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ADR-0839: C++23 wave — shadow-identifier and implicit-cast cleanup

  • Status: Accepted
  • Date: 2026-05-29
  • Deciders: lusoris
  • Tags: cpp23, lint, core, sycl, fork-local

Context

The C++23 wave (ADR-0708 and follow-on waves) converted several .c files to .cpp. After conversion, three categories of C-allows-but-C++-warns issues remained unaddressed:

  1. Shadowed identifier — fex_ctx_vector.cpp: local variable capacity at the realloc growth path shares a name with the rfe->capacity struct member. Clang's -Wshadow and clang-tidy bugprone-shadow flag this.
  2. Implicit C-style casts — feature_collector.cpp: (char*), (void*), (FeatureVector**), (VmafFeatureCollector*), (VmafPredictModel*), and the (decltype(...)) pattern were carried forward from the C original. In C++ these are reinterpret_cast-equivalent and defeat the narrowing checks static_cast provides. (VmafModelFlags)0 is a C-style enum cast where static_cast<VmafModelFlags>(0) is the idiomatic C++ form.
  3. Implicit C-style casts — core/src/sycl/common.cpp: (uint8_t*), (size_t), and (unsigned) casts in the frame-upload and plane-copy paths.

None of these caused incorrect behaviour on existing platforms, but they produced clang-tidy cppcoreguidelines-pro-type-cstyle-cast hits and blocked the -Wshadow clean-compile goal for the C++23-wave files.

Decision

Apply mechanical fixes to the three files:

  • Rename capacity → new_capacity in fex_ctx_vector.cpp to eliminate the shadow.
  • Replace every C-style cast with the appropriate static_cast<> in feature_collector.cpp and common.cpp.
  • No behaviour change; the fixes are type-system-only.

Alternatives considered

Option Pros Cons Why not chosen
NOLINT suppressions Zero-diff to logic Does not fix the underlying issue; NOLINT requires ADR citation per ADR-0278 Against policy: refactor first
Leave as-is until a dedicated lint sweep Minimal churn ADR-0141 (touched-file cleanup rule) requires fixing warnings in files we touch Violated project rule

Consequences

  • Positive: three files now compile clean under -Wshadow and without cppcoreguidelines-pro-type-cstyle-cast hits; static_cast<> makes the intent of each conversion auditable.
  • Negative: none — the changes are purely syntactic.
  • Neutral: the (decltype(...)) idiom in feature_collector.cpp is replaced with static_cast<decltype(...)>(...) which is slightly more verbose but standard C++.

References

  • ADR-0708 (C++23 pilot), ADR-0723 (fex_ctx_vector wave), ADR-0727 (dict wave), ADR-0729 (model/feature_name/picture_copy wave), ADR-0733 (output wave), ADR-0735 (cpu/ref/thread_locale/log wave).
  • ADR-0141 (touched-file cleanup rule).
  • ADR-0278 (NOLINT citation closeout).