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