diff --git a/doc/html/variant2.html b/doc/html/variant2.html new file mode 100644 index 0000000..0b63524 --- /dev/null +++ b/doc/html/variant2.html @@ -0,0 +1,2153 @@ + + + + + + + + +Boost.Variant2: A never valueless variant type + + + + + +
+
+

Overview

+
+
+

This library implements a type-safe discriminated union (variant) type, +variant<T…​>, that almost conforms to the C++17 Standard’s +std::variant<T…​>. The +main differences between the two are:

+
+
+
    +
  • +

    variant<T…​> does not have a valueless-by-exception state;

    +
    • +
  • +

    A converting constructor from, e.g. variant<int, float> to +variant<float, double, int> is provided as an extension;

    +
    • +
  • +

    The reverse operation, going from variant<float, double, int> to +variant<int, float> is provided as the member function subset<U…​>. +(This operation can throw if the current state of the variant cannot be +represented.)

    +
    • +
  • +

    variant<T…​> is not trivial when all contained types are trivial, as +mandated by C++17.

    +
    • +
+
+
+

To avoid the valueless-by-exception state, this implementation falls +back to using double storage unless

+
+
+
    +
  • +

    one of the alternatives is the type monostate,

    +
    • +
  • +

    one of the alternatives has a nonthrowing default constructor, or

    +
    • +
  • +

    all the contained types are nothrow move constructible.

    +
    • +
+
+
+

If the first two bullets don’t hold, but the third does, the variant uses +single storage, but emplace constructs a temporary and moves it into place +if the construction of the object can throw. In case this is undesirable, one +can force emplace into always constructing in-place by adding monostate as +one of the alternatives.

+
+
+
+
+

Reference

+
+
+

<boost/variant2/variant.hpp>

+
+

Synopsis

+
+
+
namespace boost {
+namespace variant2 {
+
+// in_place_type
+
+template<class T> struct in_place_type_t {};
+template<class T> constexpr in_place_type_t<T> in_place_type{};
+
+// in_place_index
+
+template<std::size_t I> struct in_place_index_t {};
+template<std::size_t I> constexpr in_place_index_t<I> in_place_index{};
+
+// variant
+
+template<class... T> class variant;
+
+// variant_size
+
+template<class T> struct variant_size {};
+
+template<class T> struct variant_size<T const>: variant_size<T> {};
+template<class T> struct variant_size<T volatile>: variant_size<T> {};
+template<class T> struct variant_size<T const volatile>: variant_size<T> {};
+
+template<class T> struct variant_size<T&>: variant_size<T> {}; // extension
+template<class T> struct variant_size<T&&>: variant_size<T> {}; // extension
+
+template<class T>
+  inline constexpr size_t variant_size_v = variant_size<T>::value;
+
+template<class... T>
+  struct variant_size<variant<T...>>:
+    std::integral_constant<std::size_t, sizeof...(T)> {};
+
+// variant_alternative
+
+template<size_t I, class T> struct variant_alternative {};
+
+template<size_t I, class T> struct variant_alternative<I, T const>;
+template<size_t I, class T> struct variant_alternative<I, T volatile>;
+template<size_t I, class T> struct variant_alternative<I, T const volatile>;
+
+template<size_t I, class T> struct variant_alternative<I, T&>; // extension
+template<size_t I, class T> struct variant_alternative<I, T&&>; // extension
+
+template<size_t I, class T>
+  using variant_alternative_t = typename variant_alternative<I, T>::type;
+
+template<size_t I, class... T>
+  struct variant_alternative<I, variant<T...>>;
+
+// variant_npos
+
+constexpr std::size_t variant_npos = -1;
+
+// holds_alternative
+
+template<class U, class... T>
+  constexpr bool holds_alternative(const variant<T...>& v) noexcept;
+
+// get
+
+template<size_t I, class... T>
+  constexpr variant_alternative_t<I, variant<T...>>&
+    get(variant<T...>& v);
+template<size_t I, class... T>
+  constexpr variant_alternative_t<I, variant<T...>>&&
+    get(variant<T...>&& v);
+template<size_t I, class... T>
+  constexpr const variant_alternative_t<I, variant<T...>>&
+    get(const variant<T...>& v);
+template<size_t I, class... T>
+  constexpr const variant_alternative_t<I, variant<T...>>&&
+    get(const variant<T...>&& v);
+
+template<class U, class... T>
+  constexpr U& get(variant<T...>& v);
+template<class U, class... T>
+  constexpr U&& get(variant<T...>&& v);
+template<class U, class... T>
+  constexpr const U& get(const variant<T...>& v);
+template<class U, class... T>
+  constexpr const U&& get(const variant<T...>&& v);
+
+// get_if
+
+template<size_t I, class... T>
+  constexpr add_pointer_t<variant_alternative_t<I, variant<T...>>>
+    get_if(variant<T...>* v) noexcept;
+template<size_t I, class... T>
+  constexpr add_pointer_t<const variant_alternative_t<I, variant<T...>>>
+    get_if(const variant<T...>* v) noexcept;
+
+template<class U, class... T>
+  constexpr add_pointer_t<U>
+    get_if(variant<T...>* v) noexcept;
+template<class U, class... T>
+  constexpr add_pointer_t<const U>
+    get_if(const variant<T...>* v) noexcept;
+
+// relational operators
+
+template<class... T>
+  constexpr bool operator==(const variant<T...>& v, const variant<T...>& w);
+template<class... T>
+  constexpr bool operator!=(const variant<T...>& v, const variant<T...>& w);
+template<class... T>
+  constexpr bool operator<(const variant<T...>& v, const variant<T...>& w);
+template<class... T>
+  constexpr bool operator>(const variant<T...>& v, const variant<T...>& w);
+template<class... T>
+  constexpr bool operator<=(const variant<T...>& v, const variant<T...>& w);
+template<class... T>
+  constexpr bool operator>=(const variant<T...>& v, const variant<T...>& w);
+
+// visit
+
+template<class F, class... V>
+  constexpr /*see below*/ visit(F&& f, V&&... v);
+
+// monostate
+
+struct monostate {};
+
+constexpr bool operator==(monostate, monostate) noexcept { return true; }
+constexpr bool operator!=(monostate, monostate) noexcept { return false; }
+constexpr bool operator<(monostate, monostate) noexcept { return false; }
+constexpr bool operator>(monostate, monostate) noexcept { return false; }
+constexpr bool operator<=(monostate, monostate) noexcept { return true; }
+constexpr bool operator>=(monostate, monostate) noexcept { return true; }
+
+// swap
+
+template<class... T>
+  void swap(variant<T...>& v, variant<T...>& w) noexcept( /*see below*/ );
+
+// bad_variant_access
+
+class bad_variant_access;
+
+} // namespace variant2
+} // namespace boost
+
+
+
+
+

variant

+
+
+
namespace boost {
+namespace variant2 {
+
+template<class... T> class variant
+{
+public:
+
+  // constructors
+
+  constexpr variant() noexcept( /*see below*/ );
+
+  constexpr variant( variant const & r ) noexcept( /*see below*/ );
+  constexpr variant( variant&& r ) noexcept( /*see below*/ );
+
+  template<class U>
+    constexpr variant( U&& u ) noexcept( /*see below*/ );
+
+  template<class U, class... A>
+    constexpr explicit variant( in_place_type_t<U>, A&&... a );
+  template<class U, class V, class... A>
+    constexpr explicit variant( in_place_type_t<U>,
+      std::initializer_list<V> il, A&&... a );
+
+  template<size_t I, class... A>
+    constexpr explicit variant( in_place_index_t<I>, A&&... a );
+  template<size_t I, class V, class... A>
+    constexpr explicit variant( in_place_index_t<I>,
+      std::initializer_list<V> il, A&&... a );
+
+  // destructor
+
+  ~variant();
+
+  // assignment
+
+  constexpr variant& operator=( variant const & r ) noexcept( /*see below*/ );
+  constexpr variant& operator=( variant&& r ) noexcept( /*see below*/ );
+
+  template<class U> constexpr variant& operator=( U&& u ) noexcept( /*see below*/ );
+
+  // modifiers
+
+  template<class U, class... A>
+    constexpr U& emplace( A&&... a );
+  template<class U, class V, class... A>
+    constexpr U& emplace( std::initializer_list<V> il, A&&... a );
+
+  template<size_t I, class... A>
+    constexpr variant_alternative_t<I, variant<T...>>&
+      emplace( A&&... a );
+  template<size_t I, class V, class... A>
+    constexpr variant_alternative_t<I, variant<T...>>&
+      emplace( std::initializer_list<V> il, A&&... a );
+
+  // value status
+
+  constexpr bool valueless_by_exception() const noexcept;
+  constexpr size_t index() const noexcept;
+
+  // swap
+
+  void swap( variant& r ) noexcept( /*see below*/ );
+
+  // converting constructors (extension)
+
+  template<class... U> variant( variant<U...> const& r )
+    noexcept( /*see below*/ );
+
+  template<class... U> variant( variant<U...>&& r )
+    noexcept( /*see below*/ );
+
+  // subset (extension)
+
+  template<class... U> constexpr variant<U...> subset() & ;
+  template<class... U> constexpr variant<U...> subset() && ;
+  template<class... U> constexpr variant<U...> subset() const& ;
+  template<class... U> constexpr variant<U...> subset() const&& ;
+};
+
+} // namespace variant2
+} // namespace boost
+
+
+
+
Constructors
+
+
+
constexpr variant() noexcept( std::is_nothrow_default_constructible_v<T0> );
+
+
+
+
    +
  • +

    +
    +
    +
    Effects:
    +
    +

    Constructs a variant holding a value-initialized value of +type T0.

    +
    +
    Ensures:
    +
    +

    index() == 0.

    +
    +
    Throws:
    +
    +

    Any exception thrown by the value-initialization of T0.

    +
    +
    Remarks:
    +
    +

    This function does not participate in overload resolution unless +std::is_default_constructible_v<T0> is true.

    +
    +
    +
    +
    • +
+
+
+
+
constexpr variant( variant const & w )
+  noexcept( mp_all<std::is_nothrow_copy_constructible<T>...>::value );
+
+
+
+
    +
  • +

    +
    +
    +
    Effects:
    +
    +

    Initializes the variant to hold the same alternative and value as +w.

    +
    +
    Throws:
    +
    +

    Any exception thrown by the initialization of the contained value.

    +
    +
    Remarks:
    +
    +

    This function does not participate in overload resolution unless +std::is_copy_constructible_v<Ti> is true for all i.

    +
    +
    +
    +
    • +
+
+
+
+
constexpr variant( variant&& w )
+  noexcept( mp_all<std::is_nothrow_move_constructible<T>...>::value );
+
+
+
+
    +
  • +

    +
    +
    +
    Effects:
    +
    +

    Initializes the variant to hold the same alternative and value as +w.

    +
    +
    Throws:
    +
    +

    Any exception thrown by the move-initialization of the contained +value.

    +
    +
    Remarks:
    +
    +

    This function does not participate in overload resolution unless +std::is_move_constructible_v<Ti> is true for all i.

    +
    +
    +
    +
    • +
+
+
+
+
template<class U> constexpr variant( U&& u ) noexcept(/*see below*/);
+
+
+
+
    +
  • +

    +
    +

    Let Tj be a type that is determined as follows: build an imaginary function +FUN(Ti) for each alternative type Ti. The overload FUN(Tj) selected by +overload resolution for the expression FUN(std::forward<U>(u)) defines the +alternative Tj which is the type of the contained value after construction.

    +
    +
    +
    +
    Effects:
    +
    +

    Initializes *this to hold the alternative type Tj and +initializes the contained value from std::forward<U>(u).

    +
    +
    Ensures:
    +
    +

    holds_alternative<Tj>(*this).

    +
    +
    Throws:
    +
    +

    Any exception thrown by the initialization of the contained value.

    +
    +
    Remarks:
    +
    +

    The expression inside noexcept is equivalent to +std::is_nothrow_constructible_v<Tj, U>. This function does not participate in +overload resolution unless

    +
    +
      +
    • +

      sizeof…​(T) is nonzero,

      +
      • +
    • +

      std::is_same_v<std::remove_cvref_t<U>, variant> is false,

      +
      • +
    • +

      std::remove_cvref_t<U> is neither a specialization of in_place_type_t nor a +specialization of in_place_index_t,

      +
      • +
    • +

      std::is_constructible_v<Tj, U> is true, and

      +
      • +
    • +

      the expression FUN(std::forward<U>(u)) is well-formed.

      +
      • +
    +
    +
    +
    +
    +
    • +
+
+
+
+
template<class U, class... A>
+  constexpr explicit variant( in_place_type_t<U>, A&&... a );
+
+
+
+
    +
  • +

    +
    +
    +
    Effects:
    +
    +

    Initializes the contained value of type U with the arguments +std::forward<A>(a)…​.

    +
    +
    Ensures:
    +
    +

    holds_alternative<U>(*this).

    +
    +
    Throws:
    +
    +

    Any exception thrown by the initialization of the contained value.

    +
    +
    Remarks:
    +
    +

    This function does not participate in overload resolution unless +there is exactly one occurrence of U in T…​ and +std::is_constructible_v<U, A…​> is true.

    +
    +
    +
    +
    • +
+
+
+
+
template<class U, class V, class... A>
+  constexpr explicit variant( in_place_type_t<U>, std::initializer_list<V> il,
+    A&&... a );
+
+
+
+
    +
  • +

    +
    +
    +
    Effects:
    +
    +

    Initializes the contained value of type U with the arguments il, +std::forward<A>(a)…​.

    +
    +
    Ensures:
    +
    +

    holds_alternative<U>(*this).

    +
    +
    Throws:
    +
    +

    Any exception thrown by the initialization of the contained value.

    +
    +
    Remarks:
    +
    +

    This function does not participate in overload resolution unless +there is exactly one occurrence of U in T…​ and +std::is_constructible_v<U, initializer_list<V>&, A…​> is true.

    +
    +
    +
    +
    • +
+
+
+
+
template<size_t I, class... A>
+  constexpr explicit variant( in_place_index_t<I>, A&&... a );
+
+
+
+
    +
  • +

    +
    +
    +
    Effects:
    +
    +

    Initializes the contained value of type TI with the arguments +std::forward<A>(a)…​.

    +
    +
    Ensures:
    +
    +

    index() == I.

    +
    +
    Throws:
    +
    +

    Any exception thrown by the initialization of the contained value.

    +
    +
    Remarks:
    +
    +

    This function does not participate in overload resolution unless +I < sizeof…​(T) and std::is_constructible_v<TI, A…​> is true.

    +
    +
    +
    +
    • +
+
+
+
+
template<size_t I, class V, class... A>
+  constexpr explicit variant( in_place_index_t<I>, std::initializer_list<V> il,
+    A&&... a );
+
+
+
+
    +
  • +

    +
    +
    +
    Effects:
    +
    +

    Initializes the contained value of type TI with the arguments +il, std::forward<A>(a)…​.

    +
    +
    Ensures:
    +
    +

    index() == I.

    +
    +
    Throws:
    +
    +

    Any exception thrown by the initialization of the contained value.

    +
    +
    Remarks:
    +
    +

    This function does not participate in overload resolution unless +I < sizeof…​(T) and +std::is_constructible_v<TI, initializer_list<V>&, A…​> is true.

    +
    +
    +
    +
    • +
+
+
+
+
Destructor
+
+
+
~variant();
+
+
+
+
    +
  • +

    +
    +
    +
    Effects:
    +
    +

    Destroys the currently contained value.

    +
    +
    +
    +
    • +
+
+
+
+
Assignment
+
+
+
constexpr variant& operator=( const variant& r )
+  noexcept( mp_all<std::is_nothrow_copy_constructible<T>...,
+    std::is_nothrow_copy_assignable<T>...>::value );
+
+
+
+
    +
  • +

    +
    +

    Let j be r.index().

    +
    +
    +
    +
    Effects:
    +
    +
    +
      +
    • +

      If index() == j, assigns the value contained in r to the value +contained in *this.

      +
      • +
    • +

      Otherwise, equivalent to emplace<j>(get<j>(r)).

      +
      • +
    +
    +
    +
    Returns:
    +
    +

    *this.

    +
    +
    Ensures:
    +
    +

    index() == r.index().

    +
    +
    Remarks:
    +
    +

    This operator does not participate in overload resolution unless +std::is_copy_constructible_v<Ti> && std::is_copy_assignable_v<Ti> is +true for all i.

    +
    +
    +
    +
    • +
+
+
+
+
constexpr variant& operator=( variant&& r )
+  noexcept( mp_all<std::is_nothrow_move_constructible<T>...,
+    std::is_nothrow_move_assignable<T>...>::value );
+
+
+
+
    +
  • +

    +
    +

    Let j be r.index().

    +
    +
    +
    +
    Effects:
    +
    +
    +
      +
    • +

      If index() == j, assigns the value contained in std::move(r) to the +value contained in *this.

      +
      • +
    • +

      Otherwise, equivalent to emplace<j>(get<j>(std::move(r))).

      +
      • +
    +
    +
    +
    Returns:
    +
    +

    *this.

    +
    +
    Ensures:
    +
    +

    index() == r.index().

    +
    +
    Remarks:
    +
    +

    This operator does not participate in overload resolution unless +std::is_move_constructible_v<Ti> && std::is_move_assignable_v<Ti> is +true for all i.

    +
    +
    +
    +
    • +
+
+
+
+
template<class U> constexpr variant& operator=( U&& u )
+  noexcept( /*see below*/ );
+
+
+
+
    +
  • +

    +
    +

    Let Tj be a type that is determined as follows: build an imaginary function +FUN(Ti) for each alternative type Ti. The overload FUN(Tj) selected by +overload resolution for the expression FUN(std::forward<U>(u)) defines the +alternative Tj which is the type of the contained value after construction.

    +
    +
    +
    +
    Effects:
    +
    +
    +
      +
    • +

      If index() == j, assigns std::forward<U>(u) to the value contained in +*this.

      +
      • +
    • +

      Otherwise, equivalent to emplace<j>(std::forward<U>(u)).

      +
      • +
    +
    +
    +
    Returns:
    +
    +

    *this.

    +
    +
    Ensures:
    +
    +

    index() == j.

    +
    +
    Remarks:
    +
    +

    The expression inside noexcept is std::is_nothrow_constructible_v<Tj, U> + && std::is_nothrow_assignable_v<Tj&, U>. +This operator does not participate in overload resolution unless

    +
    +
      +
    • +

      std::is_same_v<std::remove_cvref_t<T>, variant> is false,

      +
      • +
    • +

      std::is_constructible_v<Tj, U> && std::is_assignable_v<Tj&, U> is +true, and

      +
      • +
    • +

      the expression FUN(std::forward<U>(u)) (with FUN being the +above-mentioned set of imaginary functions) is well-formed.

      +
      • +
    +
    +
    +
    +
    +
    • +
+
+
+
+
Modifiers
+
+
+
template<class U, class... A>
+  constexpr U& emplace( A&&... a );
+
+
+
+
    +
  • +

    +
    +

    Let I be the zero-based index of U in T…​.

    +
    +
    +
    +
    Effects:
    +
    +

    Equivalent to: return emplace<I>(std::forward<A>(a)…​);

    +
    +
    Remarks:
    +
    +

    This function shall not participate in overload resolution unless +std::is_constructible_v<U, A…​> is true and U occurs exactly once +in T…​.

    +
    +
    +
    +
    • +
+
+
+
+
template<class U, class V, class... A>
+  constexpr U& emplace( std::initializer_list<V> il, A&&... a );
+
+
+
+
    +
  • +

    +
    +

    Let I be the zero-based index of U in T…​.

    +
    +
    +
    +
    Effects:
    +
    +

    Equivalent to: return emplace<I>(il, std::forward<A>(a)…​);

    +
    +
    Remarks:
    +
    +

    This function shall not participate in overload resolution unless +std::is_constructible_v<U, std::initializer_list<V>&, A…​> is true +and U occurs exactly once in T…​.

    +
    +
    +
    +
    • +
+
+
+
+
template<size_t I, class... A>
+  constexpr variant_alternative_t<I, variant<T...>>&
+    emplace( A&&... a );
+
+
+
+
    +
  • +

    +
    +
    +
    Requires:
    +
    +

    I < sizeof(T…​).

    +
    +
    Effects:
    +
    +

    Destroys the currently contained value, then initializes a new contained +value as if using the expression Ti(std::forward<A>(a)…​).

    +
    +
    Ensures:
    +
    +

    index() == I.

    +
    +
    Returns:
    +
    +

    A reference to the new contained value.

    +
    +
    Remarks:
    +
    +

    This function shall not participate in overload resolution unless +std::is_constructible_v<Ti, A…​> is true.

    +
    +
    +
    +
    • +
+
+
+
+
template<size_t I, class V, class... A>
+  constexpr variant_alternative_t<I, variant<T...>>&
+    emplace( std::initializer_list<V> il, A&&... a );
+
+
+
+
    +
  • +

    +
    +
    +
    Requires:
    +
    +

    I < sizeof(T…​).

    +
    +
    Effects:
    +
    +

    Destroys the currently contained value, then initializes a new contained +value as if using the expression Ti(il, std::forward<A>(a)…​).

    +
    +
    Ensures:
    +
    +

    index() == I.

    +
    +
    Returns:
    +
    +

    A reference to the new contained value.

    +
    +
    Remarks:
    +
    +

    This function shall not participate in overload resolution unless +std::is_constructible_v<Ti, std::initializer_list<V>&, A…​> is true.

    +
    +
    +
    +
    • +
+
+
+
+
Value Status
+
+
+
constexpr bool valueless_by_exception() const noexcept;
+
+
+
+
    +
  • +

    +
    +
    +
    Returns:
    +
    +

    false.

    +
    +
    +
    +
    • +
+
+
+
+
constexpr size_t index() const noexcept;
+
+
+
+
    +
  • +

    +
    +
    +
    Returns:
    +
    +

    The zero-based index of the active alternative.

    +
    +
    +
    +
    • +
+
+
+
+
Swap
+
+
+
void swap( variant& r ) noexcept( mp_all<std::is_nothrow_move_constructible<T>...,
+  is_nothrow_swappable<T>...>::value );
+
+
+
+
    +
  • +

    +
    +
    +
    Effects:
    +
    +
    +
      +
    • +

      If index() == r.index(), calls swap(get<I>(*this), get<I>(r)), +where I is index().

      +
      • +
    • +

      Otherwise, as if +variant tmp(std::move(*this)); *this = std::move(r); r = std::move(tmp);

      +
      • +
    +
    +
    +
    +
    +
    • +
+
+
+
+
Converting Constructors (extension)
+
+
+
template<class... U> variant( variant<U...> const& r )
+  noexcept( mp_all<std::is_nothrow_copy_constructible<U>...>::value );
+
+
+
+
    +
  • +

    +
    +
    +
    Effects:
    +
    +

    Initializes the contained value from the contained value of r.

    +
    +
    Throws:
    +
    +

    Any exception thrown by the initialization of the contained value.

    +
    +
    Remarks:
    +
    +

    This function does not participate in overload resolution unless +all types in U…​ are in T…​ and +std::is_copy_constructible_v<Ui>::value is true for all Ui.

    +
    +
    +
    +
    • +
+
+
+
+
template<class... U> variant( variant<U...>&& r )
+  noexcept( mp_all<std::is_nothrow_move_constructible<U>...>::value );
+
+
+
+
    +
  • +

    +
    +
    +
    Effects:
    +
    +

    Initializes the contained value from the contained value of +std::move(r).

    +
    +
    Throws:
    +
    +

    Any exception thrown by the initialization of the contained value.

    +
    +
    Remarks:
    +
    +

    This function does not participate in overload resolution unless +all types in U…​ are in T…​ and +std::is_move_constructible_v<Ui>::value is true for all Ui.

    +
    +
    +
    +
    • +
+
+
+
+
Subset (extension)
+
+
+
template<class... U> constexpr variant<U...> subset() & ;
+
+
+
+
+
template<class... U> constexpr variant<U...> subset() const& ;
+
+
+
+
    +
  • +

    +
    +
    +
    Returns:
    +
    +

    A variant<U…​> whose contained value is copy-initialized from +the contained value of *this and has the same type.

    +
    +
    Throws:
    +
    +
    +
      +
    • +

      If the active alternative of *this is not among the types in U…​, +bad_variant_access.

      +
      • +
    • +

      Otherwise, any exception thrown by the initialization of the contained value.

      +
      • +
    +
    +
    +
    Remarks:
    +
    +

    This function does not participate in overload resolution unless +all types in U…​ are in T…​ and +std::is_copy_constructible_v<Ui>::value is true for all Ui.

    +
    +
    +
    +
    • +
+
+
+
+
template<class... U> constexpr variant<U...> subset() && ;
+
+
+
+
+
template<class... U> constexpr variant<U...> subset() const&& ;
+
+
+
+
    +
  • +

    +
    +
    +
    Returns:
    +
    +

    A variant<U…​> whose contained value is move-initialized from +the contained value of *this and has the same type.

    +
    +
    Throws:
    +
    +
    +
      +
    • +

      If the active alternative of *this is not among the types in U…​, +bad_variant_access.

      +
      • +
    • +

      Otherwise, any exception thrown by the initialization of the contained value.

      +
      • +
    +
    +
    +
    Remarks:
    +
    +

    This function does not participate in overload resolution unless +all types in U…​ are in T…​ and +std::is_move_constructible_v<Ui>::value is true for all Ui.

    +
    +
    +
    +
    • +
+
+
+
+
+

variant_alternative

+
+
+
template<size_t I, class T> struct variant_alternative<I, T const>;
+
+
+
+
+
template<size_t I, class T> struct variant_alternative<I, T volatile>;
+
+
+
+
+
template<size_t I, class T> struct variant_alternative<I, T const volatile>;
+
+
+
+
+
template<size_t I, class T> struct variant_alternative<I, T&>; // extension
+
+
+
+
+
template<size_t I, class T> struct variant_alternative<I, T&&>; // extension
+
+
+
+
    +
  • +

    +
    +
    +
    +

    If typename variant_alternative<I, T>::type exists and is U,

    +
    +
    +
      +
    • +

      variant_alternative<I, T const>::type is U const;

      +
      • +
    • +

      variant_alternative<I, T volatile>::type is U volatile;

      +
      • +
    • +

      variant_alternative<I, T const volatile>::type is U const volatile.

      +
      • +
    • +

      variant_alternative<I, T&>::type is U&.

      +
      • +
    • +

      variant_alternative<I, T&&>::type is U&&.

      +
      • +
    +
    +
    +

    Otherwise, these structs have no member type.

    +
    +
    +
    +
    • +
+
+
+
+
template<size_t I, class... T>
+  struct variant_alternative<I, variant<T...>>;
+
+
+
+
    +
  • +

    +
    +

    When I < sizeof…​(T), the nested type type is an alias for the I-th +(zero-based) type in T…​. Otherwise, there is no member type.

    +
    +
    • +
+
+
+
+

holds_alternative

+
+
+
template<class U, class... T>
+  constexpr bool holds_alternative(const variant<T...>& v) noexcept;
+
+
+
+
    +
  • +

    +
    +
    +
    Requires:
    +
    +

    The type U occurs exactly once in T…​. Otherwise, the +program is ill-formed.

    +
    +
    Returns:
    +
    +

    true if index() is equal to the zero-based index of U +in T…​.

    +
    +
    +
    +
    • +
+
+
+
+

get

+
+
+
template<size_t I, class... T>
+  constexpr variant_alternative_t<I, variant<T...>>&
+    get(variant<T...>& v);
+
+
+
+
+
template<size_t I, class... T>
+  constexpr variant_alternative_t<I, variant<T...>>&&
+    get(variant<T...>&& v);
+
+
+
+
+
template<size_t I, class... T>
+  constexpr const variant_alternative_t<I, variant<T...>>&
+    get(const variant<T...>& v);
+
+
+
+
+
template<size_t I, class... T>
+  constexpr const variant_alternative_t<I, variant<T...>>&&
+    get(const variant<T...>&& v);
+
+
+
+
    +
  • +

    +
    +
    +
    Effects:
    +
    +

    If v.index() is I, returns a reference to the object stored in +the variant. Otherwise, throws bad_variant_access.

    +
    +
    +
    +
    • +
+
+
+
+
template<class U, class... T>
+  constexpr U& get(variant<T...>& v);
+
+
+
+
+
template<class U, class... T>
+  constexpr U&& get(variant<T...>&& v);
+
+
+
+
+
template<class U, class... T>
+  constexpr const U& get(const variant<T...>& v);
+
+
+
+
+
template<class U, class... T>
+  constexpr const U&& get(const variant<T...>&& v);
+
+
+
+
    +
  • +

    +
    +
    +
    Requires:
    +
    +

    The type U occurs exactly once in T…​. Otherwise, the +program is ill-formed.

    +
    +
    Effects:
    +
    +

    If v holds a value of type U, returns a reference to that value. +Otherwise, throws bad_variant_access.

    +
    +
    +
    +
    • +
+
+
+
+

get_if

+
+
+
template<size_t I, class... T>
+  constexpr add_pointer_t<variant_alternative_t<I, variant<T...>>>
+    get_if(variant<T...>* v) noexcept;
+
+
+
+
+
template<size_t I, class... T>
+  constexpr add_pointer_t<const variant_alternative_t<I, variant<T...>>>
+    get_if(const variant<T...>* v) noexcept;
+
+
+
+
    +
  • +

    +
    +
    +
    Requires:
    +
    +

    I < sizeof…​(U). Otherwise, the program is ill-formed.

    +
    +
    Effects:
    +
    +

    A pointer to the value stored in the variant, if +v != nullptr && v->index() == I. Otherwise, nullptr.

    +
    +
    +
    +
    • +
+
+
+
+
template<class U, class... T>
+  constexpr add_pointer_t<U>
+    get_if(variant<T...>* v) noexcept;
+
+
+
+
+
template<class U, class... T>
+  constexpr add_pointer_t<const U>
+    get_if(const variant<T...>* v) noexcept;
+
+
+
+
    +
  • +

    +
    +
    +
    Requires:
    +
    +

    The type U occurs exactly once in T…​. Otherwise, the +program is ill-formed.

    +
    +
    Effects:
    +
    +

    Equivalent to: return get_if<I>(v); with I being +the zero-based index of U in T…​.

    +
    +
    +
    +
    • +
+
+
+
+

Relational Operators

+
+
+
template<class... T>
+  constexpr bool operator==(const variant<T...>& v, const variant<T...>& w);
+
+
+
+
    +
  • +

    +
    +
    +
    Returns:
    +
    +

    v.index() == w.index && get<I>(v) == get<I>(w), where I +is v.index().

    +
    +
    +
    +
    • +
+
+
+
+
template<class... T>
+  constexpr bool operator!=(const variant<T...>& v, const variant<T...>& w);
+
+
+
+
    +
  • +

    +
    +
    +
    Returns:
    +
    +

    !(v == w).

    +
    +
    +
    +
    • +
+
+
+
+
template<class... T>
+  constexpr bool operator<(const variant<T...>& v, const variant<T...>& w);
+
+
+
+
    +
  • +

    +
    +
    +
    Returns:
    +
    +

    v.index() < w.index || (v.index() == w.index && get<I>(v) < get<I>(w)), +where I is v.index().

    +
    +
    +
    +
    • +
+
+
+
+
template<class... T>
+  constexpr bool operator>(const variant<T...>& v, const variant<T...>& w);
+
+
+
+
    +
  • +

    +
    +
    +
    Returns:
    +
    +

    w < v.

    +
    +
    +
    +
    • +
+
+
+
+
template<class... T>
+  constexpr bool operator<=(const variant<T...>& v, const variant<T...>& w);
+
+
+
+
    +
  • +

    +
    +
    +
    Returns:
    +
    +

    v.index() < w.index || (v.index() == w.index && get<I>(v) <= get<I>(w)), +where I is v.index().

    +
    +
    +
    +
    • +
+
+
+
+
template<class... T>
+  constexpr bool operator>=(const variant<T...>& v, const variant<T...>& w);
+
+
+
+
    +
  • +

    +
    +
    +
    Returns:
    +
    +

    w <= v.

    +
    +
    +
    +
    • +
+
+
+
+

visit

+
+
+
template<class F, class... V>
+  constexpr /*see below*/ visit(F&& f, V&&... v);
+
+
+
+
    +
  • +

    +
    +
    +
    Returns:
    +
    +

    std::forward<F>(f)(get<I>(std::forward<V>(v))…​), where +I…​ is v.index()…​.

    +
    +
    +
    +
    • +
+
+
+
+

swap

+
+
+
template<class... T>
+  void swap(variant<T...>& v, variant<T...>& w) noexcept( /*see below*/ );
+
+
+
+
    +
  • +

    +
    +
    +
    Effects:
    +
    +

    Equivalent to v.swap(w).

    +
    +
    +
    +
    • +
+
+
+
+

bad_variant_access

+
+
+
class bad_variant_access: public std::exception
+{
+public:
+
+    bad_variant_access() noexcept = default;
+
+    char const * what() const noexcept
+    {
+        return "bad_variant_access";
+    }
+};
+
+
+
+
+
+
+
+ +
+
+

This documentation is copyright 2018, 2019 Peter Dimov and is distributed under +the Boost Software License, Version 1.0.

+
+
+
+
+ + + + \ No newline at end of file