abstract class and virtual function

Weichao Wang

Dear all,
in a header file I've defined an abstract class (SegmentGroup) and a concrete class (Sgz) dirived from the former:

class SegmentGroup {
protected:
    QVector<QString> segments;
public:
    void appendSegment(QString);
    virtual void assignSegment() = 0;
    virtual QVector<QString> getSegments() = 0;
};
class Sgz : public SegmentGroup {
public:
    Segment cnt;
    Segment unt;
};

In the corresponding source file I've following definition:

void SegmentGroup::appendSegment(QString str) {
    segments.append(str);
}

void Sgz::assignSegment() {
    foreach (QString seg, segments) {
        if (seg.startsWith("CNT+"))
            cnt.segStr = seg;
        else if (seg.startsWith("UNT+"))
            unt.segStr = seg;
    }
}
QVector<QString> Sgz::getSegments() {
    QVector<QString> segment;
    if (!cnt.segStr.isEmpty())
        segment.append(cnt.segStr);
    if (!unt.segStr.isEmpty())
        segment.append(unt.segStr);
    return segment;
}

In my main() function I've declared an object of Sgz. But the compiler tells me that I cannot define an object of an abstract class. I think that I've made Sgz a concrete class by defining the both virtual functions in the base class. What's the error? Thanks for any hint!
Weichao

[koahnig: code tags added]

kshegunov

@Weichao-Wang said in abstract class and virtual function:

But the compiler tells me that I cannot define an object of an abstract class.

Please post the exact error. Also you're missing the functions' declarations in Sgz.

koahnig

@Weichao-Wang

That would look better:

class SegmentGroup {
protected:
    QVector<QString> segments;
public:
    void appendSegment(QString);
    virtual void assignSegment() = 0;
    virtual QVector<QString> getSegments() = 0;
};
class Sgz : public SegmentGroup {
public:
    void assignSegment();                           // redeclaration required
    QVector<QString> getSegments();    // redeclaration required

    Segment cnt;
    Segment unt;
};

Note: at least that was the syntax some standards ago. In case that became obsolete, I appreciate the hint. I was simply too lazy to check ;)

Weichao Wang

@koahnig
You've pointed out the very reason of error. After I've done the correction, the program runs well. Thank you!
Weichao

Weichao Wang

@kshegunov
Now I understand that you mean the same as koahnig. Thank you!
Weichao

koahnig

@Weichao-Wang

Please mark püost as solved then

Weichao Wang

@koahnig
Done. Thank you for the remind. Are you in Germany (for you input ü besides p)? I'm in Hamburg.
Weichao

koahnig

@Weichao-Wang

South of Germany resp south of Lake Constance (Bodensee)

SGaist

Hi,

With C++11, you can add the override keyword after the declaration of each function you re-implement. This will give the compiler a hint that will allow him to scream at you if you try to re-implement a non virtual function.