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The requirements on a bounded type are as follows:
variant template
instantiation. (See
boost::recursive_wrapper<T>
for a type wrapper that accepts incomplete types to enable recursive
variant types.)Every type specified as a template argument to
variant must at minimum fulfill the
above requirements. In addition, certain features of variant
are available only if its bounded types meet the requirements of these
following additional concepts:
variant is itself Assignable if and
only if every one of its bounded types meets the requirements of the
concept. (Note that top-level const-qualified types and
reference types do not meet these
requirements.)variant is itself
DefaultConstructible if and only if its first
bounded type (i.e., T1) meets the requirements of the
concept.variant is itself EqualityComparable
if and only if every one of its bounded types meets the requirements
of the concept.variant is itself LessThanComparable
if and only if every one of its bounded types meets the requirements
of the concept.variant is itself OutputStreamable
if and only if every one of its bounded types meets the requirements
of the concept.The requirements on a static
visitor of a type T are as follows:
operator(), unambiguously accepting any value of type
T.result_type. (See
boost::visitor_ptr for a
solution to using functions as visitors.)result_type is not void, then
each operation of the function object must return a value implicitly
convertible to result_type.The following class satisfies the requirements of a static visitor
of several types (i.e., explicitly: int and
std::string; or, e.g., implicitly: short and
const char *; etc.):
class my_visitor
: public boost::static_visitor<int>
{
public:
int operator()(int i)
{
return i * 2;
}
int operator()(const std::string& s)
{
return s.length();
}
};
Another example is the following class, whose function-call
operator is a member template, allowing it to operate on values of many
types. Thus, the following class is a visitor of any type that supports
streaming output (e.g., int, double,
std::string, etc.):
class printer
: public boost::static_visitor<>
{
template <typename T>
void operator()(const T& t)
{
std::cout << t << std::endl;
}
};
This header exists simply as a convenience to the user, including
all of the headers in the boost/variant directory.
Provides forward declarations of the
boost::variant,
boost::make_variant_over,
boost::make_recursive_variant, and
boost::make_recursive_variant_over
class templates and the boost::recursive_variant_ tag type.
Also defines several preprocessor symbols, as described below.
BOOST_VARIANT_LIMIT_TYPES BOOST_VARIANT_ENUM_PARAMS(param) BOOST_VARIANT_ENUM_SHIFTED_PARAMS(param) BOOST_VARIANT_NO_REFERENCE_SUPPORT BOOST_VARIANT_NO_TYPE_SEQUENCE_SUPPORT BOOST_VARIANT_NO_FULL_RECURSIVE_VARIANT_SUPPORT
namespace boost { template<typename T1, typename T2 = unspecified, ..., typename TN = unspecified> class variant; template<typename Sequence> class make_variant_over; template<typename T1, typename T2, ..., typename TN> void swap(variant<T1, T2, ..., TN> &, variant<T1, T2, ..., TN> &); template<typename ElemType, typename Traits, typename T1, typename T2, ..., typename TN> std::basic_ostream<ElemType,Traits> & operator<<(std::basic_ostream<ElemType,Traits> &, const variant<T1, T2, ..., TN> &); }
namespace boost { typedef unspecified recursive_variant_; template<typename T1, typename T2 = unspecified, ..., typename TN = unspecified> class make_recursive_variant; template<typename Sequence> class make_recursive_variant_over; }
namespace boost { template<typename T> class recursive_wrapper; template<typename T> class is_recursive_wrapper; template<typename T> class unwrap_recursive_wrapper; }
namespace boost { template<typename Visitor> class apply_visitor_delayed_t; template<typename Visitor, typename Variant> typename Visitor::result_type apply_visitor(Visitor &, Variant &); template<typename Visitor, typename Variant> typename Visitor::result_type apply_visitor(const Visitor &, Variant &); template<typename BinaryVisitor, typename Variant1, typename Variant2> typename BinaryVisitor::result_type apply_visitor(BinaryVisitor &, Variant1 &, Variant2 &); template<typename BinaryVisitor, typename Variant1, typename Variant2> typename BinaryVisitor::result_type apply_visitor(const BinaryVisitor &, Variant1 &, Variant2 &); template<typename Visitor> apply_visitor_delayed_t<Visitor> apply_visitor(Visitor &); }
namespace boost { class bad_get; template<typename U, typename T1, typename T2, ..., typename TN> U * get(variant<T1, T2, ..., TN> *); template<typename U, typename T1, typename T2, ..., typename TN> const U * get(const variant<T1, T2, ..., TN> *); template<typename U, typename T1, typename T2, ..., typename TN> U & get(variant<T1, T2, ..., TN> &); template<typename U, typename T1, typename T2, ..., typename TN> const U & get(const variant<T1, T2, ..., TN> &); }
namespace boost { class bad_visit; }
namespace boost { template<typename ResultType> class static_visitor; }
namespace boost { template<typename T, typename R> class visitor_ptr_t; template<typename R, typename T> visitor_ptr_t<T,R> visitor_ptr(R (*)(T)); }