Case 22 — Method Const Qualifier Changed¶

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

Verdict: 🔴 BREAKING abicheck verdict: BREAKING

What changes¶

Version	Declaration
v1	`class Widget { void get() const; };`
v2	`class Widget { void get(); };`

What breaks at binary level¶

In the Itanium C++ ABI, const qualification on a member function is part of the mangled symbol name:

Widget::get() const → _ZNK6Widget3getEv (note the K for const)
Widget::get() → _ZN6Widget3getEv

Removing (or adding) const produces a different mangled name. Existing binaries compiled against v1 import _ZNK6Widget3getEv, but v2 only exports _ZN6Widget3getEv. The dynamic linker cannot resolve the symbol → unresolved symbol error at load time.

Consumer impact¶

/* consumer compiled against v1 */
const Widget& w = get_widget();
w.get();  /* calls _ZNK6Widget3getEv */

/* v2 library exports _ZN6Widget3getEv (non-const) */
/* → undefined symbol: _ZNK6Widget3getEv */

Mitigation¶

Keep the old const-qualified method and add a non-const overload if needed.
Provide both signatures during a transition period.

Code diff¶

 class Widget {
 public:
-    void get() const;
+    void get();
 };

Real Failure Demo¶

Severity: CRITICAL

Scenario: app compiled against old header calls get() const (_ZNK6Widget3getEv). New lib exports only _ZN6Widget3getEv — undefined symbol at runtime.

# Build old lib + app
g++ -shared -fPIC -g old/lib.cpp -Iold -o libwidget.so
g++ -g app.cpp -Iold -L. -lwidget -Wl,-rpath,. -o app
./app
# → get() const called

# Swap in new lib (const removed)
g++ -shared -fPIC -g new/lib.cpp -Inew -o libwidget.so
./app
# → ./app: symbol lookup error: undefined symbol: _ZNK6Widget3getEv

Why CRITICAL: const is part of the C++ mangled name (K in _ZNK...). Removing it produces a completely different symbol. Every pre-built binary that calls widget.get() on a const reference fails to load — hard runtime crash.

Why runtime result may differ from verdict¶

const qualifier on method changes mangled name — symbol lookup error