livingtm@pc:~$ cat foo.cpp
class X {
  int i;
public:
  X();  // Constructor
}; 

int main() {
      X a;
      // ...
} 
livingtm@pc:~$ gcc foo.cpp
/tmp/ccICpVjs.o: In function `main':
foo.cpp:(.text+0x18): undefined reference to `X::X()'
/tmp/ccICpVjs.o:(.eh_frame+0x11): undefined reference to `__gxx_personality_v0'
collect2: ld returned 1 exit status