...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
boost::variant — Safe, generic, stack-based discriminated union container.
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; };
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:
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.
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.
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.
template<typename T> variant(const T & operand);
Notes: Same semantics as previous constructor, but allows construction from temporaries.
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.
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.
~variant();
Effects:
Destroys the content of *this.
Throws:
Will not throw.
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?).
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?).
template<typename T> variant & operator=(const T & rhs);
Requires:
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.
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.
const std::type_info & type() const;
Returns:
typeid(x), where x is the the
content of *this.
Throws:
Will not throw.
bool operator==(const variant & rhs) const; template<typename U> void operator==(const U & ) const;
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()andcontent_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.
bool operator<(const variant & rhs) const; template<typename U> void operator<(const U & ) const;
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.
Copyright © 2002, 2003 Eric Friedman, Itay Maman |