1Missing Key Function 2==================== 3 4If your build failed with a linker error something like this:: 5 6 foo.cc:28: error: undefined reference to 'vtable for C' 7 the vtable symbol may be undefined because the class is missing its key function 8 (see https://lld.llvm.org/missingkeyfunction) 9 10it's likely that your class C has a key function (defined by the ABI as the first 11non-pure, non-inline, virtual function), but you haven't actually defined it. 12 13When a class has a key function, the compiler emits the vtable (and some other 14things as well) only in the translation unit that defines that key function. Thus, 15if you're missing the key function, you'll also be missing the vtable. If no other 16function calls your missing function, you won't see any undefined reference errors 17for it, but you will see undefined references to the vtable symbol. 18 19When a class has no non-pure, non-inline, virtual functions, there is no key 20function, and the compiler is forced to emit the vtable in every translation unit 21that references the class. In this case, it is emitted in a COMDAT section, 22which allows the linker to eliminate all duplicate copies. This is still 23wasteful in terms of object file size and link time, so it's always advisable to 24ensure there is at least one eligible function that can serve as the key function. 25 26Here are the most common mistakes that lead to this error: 27 28Failing to define a virtual destructor 29-------------------------------------- 30 31Say you have a base class declared in a header file:: 32 33 class B { 34 public: 35 B(); 36 virtual ~B(); 37 ... 38 }; 39 40Here, ``~B`` is the first non-pure, non-inline, virtual function, so it is the key 41function. If you forget to define ``B::~B`` in your source file, the compiler will 42not emit the vtable for ``B``, and you'll get an undefined reference to "vtable 43for B". 44 45This is just an example of the more general mistake of forgetting to define the 46key function, but it's quite common because virtual destructors are likely to be 47the first eligible key function and it's easy to forget to implement them. It's 48also more likely that you won't have any direct references to the destructor, so 49you won't see any undefined reference errors that point directly to the problem. 50 51The solution in this case is to implement the missing function. 52 53Forgetting to declare a virtual function in an abstract class as pure 54--------------------------------------------------------------------- 55 56Say you have an abstract base class declared in a header file:: 57 58 class A { 59 public: 60 A(); 61 virtual ~A() {} 62 virtual int foo() = 0; 63 ... 64 virtual int bar(); 65 ... 66 }; 67 68This base class is intended to be abstract, but you forgot to mark one of the 69functions pure. Here, ``A::bar``, being non-pure, is nominated as the key function, 70and as a result, the vtable for ``A`` is not emitted, because the compiler is 71waiting for a translation unit that defines ``A::bar``. 72 73The solution in this case is to add the missing ``= 0`` to the declaration of 74``A::bar``. 75 76Key function is defined, but the linker doesn't see it 77------------------------------------------------------ 78 79It's also possible that you have defined the key function somewhere, but the 80object file containing the definition of that function isn't being linked into 81your application. 82 83The solution in this case is to check your dependencies to make sure that 84the object file or the library file containing the key function is given to 85the linker. 86