Case 110: concurrent_unordered_map API Drift¶

Field	Value
Verdict	🔴 BREAKING
Category	Breaking
Platforms	Linux, macOS
Flags	ABI break, API break
Detected `ChangeKind`s	`func_removed`
Source files	`examples/case110_concurrent_unordered_map_api_drift/`

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

What breaks¶

A public member function (insert) loses one of its parameters between versions. The mangled symbol of the old overload disappears from the .so; the new overload has a different mangled name. Both the .so symbol table and any consumer source that called the old form change.

Why this matters¶

Mirrors several documented historical signature tightenings on tbb::concurrent_unordered_map (and friends) where helper / hint parameters were dropped or reordered between releases. These changes are doubly bad: they break source AND they break linking of consumers that were built against the old headers.

How abicheck catches it¶

The diff exposes:

FUNC_REMOVED: old mangled insert(int, size_t)
FUNC_ADDED: new mangled insert(int) (informational; helps the reporter identify a candidate replacement)

FUNC_REMOVED on a public symbol is unconditionally BREAKING — there is no new detector required to flag this case.

Code diff¶

v1	v2
`void insert(int key, std::size_t rehash_hint);`	`void insert(int key);`

How to fix (as a library maintainer)¶

Keep both overloads through a deprecation window: in release N–1, mark the hint-form overload [[deprecated]] and have it call into the hint-free implementation. In release N, remove it.
If the hint argument is genuinely dead, prefer making it a default argument first (std::size_t rehash_hint = 0) — that preserves the symbol name on platforms where the mangled signature includes the parameter, and only later remove it.

Real failure demo¶

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

# v2 header, v2 .so: app.cpp's 2-arg insert is gone.
g++ -std=c++17 -I. app.cpp -L. -lmylib -o app
# → error: no matching function for call to 'insert(int, int)'

References¶

oneTBB 2021 migration guide: concurrent_unordered_map API changes.

Source files¶

CMakeLists.txt
app.cpp
v1.cpp
v1.h
v2.cpp
v2.h