printing with polymorphism and inheritance

Question

I have a problem with polymorphism and inheritance. The parent class is:

class IIR_filter {

private:
    double* a_;
    double* b_;
    unsigned int order_;

    void reverseArray(double* source, double* destination, unsigned int size);

public:
    IIR_filter();
    IIR_filter(double* a, double* b, unsigned int order);   //c'tor
    IIR_filter(const IIR_filter& filter);                   //copy c'tor
    virtual ~IIR_filter();                                          //d'tor

    virtual void filter(double* inputX, double* outputY, unsigned int size);

    virtual void zeroPhaseFilter(double* inputX, double* outputY, unsigned int size);

    friend ostream& operator<<(ostream& lhs, const IIR_filter& rhs);

protected:
    double filterSample(unsigned int indexY, double* outputY,double newSample, Queue<double>& filter);
    double getElementA(unsigned int index) const;
    double getElementB(unsigned int index) const;
    unsigned int getOrder() const;
    virtual void print(ostream& lhs, const IIR_filter* rhs)const;
};

and the cpp relevant functions are:

ostream& operator<<(ostream& lhs, const IIR_filter& rhs){
    std::cout << "IIR filter: " << std::endl;
    rhs.print(lhs, &rhs);
    return lhs;
    }

and the derived class is:

class FIR_filter : public IIR_filter {

public:
    FIR_filter(double* b, unsigned int order);              //c'tor
    friend ostream& operator<<(ostream& lhs, FIR_filter* rhs);


};

And the cpp function is:

ostream& operator<<(ostream& lhs, FIR_filter* rhs){
lhs << "FIR filter: \n";
rhs->print(lhs, rhs);
return lhs;
}

And of course there is the print function in the IIR_filter class, but it's too long and irrelevant for my opinion.

The problem is: When I call the printing function (operator<<) from an object of the derived class, the print is done from the parent class, so instead of the header "FIR filter" i get the header "IIR filter". I have tried many ways but have no success. Please help. Thank, Ran.

Don't you have an override of `print()` in the derived class ?? — Christophe, Jun 21 '16 at 21:10
This is incorrect: " When I call the printing function (operator<<) from an object of the derived class, the print is done from the parent class". That would evoke the `operator<<` for the derived class, if the argument was a pointer as it should be to match your declaration. Voting to close as lacking reproducible example. — Cheers and hth. - Alf, Jun 21 '16 at 21:40
That said, *most likely* the problem is `FIR_filter* rhs`; change that to `FIR_filter const& rhs`. — Cheers and hth. - Alf, Jun 21 '16 at 21:43

score 1 · Answer 1 · answered Jun 21 '16 at 21:14

You should move the writing of a header into a stream inside the virtual print function, and create a suitable override in the derived class. You might also want to refactor the code to have a virtual print_header function called inside a non-virtual print, since you're currently not overriding it in the derived class.

By the way, you have inconsistency - one operator<< is accepting a reference to a const object, and the other is accepting a pointer. But even if you unified your convention, the operator<< is still not virtual, so it cannot determine in runtime if the reference (or pointer) to the base type indeed points to a derived object. It means, that in current state, it will (and should) always print the "IIR filter" header.

score 0 · Answer 2 · edited May 23 '17 at 12:22

0

You can't derive friend ship. See this question for more details: friend class with inheritance

Also, like that was mentioned before, You should have one overloaded operator<< in base class which just returns only output of print function. Than in print implementations You can do all printing and other stuff for all Your classes.

edited May 23 '17 at 12:22

Community

1
1

answered Jun 21 '16 at 21:22

Piotrek

88
11

printing with polymorphism and inheritance

2 Answers2