C++ Inhertiance: function signatures for base type not working with derived type

Question

I have the following code:

class STFDataPoint {
public:

    virtual ImagePoint get_patch_top_left() const = 0;
    virtual ImagePoint get_patch_bottom_right() const = 0;
    virtual std::string get_image_filename() const = 0;

    virtual ~STFDataPoint() = 0;
};
inline STFDataPoint::~STFDataPoint() {}


class TrainingDataPoint : public STFDataPoint{
private:
    int row;
    int col;
    std::string class_label;
    ImagePoint patch_top_left;
    ImagePoint patch_bottom_right;
    std::string image_filename;
public:
    TrainingDataPoint(int row, int col, std::string class_label, 
            const ImagePoint & top_left, 
            const ImagePoint & bottom_right, 
            std::string image_filename);

    std::string get_class_label() const;

    inline bool operator==(const TrainingDataPoint& other) const{
        return other.class_label == this->class_label;
    }
    inline bool operator!=(const TrainingDataPoint& other) const{
        return !(*this == other);
    }

    virtual ImagePoint get_patch_top_left() const;
    virtual ImagePoint get_patch_bottom_right() const;
    virtual std::string get_image_filename() const;

};

And I am trying to run the following:

bool do_something(vector<STFDataPoint>& data_point){
    return true;
}


int main(int argc, char* argv[]) {

    ImagePoint left = ImagePoint(2,3);
    ImagePoint right = ImagePoint(2,3);

    TrainingDataPoint a = TrainingDataPoint(1,2,"",left, right, "");
    vector<TrainingDataPoint> b;
    b.push_back(a);

    do_something(b);
}

But get the following error:

invalid initialization of reference of type ‘std::vector<STFDataPoint>&’ from expression of type `std::vector<TrainingDataPoint>`

however if I change the signature of do_something() to take in a STFDataPoint (not a vector of them) it runs fine. Can some one please explain why this is and also if there is a work around?

Thanks

Answer 1

Since vector<TrainingDataPoint> is not a subtype of vector<STFDataPoint> you cannot do this. Vectors are not covariant in parameter type.

However you can template do_something to make it work:

template <typename T>
bool do_something(vector<T>& data_point){
   //common actions like
   ImagePoint leftPatch = data_point[0].get_patch_top_left();
   return true;
}

Answer 2

The type vector<TrainingDataPoint> is not the same as vector<STFDataPoint> and there is no conversion between the two. vector<A> is not a base type of vector<B>, even if A is a base of B.

What could work is to have a container of pointers or smart pointers to the base type, and change the function to use that:

bool do_something(vector<std::unique_ptr<STFDataPoint>>& data_point){
    return true;
}

std::vector<std::unique_ptr<STFDataPoint>> b;
b.push_back( std::unique_ptr<STFDataPoint>(new TrainingDataPoint(1,2,"",left, right, "") ); // fill with any derived types of STFDataPoint
do_something(b);