Question

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How do we explain the result of the expression (++x)+(++x)+(++x)?
Undefined Behavior and Sequence Points

I have the problem, when the code

U = C + C++;

Runs in different way for standart types and for my own types.
I have an example http://ideone.com/4S1uA where I have different values for int and my class Int, which should represent the way real Int works.

Is it possible to make my class behave the same way, as the standard int works? Has this code undefined behavior?

WHY it is undefiend behaivior? C++ has an operation priorities, so the c++ should be evaluated first, as it change the value of a, so for addition as first argument should be passed new value of a and as the second the old value. And it’s works this way for class Int, but not for standart int.

Answer 1

Has this code undefined behavior?

Yes. The order in which the operands are evaluated, with respect to the side effect, is undefined.

Section 6.5(2) of the standard says:

If a side effect on a scalar object is unsequenced relative to either a different side effect on the same scalar object or a value computation using the value of the same scalar
object, the behavior is undefined. If there are multiple allowable orderings of the
subexpressions of an expression, the behavior is undefined if such an unsequenced side
effect occurs in any of the orderings.

Since int is a scalar type, and since the side effect here is unsequenced, the behavior is undefined.

You should write your code like this:

U = 2*C;
C++;