struct ara::core::Monostate

In module core, the class ara::core::Monostate serves as a stateless placeholder type to represent an empty state in variant data structures according to its reference to [variant]. Interpretation: It is likely to function as a counterpart to std::monostate in order to make ara::core::Variant default-constructible if alternative payload types do not provide a default constructor. Question Q-b724690dfd69af81 (open) Since the structure is defined as final and contains no data fields, all lifecycle operations are trivial and take place without dynamic resource binding. The instances are predominantly used as a default alternative and can be used concurrently without synchronization overhead.

more ›

Context

The class ara::core::Monostate represents an elementary basic building block within module core of the AUTOSAR Adaptive Platform. The declaration as struct Monostate final defines a stateless unit type whose primary responsibility is to represent the absence of a usable value within typed sum types. With regard to its formal semantics and intended purpose, the specification references the corresponding usage in [variant]. Interpretation: From this, it can be deduced that ara::core::Monostate functions as a direct counterpart to std::monostate from the C++ standard and interacts closely with ara::core::Variant to provide variants with an explicitly empty state alternative. Question (open)

In standard-compliant C++ interfaces, a typed variant construct requires for parameterless construction that the type alternative listed in the first position is default-constructible. If none of the functional payload types provide a default constructor or if a variant is initially intended to explicitly represent no functional state, ara::core::Monostate in the first position of the type list assumes the role of a well-formed placeholder. By decoupling variant initialization from the actual provision of functional data, the class ensures that variant-based interfaces and function results within deterministic ECU software can always be initialized in a type-safe, well-defined, and memory-efficient manner.

Lifecycle

The lifecycle of ara::core::Monostate is characterized by maximum statelessness and minimal complexity. Since the structure is declared as final and encapsulates no data elements, an instance possesses no mutable internal states, phases, or state transitions. Interpretation: As a placeholder type declared as final without its own state data, ara::core::Monostate is likely to be trivially constructible, copyable, movable, and destructible, so that all lifecycle operations are executed without dynamic memory allocations, without the risk of exceptions, and with negligible runtime overhead. Question Q-9cf1c06bbd91b6ec (open)

Instantiation typically occurs implicitly during default construction of an enclosing variant or explicitly upon assignment of an empty state. Destruction requires no cleanup actions whatsoever, as no operating system or memory resources are bound. In terms of ownership and concurrency, the class behaves like a purely elementary value type: Instances can be created, copied, or evaluated concurrently on different execution paths without synchronization mechanisms or data races being able to occur at the class level. The lifetime of an instance is strictly bound to the local scope or to the enclosing container.

Usage Path

The typical usage path of ara::core::Monostate is inextricably linked with the declaration and handling of ara::core::Variant. When defining a variant, ara::core::Monostate is specified at the first position in order to allow an empty initial state. If the variant is subsequently created via its default constructor, it automatically assumes this empty state without functional types having to be instantiated.

In the further course of program execution, the user checks via type queries or visitor patterns whether the variant currently contains the type ara::core::Monostate, which signals an inactive or unoccupied state. As soon as payload data is available, the variant is overwritten with the corresponding functional value. Resetting to the unoccupied state takes place by assigning ara::core::Monostate{} again. Interpretation: Equality and comparison operators presumably exist for ara::core::Monostate, so that comparisons of two instances always yield equality and the relational comparison of variants in the empty state can be evaluated deterministically. Question (open)