Case 43: Base Class Member Added¶

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

Category: C++ Layout | Verdict: 🔴 BREAKING

What breaks¶

A data member is added to a base class (Base). This shifts the memory layout of all derived classes — their fields move to higher offsets. Any consumer binary compiled against v1 headers will read Derived::value at the wrong offset when linked against the v2 library, causing silent data corruption or crashes.

Why abidiff catches it¶

abidiff reports Added_Base_Class_Data_Member and exits with code 4 (ABI change). Note: abidiff exits 4 (not 12) because this is a layout change, not a symbol removal. abicheck detects: TYPE_SIZE_CHANGED on Base, TYPE_FIELD_OFFSET_CHANGED on Derived::value.

Code diff¶

v1.hpp	v2.hpp
`class Base { int base_id; ... };`	`class Base { int base_id; int extra_field; ... };`
`class Derived : public Base { int value; };`	`class Derived : public Base { int value; };`
`sizeof(Derived)=16`, `Derived::value` @ offset 12	`sizeof(Derived)=24`, `Derived::value` shifts to offset 16 (+4 bytes)

Real Failure Demo¶

Severity: 🔴 CRITICAL

Scenario: compile app against v1, swap in v2 .so — Derived::value is read at wrong offset.

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

# The base-class size shift is visible to abidiff:
abidw --out-file v1.abi libv1.so
abidw --out-file v2.abi libv2.so
abidiff v1.abi v2.abi
echo "exit: $?"   # → 4 (ABI change: base class data member added)

Reproduce manually¶

g++ -shared -fPIC -g v1.cpp -o libv1.so
g++ -shared -fPIC -g v2.cpp -o libv2.so
abidw --headers-dir . --out-file v1.abi libv1.so
abidw --headers-dir . --out-file v2.abi libv2.so
abidiff v1.abi v2.abi
echo "exit: $?"   # → 4

How to fix¶

Never add data members to a base class that has derived classes in the public API. Options: 1. Pimpl on Base — put the new field in a private Impl* struct. 2. SONAME bump — major version bump + abi_tag if breaking change is unavoidable. 3. Add to Derived, not Base — if only one derived class needs the field.