Error LNK2001

#include <iostream>
using namespace std;
 
const int MaxSize = 100;
 
class Set {
    int len;
    char members[MaxSize];
    int find (char ch);
public:
    Set() { len = 0; }
    int  getLength() { return len; }
 
    void showset();
    bool isMember(char ch);
 
    Set operator +(char ch);
    Set operator -(char ch);
 
    Set operator +(Set ob2);
    Set operator -(Set ob2);
};
 
int Set::find(char ch) {
    int i;
 
    for(i=0; i < len; i++)
        if(members[i] == ch) return -1;
}
 
void Set::showset() {
    cout << "{ ";
    for(int i=0; i<len; i++)
        cout << members[i] << " ";
 
    cout << "}\n";
}
 
Set Set::operator +(char ch) {
    Set newset;
 
    if(len == MaxSize) {
        cout << "Mnoj zapoln.\n";
        return *this;
    }
    newset = *this;
    if(find(ch) == -1) {
        newset.members[newset.len] = ch;
        newset.len++;
    }
    return newset;
}
 
Set Set::operator -(char ch) {
    Set newset;
    int i = find(ch);
 
    for(int j=0; j < len; j++)
        if(j != i) newset = newset + members[j];
 
    return newset;
}
 
 
Set Set::operator +(Set ob2) {
    Set newset = *this;
 
    for(int i=0; i < ob2.len; i++)
        newset = newset + ob2.members[i];
 
    return newset;
}
 
Set Set::operator -(Set ob2) {
    Set newset = *this;
 
    for(int i=0; i < ob2.len; i++)
        newset = newset - ob2.members[i];
 
    return newset;
}
 
int main() {
 
    Set s1;
    Set s2;
    Set s3;
 
    s1 = s1 + 'A';
    s1 = s1 + 'B';
    s1 = s1 + 'C';
 
    cout << "Mnoj s1 posle dobav simvol A B C: ";
    s1.showset();
 
    cout << "\n";
 
    cout << "Test chlenstva s pomoshuy func isMEmber().\n";
    if(s1.isMember('B'))
        cout << "B - chlen mnoj s1.\n";
    else
        cout << "B - nt chlen mnoj s1.\n";
 
    if(s1.isMember('T'))
        cout << "T - chlen mnoj s1.\n";
    else
        cout << "T - nechlen mnoj s1.\n";
 
    cout << "\n";
 
    s1 = s1 - 'B';
    cout << "Mnoj s1 posle s1 = s1 - 'B': ";
    s1.showset();
 
    s1 = s1 - 'A';
    cout << "Mnoj s1 posle s1 = s1 - 'C': ";
    s1.showset();
 
    s1 = s1 - 'C';
    cout << "Mnoj s1 posle s1 = s1 - 'C': ";
    s1.showset();
 
    cout << "\n";
 
    s1 = s1 + 'A';
    s1 = s1 + 'B';
    s1 = s1 + 'C';
    cout << "Mnoj s1 posle dobavl A B C: ";
    s1.showset();
 
    cout << "\n";
 
    s2 = s2 + 'A';
    s2 = s2 + 'X';
    s2 = s2 + 'W';
 
    cout << "Mnoj 2 posle dobav A X W: ";
    s2.showset();
 
    cout << "\n";
 
    s3 = s1 + s2;
    cout << "Mnoj s3 posle s3 = s1 + s2: ";
    s3.showset();
 
    s3 = s3 - s1;
    cout << "Mnoj s3 posle s3 - s1: ";
    s3.showset();
 
    cout << "\n";
 
    cout << "Mnoj s2 posle s2 = s2 - s2: ";
    s2 = s2 - s2;
    s2.showset();
 
    cout << "\n";
 
    s2 = s2 + 'C';
    s2 = s2 + 'B';
    s2 = s2 + 'A';
 
    cout << "Mnoj s2 posle dobav C B A: ";
    s2.showset();
 
    system("pause");
 
    return 0;
}

cout << "Test chlenstva s pomoshuy func isMEmber().\n";
    if(s1.isMember('B'))
        cout << "B - chlen mnoj s1.\n";
    else
        cout << "B - nt chlen mnoj s1.\n";
 
    if(s1.isMember('T'))
        cout << "T - chlen mnoj s1.\n";
    else
        cout << "T - nechlen mnoj s1.\n";

bool Set::isMember(char ch);