Case 123 — Default Argument Removed¶

Field	Value
Verdict	🟠 API_BREAK
Category	API Break
Platforms	Linux
Flags	API break
Detected `ChangeKind`s	`param_default_value_removed`
Source files	`examples/case123_default_argument_removed/`

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

What changes¶

Version	Declaration
v1	`int connect(const char *host, int timeout_ms = 5000);`
v2	`int connect(const char *host, int timeout_ms);`

The default value for timeout_ms is removed. The mangled symbol is identical (default arguments do not participate in name mangling).

What breaks¶

Any caller that relied on the default — connect("example.org") — no longer compiles against v2 (too few arguments to function). Already-compiled binaries keep linking and running. A pure source/API break. See app.cpp.

Why this case exists — invisible to object analysis¶

Default-argument values live only in the declaration (the header). They are not encoded in the symbol name, DWARF, or anywhere in the object file — the v1 and v2 .so are ABI-identical. abicheck detects this only in header mode, where castxml exposes the default="5000" attribute (ChangeKind param_default_value_removed). In object/DWARF mode it reports NO_CHANGE.

(Changing a default value rather than removing it is reported as the compatible-but-behavioural param_default_value_changed; adding a default is source-compatible and not flagged.)

Reproduce manually¶

g++ -shared -fPIC -g v1.cpp -o libnet_v1.so
g++ -shared -fPIC -g v2.cpp -o libnet_v2.so
abicheck compare libnet_v1.so libnet_v2.so \
    --old-header v1.h --new-header v2.h   # → API_BREAK (param_default_value_removed)
abicheck compare libnet_v1.so libnet_v2.so # → NO_CHANGE (object-only)

How to fix¶

Keep the default, or add an overload (connect(host) forwarding to connect(host, 5000)) so existing call sites keep compiling.

Source files¶

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