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Class template variant

boost::variant — Safe, generic, stack-based discriminated union container.

Synopsis

// In header: <boost/variant/variant.hpp>

template<typename T1, typename T2 = unspecified, ..., 
         typename TN = unspecified> 
class variant {
public:
  // types
  typedef unspecified types;

  // construct/copy/destruct
  variant();
  variant(const variant &);
  template<typename T> variant(T &);
  template<typename T> variant(const T &);
  template<typename U1, typename U2, ..., typename UN> 
    variant(variant<U1, U2, ..., UN> &);
  template<typename U1, typename U2, ..., typename UN> 
    variant(const variant<U1, U2, ..., UN> &);
  ~variant();

  // modifiers
  void swap(variant &);
  variant & operator=(const variant &);
  template<typename T> variant & operator=(const T &);

  // queries
  int which() const;
  bool empty() const;
  const std::type_info & type() const;

  // relational
  bool operator==(const variant &) const;
  template<typename U> void operator==(const U &) const;
  bool operator<(const variant &) const;
  template<typename U> void operator<(const U &) const;
};

Description

The variant class template (inspired by Andrei Alexandrescu's class of the same name [Ale01A]) is an efficient, recursive-capable, bounded discriminated union value type capable of containing any value type (either POD or non-POD). It supports construction from any type convertible to one of its bounded types or from a source variant whose bounded types are each convertible to one of the destination variant's bounded types. As well, through apply_visitor, variant supports compile-time checked, type-safe visitation; and through get, variant supports run-time checked, type-safe value retrieval.

Notes:

  • The bounded types of the variant are exposed via the nested typedef types, which is an MPL-compatible Sequence containing the set of types that must be handled by any visitor to the variant.
  • All members of variant satisfy at least the basic guarantee of exception-safety. That is, all operations on a variant remain defined even after previous operations have failed.
  • Each type specified as a template argument to variant must meet the requirements of the BoundedType concept.
  • Each type specified as a template argument to variant must be distinct after removal of qualifiers. Thus, for instance, both variant<int, int> and variant<int, const int> have undefined behavior.
  • Conforming implementations of variant must allow at least ten types as template arguments. The exact number of allowed arguments is exposed by the preprocessor macro BOOST_VARIANT_LIMIT_TYPES. (See make_variant_over for a means to specify the bounded types of a variant by the elements of an MPL or compatible Sequence, thus overcoming this limitation.)

variant public construct/copy/destruct

  1. variant();

    Requires:

    The first bounded type of the variant (i.e., T1) must fulfill the requirements of the DefaultConstructible [20.1.4] concept.

    Postconditions:

    Content of *this is the default value of the first bounded type (i.e, T1).

    Throws:

    May fail with any exceptions arising from the default constructor of T1.
  2. variant(const variant & other);

    Postconditions:

    Content of *this is a copy of the content of other.

    Throws:

    May fail with any exceptions arising from the copy constructor of other's contained type.
  3. template<typename T> variant(T & operand);

    Requires:

    T must be unambiguously convertible to one of the bounded types (i.e., T1, T2, etc.).

    Postconditions:

    Content of *this is the best conversion of operand to one of the bounded types, as determined by standard overload resolution rules.

    Throws:

    May fail with any exceptions arising from the conversion of operand to one of the bounded types.
  4. template<typename T> variant(const T & operand);

    Notes:

    Same semantics as previous constructor, but allows construction from temporaries.
  5. template<typename U1, typename U2, ..., typename UN> 
      variant(variant<U1, U2, ..., UN> & operand);

    Requires:

    Every one of U1, U2, ..., UN must have an unambiguous conversion to one of the bounded types (i.e., T1, T2, ..., TN).

    Postconditions:

    If variant<U1, U2, ..., UN> is itself one of the bounded types, then content of *this is a copy of operand. Otherwise, content of *this is the best conversion of the content of operand to one of the bounded types, as determined by standard overload resolution rules.

    Throws:

    If variant<U1, U2, ..., UN> is itself one of the bounded types, then may fail with any exceptions arising from the copy constructor of variant<U1, U2, ..., UN>. Otherwise, may fail with any exceptions arising from the conversion of the content of operand to one of the bounded types.
  6. template<typename U1, typename U2, ..., typename UN> 
      variant(const variant<U1, U2, ..., UN> & operand);

    Notes:

    Same semantics as previous constructor, but allows construction from temporaries.
  7. ~variant();

    Effects:

    Destroys the content of *this.

    Throws:

    Will not throw.

variant modifiers

  1. void swap(variant & other);

    Requires:

    Every bounded type must fulfill the requirements of the Assignable concept.

    Effects:

    Interchanges the content of *this and other.

    Throws:

    If the contained type of other is the same as the contained type of *this, then may fail with any exceptions arising from the swap of the contents of *this and other. Otherwise, may fail with any exceptions arising from either of the copy constructors of the contained types. Also, in the event of insufficient memory, may fail with std::bad_alloc (why?).
  2. variant & operator=(const variant & rhs);

    Requires:

    Every bounded type must fulfill the requirements of the Assignable concept.

    Effects:

    If the contained type of rhs is the same as the contained type of *this, then assigns the content of rhs into the content of *this. Otherwise, makes the content of *this a copy of the content of rhs, destroying the previous content of *this.

    Throws:

    If the contained type of rhs is the same as the contained type of *this, then may fail with any exceptions arising from the assignment of the content of rhs into the content *this. Otherwise, may fail with any exceptions arising from the copy constructor of the contained type of rhs. Also, in the event of insufficient memory, may fail with std::bad_alloc (why?).
  3. template<typename T> variant & operator=(const T & rhs);

    Requires:

    • T must be unambiguously convertible to one of the bounded types (i.e., T1, T2, etc.).
    • Every bounded type must fulfill the requirements of the Assignable concept.

    Effects:

    If the contained type of *this is T, then assigns rhs into the content of *this. Otherwise, makes the content of *this the best conversion of rhs to one of the bounded types, as determined by standard overload resolution rules, destroying the previous content of *this.

    Throws:

    If the contained type of *this is T, then may fail with any exceptions arising from the assignment of rhs into the content *this. Otherwise, may fail with any exceptions arising from the conversion of rhs to one of the bounded types. Also, in the event of insufficient memory, may fail with std::bad_alloc (why?).

variant queries

  1. int which() const;

    Returns:

    The zero-based index into the set of bounded types of the contained type of *this. (For instance, if called on a variant<int, std::string> object containing a std::string, which() would return 1.)

    Throws:

    Will not throw.
  2. bool empty() const;

    Returns:

    false: variant always contains exactly one of its bounded types. (See the section called “"Never-Empty" Guarantee” for more information.)

    Rationale:

    Facilitates generic compatibility with boost::any.

    Throws:

    Will not throw.
  3. const std::type_info & type() const;

    Returns:

    typeid(x), where x is the the content of *this.

    Throws:

    Will not throw.

    Notes:

    Not available when BOOST_NO_TYPEID is defined.

variant relational

  1. bool operator==(const variant & rhs) const;
    template<typename U> void operator==(const U &) const;
    Equality comparison.

    Notes:

    The overload returning void exists only to prohibit implicit conversion of the operator's right-hand side to variant; thus, its use will (purposefully) result in a compile-time error.

    Requires:

    Every bounded type of the variant must fulfill the requirements of the EqualityComparable concept.

    Returns:

    true iff which() == rhs.which() and content_this == content_rhs, where content_this is the content of *this and content_rhs is the content of rhs.

    Throws:

    If which() == rhs.which() then may fail with any exceptions arising from operator==(T,T), where T is the contained type of *this.
  2. bool operator<(const variant & rhs) const;
    template<typename U> void operator<(const U &) const;
    LessThan comparison.

    Notes:

    The overload returning void exists only to prohibit implicit conversion of the operator's right-hand side to variant; thus, its use will (purposefully) result in a compile-time error.

    Requires:

    Every bounded type of the variant must fulfill the requirements of the LessThanComparable concept.

    Returns:

    If which() == rhs.which() then: content_this < content_rhs, where content_this is the content of *this and content_rhs is the content of rhs. Otherwise: which() < rhs.which().

    Throws:

    If which() == rhs.which() then may fail with any exceptions arising from operator<(T,T), where T is the contained type of *this.

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