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When linker says "undefined reference to vtable for IBase" it basically means that Derived has vtable reference to IBase but it can't find any compiled object code of IBase to look up to. So the bottom line is that class IBase has declarations without implementations.
V-tables (or virtual tables) are how most C++ implementations do polymorphism. For each concrete implementation of a class, there is a table of function pointers to all the virtual methods. A pointer to this table (called the virtual table) exists as a data member in all the objects.
You can fix undefined reference in C++ by investigating the linker error messages and then providing the missing definition for the given symbols. Note that not all linker errors are undefined references, and the same programmer error does not cause all undefined reference errors.
You're declaring a virtual function and not defining it:
virtual void calculateCredits();
Either define it or declare it as:
virtual void calculateCredits() = 0;
Or simply:
virtual void calculateCredits() { };
Read more about vftable: http://en.wikipedia.org/wiki/Virtual_method_table
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