Question

That’s my program written in C++ (OOP):

#include <iostream>
#include <string>

using namespace std;

class  book {
    string name,gvari;
    double cost;
    int year;

    public:
        book(){};

        book(string a, string b, double c, int d) { a=name;b=gvari;c=cost;d=year; }
        ~book() {}
        double setprice(double a) { return a=cost; }
        friend ostream& operator <<(ostream& , book&);
        void printbook(){
            cout<<"wignis saxeli "<<name<<endl;
            cout<<"wignis avtori "<<gvari<<endl;
            cout<<"girebuleba "<<cost<<endl;
            cout<<"weli "<<year<<endl;
        }
};

ostream& operator <<(ostream& out, book& a){
    out<<"wignis saxeli "<<a.name<<endl;
    out<<"wignis avtori "<<a.gvari<<endl;
    out<<"girebuleba "<<a.cost<<endl;
    out<<"weli "<<a.year<<endl;
    return out;
}

class library_card : public book {
    string nomeri;
    int raod;

    public:
        library_card(){};
        library_card( string a, int b){a=nomeri;b=raod;}
        ~library_card();
        void printcard(){
            cout<<"katalogis nomeri "<<nomeri<<endl;
            cout<<"gacemis raodenoba "<<raod<<endl;
        }
        friend ostream& operator <<(ostream& , library_card&);
};

ostream& operator <<(ostream& out, library_card& b) {
    out<<"katalogis nomeri "<<b.nomeri<<endl;
    out<<"gacemis raodenoba "<<b.raod<<endl;
    return out;
}


int main() {
    book A("robizon kruno","giorgi",15,1992);
    library_card B("910CPP",123);
    cout<<A;
    cout<<B;
    A.setprice(15);
    cout<<B;

    system("pause");
    return 0;
}

//P.S.
//B.printcard();
//A.printbook();
// As I've overloaded the "<<" operators, I've removed these two lines and put "cout<<A" "cout<<B". I hope, I'm right. By the way, error isn't related to these ones.

Here is the error, it may help you:

1>Neyw.obj : error LNK2019: unresolved external symbol "public: __thiscall library_card::~library_card(void)" (??1library_card@@QAE@XZ) referenced in function _main
1>c:\users\geo\documents\visual studio 2010\Projects\Newww\Debug\Newww.exe : fatal error LNK1120: 1 unresolved externals

1>Build FAILED.

The problem is I can’t figure out what’s wrong with the code.. There are two classes, the main one, book, and the successor class, library_card. Why does it give me such an error?

Answer 1

Replace

~library_card();

with

~library_card() {}

You haven’t implemented the destructor and this is exactly what your linker is saying.

Or you can remove this string as well if you don’t know why you need this destructor.