/tmp/ccxWE2Mp.o(.text+0x3c7): In function `main':
lab2.cc: undefined reference to `Triple<int, float, std::basic_string<char, std::char_traits<char>, std::allocator<char> > >::Triple(Triple<int, float, std::basic_string<char, std::char_traits<char>, std::allocator<char> > > const&)'
collect2: ld returned 1 exit status

template<class T1, class T2, class T3>
class Triple
{
public:
        // default construction
        Triple() {}
        
        // construction with initial values
        Triple(T1 v1, T2 v2, T3 v3) : _value1(v1), _value2(v2), _value3(v3) { }
        
        // copy construction
        Triple(const Triple& t);  
        
        // accessors
        T1 value1() { return _value1; }
        T2 value2() { return _value2; }
        T3 value3() { return _value3; }
        
        // modifiers
        void set_value1(T1 v) { _value1 = v; }
        void set_value2(T2 v) { _value2 = v; }
        void set_value3(T3 v) { _value3 = v; }
private:
        T1 _value1;
        T2 _value2;
        T3 _value3;
};
// copy constructor
template< class T1, class T2, class T3 >
Triple< T1, T2, T3 >::Triple(const Triple< T1, T2, T3 >& t)
{
        
        _value1 = t.value1();
        _value2 = t.value2();
        _value3 = t.value3();
        
}

int 
main()
{
         
        // TEST TEMPLATE CLASS
        Triple<int, float, string> ifs_triple1;

        // test modifiers
        ifs_triple1.set_value1(5);
        ifs_triple1.set_value2(5.34);
        ifs_triple1.set_value3("Hello!");
             
        // test copy constructor     
        Triple<int, float, string> ifs_triple2(ifs_triple1);
        
        // print values (should be the same)
        cout << "TEST TEMPLATE CLASS:" << endl;
        cout << "ifs_triple1.value1()=" << ifs_triple1.value1();
        cout << " ifs_triple1.value2()=" << ifs_triple1.value2();
        cout << " ifs_triple1.value3()=" << ifs_triple1.value3();
        cout << endl;

        cout << "ifs_triple2.value1()=" << ifs_triple2.value1();
        cout << " ifs_triple2.value2()=" << ifs_triple2.value2();
        cout << " ifs_triple2.value3()=" << ifs_triple2.value3();
        cout << endl;
        
        return 0;               // exit successfull
}

       // copy construction
        Triple(const Triple& t);

       // copy construction
        Triple(const Triple<T1,T2,T3> & t);