Abstract method inheritance

[space_otter]

I have two classes of the form:

class DNumber
{
    static int Digits;
    virtual DNumber* add(const DNumber* a, unsigned digits) const = 0;
    DNumber* add(const DNumber* a) const {return add(a, Digits);}
}
 
class DInteger : public DNumber
{
    // implementation uses runtime type checking to perform addition
    DInteger(int value);
    DNumber* add(const DNumber* a, unsigned digits) const;
}

class DNumber
{
    static int Digits;
    virtual DNumber* add(const DNumber* a, unsigned digits) const = 0;
    DNumber* add(const DNumber* a) const {return add(a, Digits);}
}

class DInteger : public DNumber
{
    // implementation uses runtime type checking to perform addition
    DInteger(int value);
    DNumber* add(const DNumber* a, unsigned digits) const;
}

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Attempting to use this:

DInteger* cien = new DInteger(100);
    DInteger* dos = cien->add(cien);
    qDebug() << dos->toString();
 
    delete cien, delete dos;

DInteger* cien = new DInteger(100);
    DInteger* dos = cien->add(cien);
    qDebug() << dos->toString();

    delete cien, delete dos;

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Results in a compile error:

error: no matching function for call to 'DInteger::add(DInteger*&)'

DInteger should inherit add(DNumber*) right?