undefined and implementation specific constructs

As an exercise in Barjne Strousups book he asks to make a list of five constructs which are undefined in C++ and five constructs which are implementation specific.

I couldn't come up with five of each, but thought it would be a good resource if you could list any examples you can think of.

Here is what I came up with from the text.

Undfined:

1. Overflow/Underflow

incrementing/decrementing past the max size of the argument type.

2. Division by Zero

3. subexpression evaluation

int i=f(1)+g(2);// cannot assume left to right evaluation.

The only one I could come up with that is implementation dependant is:

The definition of NULL;

Feel free to add your own :)