Comments by Cashey Wen (Top 4 by date)

I think you might be inserting 4 duplicated elements. STL set automatically removes all duplicated elements, this guarantees elements stored are unique.

According to your classcomp, if lhs.getID() is equal to rhs.getID(), the set will regard lhs and rhs as two duplicated elements, only one of them will be stored.

If you have to store dumplicated elements, set is not the solution. Sorry for my mistake, I thought "delete a *specific* element from a set" means a set with no dumplicated elements.

Well, if it is a question like this, this could be easier. Just use list, I got an example for you.

http://www.cplusplus.com/reference/list/list/

#include <iostream>
#include <list>
using namespace std;

class MyClass
{
public:
MyClass(int _a, int _b, string _c): a(_a), b(_b), c(_c){}
bool operator ==(MyClass mc)
{
if (a == mc.a && b == mc.b && c == mc.c)
{
return true;
}
return false;
}

int a;
int b;
string c;
};

int main()
{
list<myclass> mylist;
list<myclass>::iterator iter;

MyClass mc1(1, 2, "abc");
MyClass mc2(2, 3, "ccc");
MyClass mc3(3, 4, "bbb");

mylist.push_back(mc1);
mylist.push_back(mc2);
mylist.push_back(mc3);

cout << "before deleting:" << endl;
for (iter = mylist.begin(); iter != mylist.end(); iter++)
{
cout << (*iter).a << " " << (*iter).b << " " << (*iter).c << endl;
}

mylist.remove(mc3);
cout << "after deleting:" << endl;
for (iter = mylist.begin(); iter != mylist.end(); iter++)
{
cout << (*iter).a << " " << (*iter).b << " " << (*iter).c << endl;
}

cin >> ws;
return 0;
}