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 :)

Go read the language standard - there are lots

These are implementation defined.

The number of bits in a byte (CHAR_BIT)

The sign of the result of % on negative integers

The size of all non-char data types (eg sizeof(int) is 1, 2, 4, 8 depending on your machine)

Everything about bit-fields is implementation specific

The effect of shift left/right on negative integers

> 3. subexpression evaluation
That's more unspecified than undefined
http://www.eskimo.com/~scs/C-faq/q11.33.html

Undefined means absolutely anything, like
array[i++] = i;