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+Missing Key Function
+====================
+
+If your build failed with a linker error something like this::
+
+  foo.cc:28: error: undefined reference to 'vtable for C'
+  the vtable symbol may be undefined because the class is missing its key function
+  (see https://lld.llvm.org/missingkeyfunction)
+
+it's likely that your class C has a key function (defined by the ABI as the first
+non-pure, non-inline, virtual function), but you haven't actually defined it.
+
+When a class has a key function, the compiler emits the vtable (and some other
+things as well) only in the translation unit that defines that key function. Thus,
+if you're missing the key function, you'll also be missing the vtable. If no other
+function calls your missing function, you won't see any undefined reference errors
+for it, but you will see undefined references to the vtable symbol.
+
+When a class has no non-pure, non-inline, virtual functions, there is no key
+function, and the compiler is forced to emit the vtable in every translation unit
+that references the class. In this case, it is emitted in a COMDAT section,
+which allows the linker to eliminate all duplicate copies. This is still
+wasteful in terms of object file size and link time, so it's always advisable to
+ensure there is at least one eligible function that can serve as the key function.
+
+Here are the most common mistakes that lead to this error:
+
+Failing to define a virtual destructor
+--------------------------------------
+
+Say you have a base class declared in a header file::
+
+  class B {
+  public:
+    B();
+    virtual ~B();
+    ...
+  };
+
+Here, ``~B`` is the first non-pure, non-inline, virtual function, so it is the key
+function. If you forget to define ``B::~B`` in your source file, the compiler will
+not emit the vtable for ``B``, and you'll get an undefined reference to "vtable
+for B".
+
+This is just an example of the more general mistake of forgetting to define the
+key function, but it's quite common because virtual destructors are likely to be
+the first eligible key function and it's easy to forget to implement them. It's
+also more likely that you won't have any direct references to the destructor, so
+you won't see any undefined reference errors that point directly to the problem.
+
+The solution in this case is to implement the missing function.
+
+Forgetting to declare a virtual function in an abstract class as pure
+---------------------------------------------------------------------
+
+Say you have an abstract base class declared in a header file::
+
+  class A {
+  public:
+    A();
+    virtual ~A() {}
+    virtual int foo() = 0;
+    ...
+    virtual int bar();
+    ...
+  };
+
+This base class is intended to be abstract, but you forgot to mark one of the
+functions pure. Here, ``A::bar``, being non-pure, is nominated as the key function,
+and as a result, the vtable for ``A`` is not emitted, because the compiler is
+waiting for a translation unit that defines ``A::bar``.
+
+The solution in this case is to add the missing ``= 0`` to the declaration of
+``A::bar``.
+
+Key function is defined, but the linker doesn't see it
+------------------------------------------------------
+
+It's also possible that you have defined the key function somewhere, but the
+object file containing the definition of that function isn't being linked into
+your application.
+
+The solution in this case is to check your dependencies to make sure that
+the object file or the library file containing the key function is given to
+the linker.