
  template <class U> T value_or(U&&) &&;

private:
  bool init; // exposition only
  T*   val;  // exposition only
};

Engaged instances of optional<T> where T is of object type shall contain a value of type T within its own storage. This value is referred to as the contained value of the optional object. Implementations are not permitted to use additional storage, such as dynamic memory, to allocate its contained value. The contained value shall be allocated in a region of the optional<T> storage suitably aligned for the type T.

Members init and val are provided for exposition only. Implementations need not provide those members. init indicates whether the optional object's contained value has been initialized (and not yet destroyed); val points to (a possibly uninitialized) contained value.

T shall be an object type and shall satisfy the requirements of Destructible (Table 24).

2.4.1 Constructors [optional.object.ctor]

constexpr optional() noexcept;constexpr optional(nullopt_t) noexcept;
Postconditions:
*this is disengaged.
Remarks:
No T object referenced is initialized. For every object type T these constructors shall be constexpr constructors (C++11 §7.1.5).
 
optional(const optional<T>& rhs);
Requires:
is_copy_constructible<T>::value is true.
Effects:
If rhs is engaged initializes the contained value as if direct-non-list-initializing an object of type T with the expression *rhs.
Postconditions:
bool(rhs) == bool(*this).
Throws:
Any exception thrown by the selected constructor of T.
 
optional(optional<T>&& rhs) noexcept(see below);
Requires:
is_move_constructible<T>::value is true.
Effects:
If rhs is engaged initializes the contained value as if direct-non-list-initializing an object of type T with the expression std::move(*rhs). bool(rhs) is unchanged.
Postconditions:
bool(rhs) == bool(*this).
Throws:
Any exception thrown by the selected constructor of T.
Remarks:
The expression inside noexcept is equivalent to:
is_nothrow_move_constructible<T>::value
 
optional(const T& v);
Requires:
is_copy_constructible<T>::value is true.
Effects:
Initializes the contained value as if direct-non-list-initializing an object of type T with the expression v.
Postconditions:
*this is engaged.
Throws:
Any exception thrown by the selected constructor of T.
Remarks:
If T's selected constructor is a constexpr constructor, this constructor shall be a constexpr constructor.
 
optional(T&& v);
Requires:
is_move_constructible<T>::value is true.
Effects:
Initializes the contained value as if direct-non-list-initializing an object of type T with the expression std::move(v).
Postconditions:
*this is engaged.
Throws:
Any exception thrown by the selected constructor of T.
Remarks:
If T's selected constructor is a constexpr constructor, this constructor shall be a constexpr constructor.
 
template <class... Args> constexpr explicit optional(in_place_t, Args&&... args);
Requires:
is_constructible<T, Args&&...>::value is true.
Effects:
Initializes the contained value as if constructing an object of type T with the arguments std::forward<Args>(args)....
Postconditions:
*this is engaged.
Throws:
Any exception thrown by the selected constructor of T.
Remarks:
If T's constructor selected for the initialization is a constexpr constructor, this constructor shall be a constexpr constructor.
 
template <class U, class... Args> explicit optional(in_place_t, initializer_list<U> il, Args&&... args);
Requires:
is_constructible<T, initializer_list<U>&, Args&&...>::value is true.
Effects:
Initializes the contained value as if constructing an object of type T with the arguments il, std::forward<Args>(args)....
Postconditions:
*this is engaged.
Throws:
Any exception thrown by the selected constructor of T.
Remarks:
The function shall not participate in overload resolution unless is_constructible<T, initializer_list<U>&, Args&&...>::value is true.
Remarks:
If T's constructor selected for the initialization is a constexpr constructor, this constructor shall be a constexpr constructor.

2.4.2 Destructor [optional.object.dtor]

~optional();
Effects:
If is_trivially_destructible<T>::value != true and *this is engaged, calls val->T::~T().
Remarks:
If is_trivially_destructible<T>::value == true then this destructor shall be a trivial destructor.

2.4.3 Assignment [optional.object.assign]

optional<T>& operator=(nullopt_t) noexcept;
Effects:
If *this is engaged calls val->T::~T() to destroy the contained value; otherwise no effect.
Returns:
*this.
Postconditions:
*this is disengaged.
 
optional<T>& operator=(const optional<T>& rhs);
Requires:
is_copy_constructible<T>::value is true and is_copy_assignable<T>::value is true.
Effects:
  • If *this is disengaged and rhs is disengaged, no effect, otherwise
  • if *this is engaged and rhs is disengaged, destroys the contained value by calling val->T::~T(), otherwise
  • if *this is disengaged and rhs is engaged, initializes the contained value as if direct-non-list-initializing an object of type T with *rhs, otherwise
  • (if both *this and rhs are engaged) assigns *rhs to the contained value.
Returns:
*this.
Postconditions:
bool(rhs) == bool(*this).
 
optional<T>& operator=(optional<T>&& rhs) noexcept(see below);
Requires:
is_move_constructible<T>::value is true and is_move_assignable<T>::value is true.
Effects:
  • If *this is disengaged and rhs is disengaged, no effect, otherwise
  • if *this is engaged and rhs is disengaged, destroys the contained value by calling val->T::~T(), otherwise
  • if *this is disengaged and rhs is engaged, initializes the contained value as if direct-non-list-initializing an object of type T with std::move(*rhs), otherwise
  • (if both *this and rhs are engaged) assigns std::move(*rhs) to the contained value.
Returns:
*this.
Postconditions:
bool(rhs) == bool(*this).
Remarks:
The expression inside noexcept is equivalent to: 
is_nothrow_move_assignable<T>::value && is_nothrow_move_constructible<T>::value
 
template <class U> optional<T>& operator=(U&& v);
Requires:
is_constructible<T, U>::value is true and is_assignable<U, T>::value is true.
Effects:
If *this is engaged assigns std::forward<U>(v) to the contained value; otherwise initializes the contained value as if direct-non-list-initializing object of type T with std::forward<U>(v).
Returns:
*this.
Postconditions:
*this is engaged.
Remarks:
The function shall not participate in overload resolution unless is_same<typename remove_reference<U>::type, T>::value is true.
Notes:
The reason for providing such generic assignment and then constraining it so that effectively T == U is to guarantee that assignment of the form o = {} is unambiguous.
 
template <class... Args> void emplace(Args&&... args);
Requires:
is_constructible<T, Args&&...>::value is true.
Effects:
Calls *this = nullopt. Then initializes the contained value as if constructing an object of type T with the arguments std::forward<Args>(args)....
Postconditions:
*this is engaged.
Throws:
Any exception thrown by the selected constructor of T.
 
template <class U, class... Args> void emplace(initializer_list<U> il, Args&&... args);
Requires:
is_constructible<T, initializer_list<U>&, Args&&...>::value is true.
Effects:
Calls *this = nullopt. Then initializes the contained value as if constructing an object of type T with the arguments il, std::forward<Args>(args)....
Postconditions:
*this is engaged.
Throws:
Any exception thrown by the selected constructor of T.
Remarks:
The function shall not participate in overload resolution unless is_constructible<T, initializer_list<U>&, Args&&...>::value is true.

2.4.4 Swap [optional.object.swap]

void swap(optional<T>& rhs) noexcept(see below);
Requires:
LValues of type T shall be swappable and is_move_constructible<T>::value is true.
Effects:
  • If *this is disengaged and rhs is disengaged, no effect, otherwise
  • if *this is engaged and rhs is disengaged, initializes the contained value of rhs by direct-initialization with std::move(*(*this)), followed by val->T::~T(), swap(init, rhs.init), otherwise
  • if *this is disengaged and rhs is engaged, initializes the contained value of *this by direct-initialization with std::move(*rhs), followed by rhs.val->T::~T(), swap(init, rhs.init), otherwise
  • (if both *this and rhs are engaged) calls swap(*(*this), *rhs).
Throws:
Any exceptions that the expressions in the Effects clause throw.
Remarks:
The expression inside noexcept is equivalent to: 
is_nothrow_move_constructible<T>::value && noexcept(swap(declval<T&>(), declval<T&>()))

2.4.5 Observers [optional.object.observe]

constexpr T const* operator->() const;T* operator->();
Requires:
*this is engaged.
Returns:
val.
Throws:
Nothing.
Remarks:
Unless T is a user-defined type with overloaded unary operator&, the first function shall be a constexpr function.
 
constexpr T const& operator*() const;T& operator*();
Requires:
*this is engaged.
Returns:
*val.
Throws:
Nothing.
Remarks:
The first function shall be a constexpr function.
 
constexpr explicit operator bool() noexcept;
Returns:
init.
Remarks:
this function shall be a constexpr function.
 
constexpr T const& value() const;T& value();
Returns:
*val, if bool(*this).
Throws:
bad_optional_access if !*this.
Remarks:
The first function shall be a constexpr function.
 
template <class U> constexpr T value_or(U&& v) const&;
Requires:
is_copy_constructible<T>::value is true and is_convertible<U&&, T>::value is true.
Returns:
bool(*this) ? **this : static_cast<T>(std::forward<U>(v)).
Throws:
Any exception thrown by the selected constructor of T.
Remarks:
If the selected constructor of T is a constexpr constructor, this function shall be a constexpr function.
 
template <class U> T value_or(U&& v) &&;
Requires:
is_move_constructible<T>::value is true and is_convertible<U&&, T>::value is true.
Returns:
bool(*this) ? std::move(**this) : static_cast<T>(std::forward<U>(v)).
Throws:
Any exception thrown by the selected constructor of T.

2.5 In-place construction [optional.inplace]

struct in_place_t{};constexpr in_place_t in_place{};

The struct in_place_t is an empty structure type used as a unique type to disambiguate constructor and function overloading. Specifically, optional<T> has a constructor with in_place_t as the first argument followed by an argument pack; this indicates that T should be constructed in-place (as if by a call to placement new expression) with the forwarded argument pack as parameters.

2.6 Disengaged state indicator [optional.nullopt]

struct nullopt_t{see below};constexpr nullopt_t nullopt(unspecified);

The struct nullopt_t is an empty structure type used as a unique type to indicate a disengaged state for optional objects. In particular, optional<T> has a constructor with nullopt_t as single argument; this indicates that a disengaged optional object shall be constructed.

Type nullopt_t shall not have a default constructor. It shall be a literal type. Constant nullopt shall be initialized with an argument of literal type.

2.7 Class bad_optional_access [optional.bad_optional_access]

class bad_optional_access : public logic_error {
public:
  explicit bad_optional_access(const string& what_arg);
  explicit bad_optional_access(const char* what_arg);
};

The class bad_optional_access defines the type of objects thrown as exceptions to report the situation where an attempt is made to access the value of a disengaged optional object.

bad_optional_access(const string& what_arg);
Effects:
Constructs an object of class bad_optional_access.
 
bad_optional_access(const char* what_arg);
Effects:
Constructs an object of class bad_optional_access.

2.8 Relational operators [optional.relops]

template <class T> constexpr bool operator==(const optional<T>& x, const optional<T>& y);
Requires:
T shall meet the requirements of EqualityComparable.
Returns:
If bool(x) != bool(y), false; otherwise if bool(x) == false, true; otherwise *x == *y.
Remarks:
Instantiations of this function template for which *x == *y is a core constant expression, shall be constexpr functions.
 
template <class T> constexpr bool operator<(const optional<T>& x, const optional<T>& y);
Requires:
Expression less<T>{}(*x, *y) shall be well-formed.
Returns:
If (!y), false; otherwise, if (!x), true; otherwise less<T>{}(*x, *y).
Remarks:
Instantiations of this function template for which less<T>{}(*x, *y) is a core constant expression, shall be constexpr functions.

2.9 Comparison with nullopt [optional.nullops]

template <class T> constexpr bool operator==(const optional<T>& x, nullopt_t) noexcept;template <class T> constexpr bool operator==(nullopt_t, const optional<T>& x) noexcept;
Returns:
(!x).
 
template <class T> constexpr bool operator<(const optional<T>& x, nullopt_t) noexcept;
Returns:
false.
 
template <class T> constexpr bool operator<(nullopt_t, const optional<T>& x) noexcept;
Returns:
bool(x).

2.10 Comparison with T [optional.comp_with_t]

template <class T> constexpr bool operator==(const optional<T>& x, const T& v);
Returns:
bool(x) ? *x == v : false.
 
template <class T> constexpr bool operator==(const T& v, const optional<T>& x);
Returns:
bool(x) ? v == *x : false.
 
template <class T> constexpr bool operator<(const optional<T>& x, const T& v);
Returns:
bool(x) ? less<T>{}(*x, v) : true.

2.11 Specialized algorithms [optional.specalg]

template <class T> void swap(optional<T>& x, optional<T>& y) noexcept(noexcept(x.swap(y)));
Effects:
calls x.swap(y).
 
template <class T> constexpr optional<typename decay<T>::type> make_optional(T&& v);
Returns:
optional<typename decay<T>::type>(std::forward<T>(v)).

2.12 Hash support [optional.hash]

template <class T> struct hash<experimental::optional<T>>;
Requires:
the template specialization hash<T> shall meet the requirements of class template hash (C++11 §20.8.12). The template specialization hash<optional<T>> shall meet the requirements of class template hash. For an object o of type optional<T>, if bool(o) == true, hash<optional<T>>()(o) shall evaluate to the same value as hash<T>()(*o).