I mean take the stringstream declaration, and use, and put it in a different function entirely, just for now.
If I copy this function below main it makes no difference.
Are you suggesting that the line "ss << 15" forces instantiation of operator<< where-as without it the member function is not?
When it encounters them, yes, but with it commented out, it is never used; the compiler never processes what's in comments. I'm not sure if it is implementation defined, but my compiler does not instantiate a member function if it is never called, in order to cut down on executable size. This leads to frustrating bugs that I overlook, because I never used the function, thus the compiler never instantiates it and points out an obvious bug.
Now isn't the compiler supposed to instantiate the functions automatically when it encounters them? So would this be a compiler issue?
So, call it a long shot, but perhaps something relies on this operator <<() function being instantiated, and when it isn't instantiated in any of the objects linked together, the program crashes. This reminds me of a compiler bug discovered not too long ago, where virtual functions were being declared as inline, and thus the compiler never instantiated the function as a normal function, leading to a crash when the program attempted to access the function through the vtable.
Defining a virtual function as an inline function is legal, and has uses, but the compiler is supposed to make a normal function instantiation as well, so that it can be called via function pointer. The bug was that the normal out-of-line instantiation wasn't made, thus the function pointer pointed to garbage.