struct ara::core::variant_size

The class template ara::core::variant_size provides a metaprogramming construct for statically querying the type count in variants for the AUTOSAR Adaptive Platform. According to the specification, the template aligns with the corresponding usage in [variant]. Interpretation: The evaluation takes place exclusively at compile time by the compiler, so that neither memory allocations nor dynamic state transitions occur at runtime. Question Q-9438291a39a7af27 (open) An essential constraint is that the passed data type must possess a valid variant specialization, as unsuitable types lead to a compilation error.

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Context

Within the AUTOSAR Adaptive Platform, module core provides basic data structures, type definitions, and helper constructs for adaptive applications. The class template ara::core::variant_size is syntactically declared as template <typename T> struct variant_size; and addresses the handling of variant-based types in the system. According to the specification description, the functionality aligns with the corresponding usage in [variant]. Interpretation: Thereby, the structure functions as a typical compile-time type trait whose central responsibility consists in querying the number of alternatives contained in a variant type at compile time. Question Q-7b29532c4a4dbfb1 (open)

The interaction partners are restricted to software components, algorithms, and function templates within the platform that evaluate type-safe union structures. Particularly in combination with variant types such as ara::core::variant, ara::core::variant_size enables developers and library functions to query meta-information about type lists without having to create instances of the underlying types. Since the declaration is anchored in namespace ara::core, it forms a standardized building block for adaptive software components that operate according to deterministic and resource-conserving patterns.

Lifecycle

Because ara::core::variant_size is designed as a pure template construct without runtime state, its lifecycle differs fundamentally from regular runtime objects of the Adaptive Platform. Interpretation: Neither dynamic allocations nor ownership relationships, state transitions, or an explicit destruction process exist, as the structure is evaluated exclusively within the scope of template instantiation by the compiler. Question (open)

Instance variables or internal buffers are not provided for in this concept. The type is typically not allocated as a dynamic object on the heap or stack, but serves as a compile-time constant source. With respect to concurrency, no synchronization requirements arise: Because access takes place purely statically and no shared mutable states exist, the query is inherently concurrency-safe and possible without any runtime overhead.

Usage Path

The typical usage path of ara::core::variant_size takes place entirely during the compilation of C++ source code. A developer or generic algorithm uses the trait to perform branching or static assertions based on the number of permissible types of a variant.

First, the desired target type, typically a variant with multiple type alternatives, is passed as a template argument to ara::core::variant_size. In the next step, the calling code accesses the static value of the structure to obtain the numeric value of the type count. This step frequently takes place in the context of static_assert checks, generic loop constructs, or template metaprogramming chains to ensure that only variants with an expected minimum or maximum number of elements are processed.

Interpretation: If an unsuitable data type is passed for which no specialization exists, the process aborts directly in the compiler with a compilation error, so that invalid type usages are detected and averted prior to program execution. Question (open)