C++ in G++ - Segmentation Fault when not using pointers

I'm trying to use G++ to compile some C++ code. It seems to work fine in other compilers, but for whatever reason, G++ won't produce working output.

Disclosure: This is part of a homework assignment, but I feel like it's more of a compiler issue, since it works in other compilers.

Here's the snippet that's wreaking havoc:

set<int> t1, t2;

It's strange because the following code works just fine:

set<int> *t1 = new set<int>();
set<int> *t2 = new set<int>();

Granted, I have to use -> instead of ., but that's expected. The first snippet produces a segmentation fault at runtime. The second does intuitively what I'd expect it to.

Anyhow, behind the scenes, the .cpp for set has this:

#include <cstdlib>
#include <iostream>

using namespace std;

template <class T>
set<T>::set() : bag<T>() {}

template <class T>
set<T>::set(const set<T>& b) : bag<T>(b) {}

The .h looks like this:

#include "bag.h"

template <class T>
class set : public bag<T>
{ 
    public:
        set( );
        set(const set &b);

// ...

};
#include "set.cpp"

And last but not least, the bag.cpp and bag.h files looks like this:

using namespace std;

template <class T>
bag<T>::bag() {
    head = NULL;
}

template <class T>
bag<T>::bag(const bag<T>& b) {

    // ...

}

and bag.h:

template <class T>
class bag
{ 
    public:
        bag( );
        bag(const bag &b);

    // ...
};
#include "bag.cpp"

Again, I feel like G++ just hates me, but then again I could be doing something dumb. Just a simple nudge in the right direction would be great.