[c++] GCC C++ Linker errors: Undefined reference to 'vtable for XXX', Undefined reference to 'ClassName::ClassName()'

In my case the problem occured when i forgot to add the =0 on one function in my pure virtual class. It was fixed when the =0 was added. The same as for Frank above.

class ISettings
{
public: 
    virtual ~ISettings() {};
    virtual void OKFunction() =0;
    virtual void ProblemFunction(); // missing =0   
};

class Settings : ISettings
{
    virtual ~Settings() {};
    void OKFunction();
    void ProblemFunction(); 
};

void Settings::OKFunction()
{
    //stuff
}

void Settings::ProblemFunction()
{
    //stuff
}

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