The ISO C++ Standard specifies that all virtual methods of a class that are not pure-virtual must be defined and compilers are not bound (by standards) to warn you if you don't follow this rule. Based on this assumption, GCC will only emit the implicitly defined constructors, the assignment operator, the destructor and the virtual table of a class in the translation unit that defines its first such non-inline method.
Therefore, if you fail to define this particular method, the linker complains. In case of gcc and ld (linker on Linux), the linker gives out an error message saying "undefined reference to `vtable for function_name' ". This error message is quite misleading. The solution is to ensure that all virtual methods that are not pure are defined. An exception to this rule is a pure-virtual destructor, which must be defined (empty body) in any case. Ch. 12, [class.dtor]/7.