struct std::hash< ara::core::Optional< T >

In module core of the AUTOSAR Adaptive Platform (release R25-11), the template specialization std::hash<ara::core::Optional<T>> provides a standard-compliant hash function for ara::core::Optional. It enables the direct use of optional values as keys in unordered associative containers such as std::unordered_map by calculating the hash value based on the engaged state and the contained value. Interpretation: As the most important constraint, it is likely to be required that a functional specialization std::hash<T> exists for the parameterized data type T and that the stateless function object can be called concurrently without side effects. Question Q-5e98f9abd214d511 (open) The absence of a value in the optional does not represent an error state, but rather leads to a deterministic result.

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Context

In module core of the AUTOSAR Adaptive Platform (release R25-11), the template class std::hash<ara::core::Optional<T>> provides a specialization of the standardized function object std::hash for the generic data type ara::core::Optional. The primary task of this specialization is to transparently integrate hash value computations for instances of ara::core::Optional<T> into the standard-compliant hashing ecosystem of C++. In this way, optional values can be used directly as keys in unordered associative containers such as std::unordered_map or std::unordered_set. Interpretation: The specialization of std::hash for ara::core::Optional<T> is likely to require that a valid specialization of std::hash<T> is also available for the underlying type T in order to determine the hash value of an engaged value. Question Q-1ada0d8edc4f7690 (open)

Lifecycle

As a specialization of a struct according to syntax template <typename T> struct hash<ara::core::Optional<T>>, this type functions as a regular function object. The object typically has no internal state of its own and is not subject to complex lifecycle phases or expensive resource reservations. Instantiation occurs on demand, usually implicitly by the managing associative container or explicitly upon direct execution of a hashing operation in application code.

Ownership of the passed data is not established during the call, as the instance of ara::core::Optional<T> is merely consulted as a const reference for evaluation. After completion of the function call or upon leaving the scope of the container, the instance of std::hash is trivially destroyed. Interpretation: The function object is stateless and is therefore likely to be usable without side effects and in a thread-safe manner for concurrent hashing operations across multiple execution contexts, as long as the referenced value is not modified concurrently. Question (open)

Usage Path

Typical use occurs within the context of using ara::core::Optional in hash-based data structures. Developers include the definition and declare, for example, a set or a map whose key type corresponds to ara::core::Optional<T>. During insertion, search, or deletion operations, the container autonomously invokes the call operator operator() of the specialization.

When evaluating the hash value, a distinction is made between the presence and the absence of a value. Interpretation: For an empty ara::core::Optional (without a value), a fixed, deterministic hash value is likely to be calculated, whereas if a value is present, the hash value of the contained object is presumably determined via the corresponding hash function of T. Question (open) The determined numerical value of type std::size_t is subsequently returned to the calling container in order to address the storage bucket.

Error Scenarios

At the level of hash calculation, no error returns via error codes such as ara::core::ErrorCode or the like are regularly provided. A function object for hash values must operate deterministically and reliably to maintain the integrity of the container invariants. An absence of the value in ara::core::Optional does not represent an error state, but is a regular state of the object that must be mapped to a defined hash value.

Possible compile errors occur if the type T is not hashable and no corresponding std::hash<T> specialization exists. Runtime errors can arise if user-defined hash functions for the type T throw exceptions or induce undefined behavior. A precondition for an error-free call is the validity of the referenced ara::core::Optional object during execution of the operation.