What does it mean that the “virtual table” is an unresolved external?
If you get a link error of the form “Error: Unresolved or undefined symbols detected: virtual table for class Fred,” you probably have an undefined virtual[20] member function in class Fred. The compiler typically creates a magical data structure called the “virtual table” for classes that have virtual functions (this is how it handles dynamic binding[20.2]). Normally you don’t have to know about it at all. But if you forget to define a virtual function for class Fred, you will sometimes get this linker error. Here’s the nitty gritty: Many compilers put this magical “virtual table” in the compilation unit that defines the first non-inline virtual function in the class. Thus if the first non-inline virtual function in Fred is wilma(), the compiler will put Fred’s virtual table in the same compilation unit where it sees Fred::wilma(). Unfortunately if you accidentally forget to define Fred::wilma(), rather than getting a Fred::wilma() is undefined, you may get a “Fred’s virtual table is
