Case 112: task_arena::attach Tag Type Replaces Enum¶

Field	Value
Verdict	🔴 BREAKING
Category	Breaking
Platforms	Linux, macOS, Windows
Flags	ABI break, API break
Detected `ChangeKind`s	`func_removed`, `type_removed`, `enum_member_removed`
Source files	browse on GitHub

Category: ABI + source break / oneTBB regression suite | Verdict: 🔴 BREAKING

What breaks¶

An attach_mode_t enum and the constructor task_arena(attach_mode_t) are replaced by an empty tag struct task_arena::attach and a task_arena(attach) constructor. The old enum is removed entirely, the old constructor's mangled name is gone, and consumer source that wrote task_arena ta(attach_to_current); no longer compiles.

Why this is in the oneTBB regression suite¶

Mirrors a documented oneTBB API move from enum-value-based mode selection to tag-type-based selection. The motivation upstream was to align with modern C++ idioms (tag dispatch, std::piecewise_construct, std::nothrow), but the transition broke both source and ABI for any consumer that referenced the old API.

How abicheck catches it¶

The diff exposes:

TYPE_REMOVED: attach_mode_t (the enum)
ENUM_MEMBER_REMOVED: no_attach, attach_to_current
FUNC_REMOVED: old mangled task_arena(attach_mode_t)
TYPE_ADDED: task_arena::attach
FUNC_ADDED: new mangled task_arena(task_arena::attach)

Any of FUNC_REMOVED / TYPE_REMOVED / ENUM_MEMBER_REMOVED on a public symbol is BREAKING under default policy, so no new detector is needed.

Code diff¶

v1	v2
`enum attach_mode_t { no_attach=0, attach_to_current=1 };`	(removed)
`task_arena(attach_mode_t mode);`	`task_arena(attach tag);`
(no nested types)	`struct attach {};` (nested)

How to fix (as a library maintainer)¶

Ship a deprecation cycle: in release N–1, add the tag type and the tag-form constructor while keeping the enum + old constructor; mark the enum and old constructor [[deprecated]]. In release N, remove the deprecated path.
Resist the urge to remove the old enum aggressively — tag-dispatch cleanups feel "purely additive" until you measure the downstream consumer fleet.

Real failure demo¶

# v1 header, v1 .so:
g++ -std=c++17 -I. app.cpp -L. -lmylib -o app   # compiles, links

# v2 header, v2 .so:
g++ -std=c++17 -I. app.cpp -L. -lmylib -o app
# → error: 'attach_to_current' was not declared in this scope
#   error: no matching function for call to 'mylib::task_arena::task_arena(<unknown>)'

References¶

oneTBB 2021 release notes — task_arena::attach tag introduction.