Fatal error LNK1120 in C++
<p>That's my program written in C++ (OOP):</p>

<pre><code>#include &lt;iostream&gt;
#include &lt;string&gt;

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&amp; operator &lt;&lt;(ostream&amp; , book&amp;);
        void printbook(){
            cout&lt;&lt;"wignis saxeli "&lt;&lt;name&lt;&lt;endl;
            cout&lt;&lt;"wignis avtori "&lt;&lt;gvari&lt;&lt;endl;
            cout&lt;&lt;"girebuleba "&lt;&lt;cost&lt;&lt;endl;
            cout&lt;&lt;"weli "&lt;&lt;year&lt;&lt;endl;
        }
};

ostream&amp; operator &lt;&lt;(ostream&amp; out, book&amp; a){
    out&lt;&lt;"wignis saxeli "&lt;&lt;a.name&lt;&lt;endl;
    out&lt;&lt;"wignis avtori "&lt;&lt;a.gvari&lt;&lt;endl;
    out&lt;&lt;"girebuleba "&lt;&lt;a.cost&lt;&lt;endl;
    out&lt;&lt;"weli "&lt;&lt;a.year&lt;&lt;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&lt;&lt;"katalogis nomeri "&lt;&lt;nomeri&lt;&lt;endl;
            cout&lt;&lt;"gacemis raodenoba "&lt;&lt;raod&lt;&lt;endl;
        }
        friend ostream&amp; operator &lt;&lt;(ostream&amp; , library_card&amp;);
};

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


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

    system("pause");
    return 0;
}

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

<p>Here is the error, it may help you:</p>

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

1&gt;Build FAILED.
</code></pre>

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