C++ New Added 10 September 2026

C++: undefined reference to `vtable for X'

This is not a missing call to the named function. The compiler emits a class's vtable alongside the definition of its first non inline virtual function, so if that function is declared and never defined, or its translation unit is missing from the link, the vtable is never emitted. A virtual destructor declared in the header and never written is the classic case.

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

Quick fix

// Declared, never defined: the vtable has nowhere to be emitted
class Shape {
public:
    virtual ~Shape();        // needs a definition somewhere
    virtual void draw() = 0; // pure virtual is fine, no definition needed
};

// Either define it in the .cpp
Shape::~Shape() = default;

// Or define it inline in the header
virtual ~Shape() = default;

# Or the definition exists but the object file was never linked
nm -C build/shape.o | grep -i vtable
ls build/CMakeFiles/app.dir/*.o

# CMake: the source is missing from the target
# add_executable(app main.cpp shape.cpp)

# Qt classes need moc output linked in too: rerun the generator
cmake --build build --clean-first

How to diagnose C++ errors

C++ errors fall into two very different worlds. Compile and link errors (undefined reference, multiple definition, template substitution failure) are deterministic and are almost always about declarations, the One Definition Rule, or link order. Runtime memory errors (segfaults, double free, use-after-free) are non-deterministic and should never be debugged by reading code alone; a sanitizer will find in seconds what code review misses for days.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

Rebuild with sanitizers before anything else: -fsanitize=address,undefined -fno-omit-frame-pointer -g. AddressSanitizer reports the exact allocation and free sites for use-after-free and double-free.
For undefined-reference errors, check link order, remembering that with GNU ld libraries must come after the objects that use them, and check for a C/C++ linkage mismatch that needs extern "C".
Use nm -C libfoo.a | grep symbol to confirm the symbol is actually present and to see the demangled signature. A signature that differs by a const is a different symbol.
Enable core dumps (ulimit -c unlimited) and open them in gdb with gdb ./binary core, then bt full. A stack trace beats speculation.
For template errors, read the message from the bottom up. The final line is usually the real constraint that failed; everything above is instantiation context.

Tools worth reaching for

-fsanitize=address,undefined
valgrind
gdb / lldb
nm -C
ldd

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

gcc.gnu.org

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