struct std::hash< ara::core::Monostate >

The specialization std::hash<ara::core::Monostate> provides a standard-compliant function object for calculating hash values for ara::core::Monostate [SWS_CORE_01671]. Via the call operator operator(), the struct accepts a const reference to a Monostate object and returns the associated hash value as size_t [SWS_CORE_01671]. Interpretation: Since the Monostate object has no internal state, a uniform, deterministic value is likely to be determined for each instance. Question (open) As a central constraint, the operation is specified as noexcept as well as exception-safe, completely precluding runtime errors.

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Context

The template specialization std::hash<ara::core::Monostate> represents the functional link between the AUTOSAR Adaptive base type ara::core::Monostate and the standard hashing infrastructure [SWS_CORE_01671]. Within module core, ara::core::Monostate is used to represent an empty state, for example as an alternative type in variants. To allow such types to be used seamlessly in associative hash containers or generic hashing algorithms, the specification defines an explicit specialization of the standard functor class std::hash [SWS_CORE_01671].

The struct is provided via the header file ara/core/variant.h [SWS_CORE_01671]. As a hashing function object, the specialization primarily interacts with generic container implementations such as hash tables and with instances of ara::core::Monostate. By adhering to the interface conventions of the C++ Standard Library, it acts as an adapter that ensures uniform access to hash values even for stateless components [SWS_CORE_01671].

Lifecycle

The struct std::hash<ara::core::Monostate> is designed as a regular function object [SWS_CORE_01671]. Since it requires no instance-specific data to calculate the hash value, it has no mutable state of its own. Creation and destruction of instances of the specialization require no special precautions and incur no dynamic memory overhead.

Regarding concurrency and ownership, the function object is completely stateless. The function call operator operator() is declared as const [SWS_CORE_01671]. This means that the same instance can be used safely and concurrently by multiple execution contexts or threads in parallel. There are no exclusive ownership claims to the passed Monostate objects, as these are merely read via a const reference [SWS_CORE_01671].

Usage Path

The typical usage path begins with including the header file ara/core/variant.h [SWS_CORE_01671] in the respective translation unit. Once the header file is included, the specialization is automatically available in namespace std and is resolved by hash-based data structures as needed.

The actual execution takes place via the overloaded function call operator operator(), which accepts a const reference to an ara::core::Monostate object [SWS_CORE_01671]: size_t operator() (ara::core::Monostate const &v) const noexcept; [SWS_CORE_01671]. As a result, the call returns a numeric hash value of type size_t [SWS_CORE_01671]. Interpretation: Since ara::core::Monostate by definition carries no variable state, the function is likely to calculate an identical, deterministic hash value for all passed instances. Question Q-a9e7ab0092428d3c (open) In practice, this call usually occurs transparently as part of lookup or insertion operations of higher-level data structures.

Error Scenarios

The specification defines strict behavior regarding exceptions and robustness for method operator(): the operator is declared with the specifier noexcept and explicitly classified as exception-safe (exception safe) [SWS_CORE_01671]. This guarantees that under no circumstances are C++ exceptions thrown during the hashing process.

Specific error codes, return values for error signaling, or preconditions beyond a syntactically valid object reference do not exist [SWS_CORE_01671]. The call can therefore never fail. The postcondition of the call is always the successful provision of an integer hash value of type size_t, so callers do not need to implement fallback or error handling branches [SWS_CORE_01671].