20 General utilities library [utilities]
20.8 Storage for any type [any]
namespace std {
class any {
public:
constexpr any() noexcept;
any(const any& other);
any(any&& other) noexcept;
template<class T>
any(T&& value);
template<class T, class... Args>
explicit any(in_place_type_t<T>, Args&&...);
template<class T, class U, class... Args>
explicit any(in_place_type_t<T>, initializer_list<U>, Args&&...);
~any();
any& operator=(const any& rhs);
any& operator=(any&& rhs) noexcept;
template<class T>
any& operator=(T&& rhs);
template<class T, class... Args>
decay_t<T>& emplace(Args&&...);
template<class T, class U, class... Args>
decay_t<T>& emplace(initializer_list<U>, Args&&...);
void reset() noexcept;
void swap(any& rhs) noexcept;
bool has_value() const noexcept;
const type_info& type() const noexcept;
};
}
An object of class
any stores an instance of any type that meets the constructor requirements or it has no value,
and this is referred to as the
state of the class
any object
.Two states are equivalent if either they both have no value, or they both have a value and the contained values are equivalent
.The non-member
any_cast functions provide type-safe access to the contained value
.Implementations should avoid the use of dynamically allocated memory for a small contained value
.However, any such small-object optimization shall only be applied to types
T for which
is_nothrow_move_constructible_v<T> is
true.[
Example 1:
A contained value of type
int could be stored in an internal buffer,
not in separately-allocated memory
. —
end example]