C ++转发类声明的问题

Question

我在使用纯虚方法和正向类声明时遇到问题。 以下程序可以编译，但不会链接。

class SubClass;

class SuperClass {
    public:
    virtual SubClass& GetSomething() = 0;
};

class SubClass: public SuperClass {
    public:
    SubClass& GetSomething();
};


SubClass& SubClass::GetSomething()
{
    return *this;
}

int main()
{
    SuperClass *obj = new SubClass();
    SubClass result = obj->GetSomething();
    return 0;
}

我遇到以下错误：

Undefined symbols for architecture x86_64:
  "vtable for __cxxabiv1::__class_type_info", referenced from:
      typeinfo for SuperClass in classtest-56e822.o
  NOTE: a missing vtable usually means the first non-inline virtual member function has no definition.
  "vtable for __cxxabiv1::__si_class_type_info", referenced from:
      typeinfo for SubClass in classtest-56e822.o
  NOTE: a missing vtable usually means the first non-inline virtual member function has no definition.
  "operator new(unsigned long)", referenced from:
      _main in classtest-56e822.o
  "___cxa_pure_virtual", referenced from:
      vtable for SuperClass in classtest-56e822.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

我在做什么错了？