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Case 106: Conversion Operator Became explicit

Field Value
Verdict 🟠 API_BREAK
Category API Break
Platforms Linux, macOS, Windows
Flags API break
Detected ChangeKinds ctor_explicit_added
Source files browse on GitHub

Category: Source API contract | Verdict: 🟠 API_BREAK

What breaks

A user-defined conversion operator (operator int() const) that previously allowed implicit conversion gains the explicit specifier. The mangled name is unchanged, so previously-compiled consumers keep linking — but every source TU that relied on implicit conversion fails to compile against the new headers.

The case applies equally to converting constructors (explicit Foo(int)), but conversion operators are used in the example because they receive mangled names from both castxml and DWARF, while constructors only mangle via DWARF — a quirk that would otherwise prevent integration tests from exercising the detector through the default castxml dump path.

Why this is a oneTBB-flavored break

oneTBB handle types (task_arena, task_group, global_control) frequently wrap a primitive (concurrency, version, count) and expose conversions to/from the underlying integer. Tightening these with explicit per the modern C++ Core Guidelines is a tempting cleanup, but it silently breaks every consumer that used the implicit conversion at a function-call argument boundary or in copy-initialization.

Code diff

v1 v2
operator int() const; explicit operator int() const;
int n = ta; (compiles) same line: error — explicit cast needed

How abicheck catches it

New ChangeKind CTOR_EXPLICIT_ADDED (API_BREAK). The detector reads DW_AT_explicit on the function's DW_TAG_subprogram (and the equivalent explicit="1" attribute on castxml Constructor / Method / Converter elements) and emits a finding when the bit transitions from absent to present.

The symmetric kind CTOR_EXPLICIT_REMOVED is COMPATIBLE_WITH_RISK — the implicit-conversion path may now select a different overload silently.

The detector is tri-state: a missing is_explicit field in either snapshot suppresses the finding. This prevents false positives against older baseline snapshots that predate the field.

Real Failure Demo

Severity: BAD PRACTICE / API BREAK

# v1 header, v1 .so: compiles and runs.
# Note linker order: object/source inputs must precede -l flags on modern
# Linux toolchains (-Wl,--as-needed is the default).
g++ -std=c++17 -I. app.cpp -L. -lmylib -o app
./app   # → concurrency = 4 (expect 4)

# v2 header, v2 .so: app.cpp does `int n = ta;`, an implicit conversion
# via `operator int() const`. v2 declares the operator `explicit`, so the
# same app.cpp source no longer compiles:
g++ -std=c++17 -I. app.cpp -L. -lmylib -o app
# → error: cannot convert 'mylib::task_arena' to 'int' in initialization

How to fix

  • Provide a non-explicit accessor (int task_arena::concurrency() const) as the migration path for any users who relied on implicit conversion.
  • Stage the explicit tightening across a deprecation window: warn users for one release, error in the next.

References

Source files

See also: Examples overview · All API_BREAK cases · Category: API Break.