"undefined reference" to Virtual Base class destructor [duplicate]

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What is an undefined reference/unresolved external symbol error and how do I fix it?

I have some experience with Java, and am now doing a C++ course. I wanted to try writing an interface, but I have run into some trouble with destructors which I have not been able to resolve, even with the help on the Internet... Here's my code:

 class Force { public: virtual ~Force(); virtual VECTOR eval(VECTOR x, double t); }; class InvSquare : public Force { public: InvSquare(double A) { c = A; } ~InvSquare(){}; VECTOR eval(VECTOR x, double t) { // omitted stuff } private: double c; };

I have tried to declare a virtual destructor for the base class, and a non-virtual one for the derived class, but I get an error saying "undefined reference to `Force::~Force()'". What does it mean, and how can I fix it?

Forgive me if this is a silly question!

Thank you very much for your help, noctilux

Mike Seymour · Accepted Answer · 2012-11-18 21:46:45Z

You've declared the destructor, but not defined it. Change the declaration to:

virtual ~Force() {}

to define it to do nothing.

You also want to make all the functions in the abstract interface pure virtual, otherwise they will need to be defined too:

virtual VECTOR eval(VECTOR x, double t) = 0;

A, so called, C++ interface is a class in which all methods are pure virtual, all but the destructor?

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