Investigate C++17 dependencies that would break C++14 compatibility

[vtz]

Summary

Investigate all C++17-specific features used in opensomeip's core library (src/ and include/) to determine which would prevent compilation under a C++14 toolchain. The goal is to produce a clear inventory and, where feasible, identify drop-in C++14 alternatives. The output of this investigation is one or more follow-up tickets with concrete changes — this ticket itself involves no code changes.

Motivation

Embedded BSW frameworks such as Eclipse OpenBSW standardize on C++14. Integrating opensomeip into such a codebase currently requires the entire project to raise its language standard to C++17, which may conflict with existing code, toolchain constraints, or coding guidelines. Understanding and potentially reducing our C++17 surface would make opensomeip accessible to a broader embedded audience.

Scope

Audit src/ and include/ only (not tests/, examples/, or zephyr/). Platform backends under include/platform/ are in scope since they ship with the library.

Known C++17 Features in Use (starting point)

Based on an initial scan, the following C++17 features are used in the core library:

std::optional

File	Usage
include/someip/message.h	Lines 149, 169
include/serialization/serializer.h	Lines 92, 227–234
include/e2e/e2e_protection.h	Line 73
src/serialization/serializer.cpp	Lines 436, 447, 458, 478, 486, 494, 502, 512
src/e2e/e2e_protection.cpp	Line 86
src/e2e/e2e_profiles/standard_profile.cpp	Line 135

if constexpr

File	Usage
include/serialization/serializer.h	Lines 246–268, 285–307

Structured bindings (auto [a, b] = ...)

File	Usage
src/transport/tcp_transport.cpp	Line 90

[[nodiscard]]

File	Usage
include/transport/transport.h	Lines 78, 91, 97, 121, 127
include/transport/tcp_transport.h	Lines 101, 109
include/transport/udp_transport.h	Line 72

[[maybe_unused]]

File	Usage
src/sd/sd_server.cpp	Line 482
src/someip/message.cpp	Lines 300, 356, 369

std::apply

File	Usage
include/platform/zephyr/thread_impl.h	Line 92
include/platform/threadx/thread_impl.h	Line 141
include/platform/freertos/thread_impl.h	Line 155

Features not used (confirmed absent)

std::string_view, std::variant, std::any, std::shared_mutex, std::filesystem, std::clamp, std::invoke, fold expressions, CTAD, nested namespace declarations (namespace a::b).

Investigation Tasks

• For each C++17 feature above, assess whether a C++14-compatible alternative exists:
  • std::optional → custom Optional<T> wrapper, pointer + bool, or ETL etl::optional?
  • if constexpr → SFINAE / std::enable_if / tag dispatch?
  • Structured bindings → explicit member access?
  • [[nodiscard]] / [[maybe_unused]] → compiler-specific __attribute__ or remove?
  • std::apply → manual tuple unpacking with index sequence?
• Evaluate whether a compile-time switch (e.g. #if __cplusplus >= 201703L) is viable vs. a full migration to C++14-only constructs
• Assess impact on code readability and maintainability for each replacement
• Identify any implicit C++17 dependencies (e.g. guaranteed copy elision, relaxed enum init, std::shared_ptr for arrays) that may not show up in a text search
• Check if any third-party headers pulled via FetchContent require C++17
• Document findings and create follow-up ticket(s) with the specific changes needed

Acceptance Criteria

• Complete inventory of C++17 features with file locations, and assessment of C++14 alternatives
• Follow-up ticket(s) created with concrete, actionable tasks
• Decision documented: full C++14 compatibility vs. dual-standard support vs. "not feasible"

Related

• OpenBSW integration planning (see related tickets)

[vtz]

C++17 audit (core: include/ + src/)

Investigation results are documented in docs/investigations/cpp17-cpp14-compatibility.md (see PR adding that file).

Confirmed C++17 usage

• std::optional: Message E2E header + return type of get_e2e_header(), DeserializationResult / Deserializer::read_be_*, E2EProtection::extract_header() and implementations. This is the main public API surface to migrate for C++14 (e.g. bundled optional, Boost, or Abseil).
• if constexpr + std::is_same_v: include/serialization/serializer.h (serialize_array / deserialize_array) — replace with tag dispatch / overloads (or SFINAE).
• std::apply: include/platform/freertos/thread_impl.h, threadx/thread_impl.h, zephyr/thread_impl.h — replace with a small C++14 tuple + index_sequence apply helper.
• Structured binding: src/transport/tcp_transport.cpp — use pair::first / second.
• inline static member: include/platform/threadx/thread_impl.h (s_registry) — one definition in a .cpp for C++14.
• [[nodiscard]] / [[maybe_unused]]: transport headers + a few .cpp locals — optional attributes or (void)x.

Not found in core

<string_view>, <variant>, <any>, <filesystem>, <shared_mutex>, nested namespace a::b, std::byte, fold expressions, [[fallthrough]], etc.

Recommendation

Feasible to support C++14 with moderate effort; best path is a dual-standard optional typedef + serializer/thread refactors + CI C++14 build. Root CMakeLists.txt and Zephyr currently force C++17.