Case 125 — Class Became `final`¶

Field	Value
Verdict	🟠 API_BREAK
Category	API Break
Platforms	Linux, macOS, Windows
Flags	API break
Detected `ChangeKind`s	`type_became_final`
Source files	`examples/case125_class_became_final/`

Verdict: 🟠 API_BREAK abicheck verdict: API_BREAK (with headers) / NO_CHANGE (object/ELF-only)

What changes¶

Version	Declaration
v1	`class Shape { ... };`
v2	`class Shape final { ... };`

The class gains the final specifier. Its size, alignment, vtable and the mangled names of all its members are unchanged.

What breaks¶

Any consumer that derives from Shape (struct MyShape : public Shape) no longer compiles against v2 — error: cannot derive from 'final' base. See app.cpp, which compiles against v1.h and fails against v2.h. Already-compiled binaries keep linking and running (the ABI is identical), so this is a pure source / API break, not a runtime ABI break.

Why this case exists — a change no object inspection can detect¶

final is a C++ source-level specifier. It is not recorded in DWARF, in the symbol table, or anywhere in the object file — the v1 and v2 .so files are ABI-identical. A tool that compares only binaries (or stripped/DWARF-only builds) cannot detect this change, and reports NO_CHANGE.

abicheck catches it only in header mode, where castxml parses the declaration and records the final class-key (ChangeKind type_became_final). This case demonstrates why source/header analysis — not just object comparison — is required for the full API picture. See Limitations → Source-only changes.

Reproduce manually¶

g++ -shared -fPIC -g v1.cpp -o libshape_v1.so
g++ -shared -fPIC -g v2.cpp -o libshape_v2.so

# Header mode — detected:
abicheck compare libshape_v1.so libshape_v2.so \
    --old-header v1.h --new-header v2.h        # → API_BREAK (type_became_final)

# Object-only mode — invisible:
abicheck compare libshape_v1.so libshape_v2.so # → NO_CHANGE

How to fix¶

Keep public base classes non-final, or document the inheritance contract. Adding final to a previously-extensible public class is a breaking API change and warrants a major version bump.

Source files¶

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

Case 125 — Class Became final¶