How does base pointer to derived object access its private members?

class IErrorLog
{
    public:
    virtual bool OpenLog(const char *strFilename) = 0;
    virtual bool CloseLog() = 0;

    virtual bool WriteError(const char *strErrorMessage) = 0;
};

class FileErrorLog : public IErrorLog
{
    bool OpenLog(const char *strFilename)
    {
        cout << strFilename <<" is opened in FileErrorLog"<<endl;
        return true;
    }
   bool CloseLog()
    {
        cout << "Close Log in FileErrorLog"<<endl;
        return true;
    }

    bool WriteError(const char *strErrorMessage)
    {
        cout << "Error: "<<strErrorMessage<<" in in FileErrorLog"<<endl;
        return true;
    }
public:
    FileErrorLog(){}


};

int main()
{
    FileErrorLog *f1= new FileErrorLog;
    IErrorLog *ptr = new FileErrorLog;
    bool er=ptr->WriteError("Memory finished");
    bool f=f1->WriteError("direct");