Case 131: RTTI Mode Flip (-fno-rtti)

Field	Value
Verdict	🟡 COMPATIBLE_WITH_RISK
Category	Risk
Platforms	Linux
Flags	—
Detected ChangeKinds	rtti_mode_changed
Source files	examples/case131_rtti_mode_flip/

Category: Build mode | Verdict: 🟡 COMPATIBLE_WITH_RISK

Same source, same symbols; v1 built with RTTI (-frtti), v2 with -fno-rtti. The generated CMake build context (L3) reveals the flip → rtti_mode_changed.

What this demonstrates

-fno-rtti omits type_info for polymorphic types, so dynamic_cast/typeid and cross-DSO exception matching that relies on RTTI identity can fail to link or silently misbehave when one side has RTTI and the other does not.

Why COMPATIBLE_WITH_RISK

A build-mode signal, not a proven binary break (ADR-028 D3). The artifact diff confirms any concrete break; this localizes the elevated risk.

How abicheck detects it

The CMake fixture builds v1 with -frtti and v2 with -fno-rtti; the generated build-dir compile_commands.json carries those flags. The L3 diff normalizes to the canonical rtti option and reports it.

Reproduce manually

cmake -S examples -B /tmp/abicheck-examples-build -DCMAKE_BUILD_TYPE=Release -DCMAKE_EXPORT_COMPILE_COMMANDS=ON
cmake --build /tmp/abicheck-examples-build --target case131_rtti_mode_flip_v1 case131_rtti_mode_flip_v2
abicheck dump /tmp/abicheck-examples-build/case131_rtti_mode_flip/libv1.so --build-info /tmp/abicheck-examples-build/compile_commands.json -o v1.abi.json
abicheck dump /tmp/abicheck-examples-build/case131_rtti_mode_flip/libv2.so --build-info /tmp/abicheck-examples-build/compile_commands.json -o v2.abi.json
abicheck compare v1.abi.json v2.abi.json   # → rtti_mode_changed

How to fix

Keep a single RTTI mode for the public ABI, or rebuild consumers in the matching mode if the public API exposes polymorphic types or dynamic_cast/typeid.

---

Source files

• CMakeLists.txt
• app.cpp
• v1.cpp
• v2.cpp

See also: Examples overview · All COMPATIBLE_WITH_RISK cases · Category: Risk.