Case 60: Base Class Position Changed (Multiple Inheritance Reorder)¶

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

Category: C++ Layout | Verdict: BREAKING

What this case is about¶

v1: struct Widget : public Drawable, public Clickable v2: struct Widget : public Clickable, public Drawable

The base class declaration order is swapped. In C++ multiple inheritance, base subobjects are laid out in declaration order, so swapping them changes every offset in the class.

What breaks at binary level¶

Subobject offsets swap: Drawable subobject was at offset 0, now Clickable is at offset 0 and Drawable is further into the object.
Vtable pointer adjustment breaks: Casting Widget* to Drawable* or Clickable* uses compile-time offsets that are now wrong.
Cross-cast corrupts: static_cast<Clickable*>(widget) compiled against v1 adjusts by the wrong offset when v2 is loaded.

What ABICheck detects¶

BASE_CLASS_POSITION_CHANGED: The base class list order changed, detected via DWARF DW_TAG_inheritance entries with different offsets.

Overall verdict: BREAKING

How to reproduce¶

g++ -shared -fPIC -g v1.cpp -o libv1.so
g++ -shared -fPIC -g v2.cpp -o libv2.so

python3 -m abicheck.cli dump libv1.so -o /tmp/v1.json
python3 -m abicheck.cli dump libv2.so -o /tmp/v2.json
python3 -m abicheck.cli compare /tmp/v1.json /tmp/v2.json
# → BREAKING: BASE_CLASS_POSITION_CHANGED

Real-world examples¶

GUI frameworks with multiple interface inheritance (Qt, GTK+) must freeze base class order once published.
COM-style interfaces on Linux also depend on base class order for vtable layout.

References¶

Itanium C++ ABI: class layout