[C++] key function

Reference:
undefined reference to vtable for X
What is a C++ "Key Function" as described by gold?

Every polymorphic class requires a virtual table or vtable describing the type and its virtual functions. Whenever possible the vtable will only be generated and output in a single object file rather than generating the vtable in every object file which refers to the class (which might be hundreds of objects that include a header but don't actually use the class.)

The cross-platform C++ ABI states that the vtable will be output in the object file that contains the definition of the key function, which is defined as the first non-inline, non-pure, virtual function declared in the class.

If you do not provide a definition for the key function (or fail to link to the file providing the definition) then the linker will not be able to find the class' vtable.

class X {
public:
  virtual ~X() = 0;
  void f();
  virtual void g() { }
  virtual void h(); // defined inline out side of class.
  virtual void i(); // <-- Key fuction.
  virtual void j();
};

inline void X::h() { }