xronos::sdk::ValueView< T > Class Template Reference

A borrowed view of a message value. More...

#include <xronos/sdk/value.hh>

Public Member Functions
constexpr	ValueView () noexcept=default
	Constructs an absent view that refers to nothing.
auto	as_shared_ptr () const -> std::shared_ptr< const T >
	Retrieve the viewed value as a std::shared_ptr.
auto	get () const noexcept -> const T *
	Retrieve a pointer to the viewed value, or nullptr if the view is absent.
	operator bool () const noexcept
	Check whether the view refers to a value.
auto	operator* () const noexcept -> const T &
	Access the viewed value.
auto	operator-> () const noexcept -> const T *
	Access members of the viewed value.

Friends
auto	operator== (const ValueView &view, std::nullptr_t) noexcept -> bool
	Check whether the view is absent (view == nullptr is equivalent to !view).

Detailed Description

template<class T>
class xronos::sdk::ValueView< T >

A borrowed view of a message value.

A ValueView does not own the value it refers to: it forwards to a value stored by the runtime, which remains valid for the duration of the current reaction handler. Reading through a view performs no copy.

Views cannot be copied or moved: a view is a scope-bound borrow, and retaining one beyond the reaction handler it was obtained in would leave it dangling. Initializing a local from a fresh view (auto view = input.get();) works – that is guaranteed copy elision, not a copy. To keep the viewed value, copy the view into a Value :

if (auto view = input_.get()) {
  self().last_input_ = Value<Message>{view};
}

A default-constructed (or absent) view refers to nothing; this is what Trigger::get() returns when no event is present.

Template Parameters

T	The type of the viewed value.

Member Function Documentation

as_shared_ptr()

template<class T>

auto xronos::sdk::ValueView< T >::as_shared_ptr ( ) const -> std::shared_ptr< const T >

nodiscard

Retrieve the viewed value as a std::shared_ptr.

Unlike the view itself, the returned pointer keeps the value alive beyond the current reaction handler, which makes it suitable for handing a received value to another framework (for example, publishing to ROS 2). Note that pointer identity is not necessarily carried through: for payload types in shared storage the returned pointer shares ownership with the stored value, while small trivially copyable payloads are stored by value and are copied into a fresh allocation on every call.

Returns

A shared pointer to the viewed value, or nullptr if the view is absent.

get()

template<class T>

auto xronos::sdk::ValueView< T >::get ( ) const -> const T *

nodiscardnoexcept

Retrieve a pointer to the viewed value, or nullptr if the view is absent.

The pointer is only valid for the duration of the current reaction handler.

operator bool()

template<class T>

xronos::sdk::ValueView< T >::operator bool ( ) const

explicitnoexcept

Check whether the view refers to a value.

For a view returned by a trigger, this indicates whether an event with a value is currently present.

operator*()

template<class T>

auto xronos::sdk::ValueView< T >::operator* ( ) const -> const T &

noexcept

Access the viewed value.

The behavior is undefined if the view is absent (asserted in debug builds).

operator->()

template<class T>

auto xronos::sdk::ValueView< T >::operator-> ( ) const -> const T *

noexcept

Access members of the viewed value.

The behavior is undefined if the view is absent (asserted in debug builds).

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The documentation for this class was generated from the following file:

• sdk/value.hh