Multiple postfix increment operator in an expressi

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Could anyone explain these undefined behaviors (i = i++ + ++i , i = i++, etc…)

I'm having my lecturer class right now and my lecturer mention an expression as follows:

int a , b;
a = 4;
b = a++ + a--;

The Question

My lecturer says this expression value can be defined: that it is 8. Which means both a value 4 is summed and assign to b before it's incremented and decremented.

But to me, I think this expression answer is kinda vague and the result will be based on the compiler implementation. That's because to me, the compiler might do the a++ part first—that is, use the value 4 and incremented the a to 5, after that the expression is 4 + 5 = 9 and is assigned to b then only the a is decremented.

It might also do the a-- part first by using the value 4 and decremented it by 1 to 3, and later on sum the value 4 with 3 and assign the value to b then only incremented the a back to 4.

My question is who's correct, my lecturer or me? Because I don't wanna get confused with it and later on will be a problem for me.

Answer 1

Your lecturer is wrong. Writing to the same variable twice without an intervening sequence point is undefined behaviour. From the spec, J.2 Undefined behaviour:

Between two sequence points, an object is modified more than once, or is modified and the prior value is read other than to determine the value to be stored (6.5).

The reference is to 6.5 Expressions, paragraph 5:

Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored.

Annex C of the C spec has a handy list of all of the sequence points:

The following are the sequence points described in 5.1.2.3:

• The call to a function, after the arguments have been evaluated (6.5.2.2).
• The end of the first operand of the following operators: logical AND && (6.5.13); logical OR || (6.5.14); conditional ? (6.5.15); comma , (6.5.17).
• The end of a full declarator: declarators (6.7.5);
• The end of a full expression: an initializer (6.7.8); the expression in an expression statement (6.8.3); the controlling expression of a selection statement (if or switch) (6.8.4); the controlling expression of a while or do statement (6.8.5); each of the expressions of a for statement (6.8.5.3); the expression in a return statement (6.8.6.4).
• Immediately before a library function returns (7.1.4).
• After the actions associated with each formatted input/output function conversion specifier (7.19.6, 7.24.2).
• Immediately before and immediately after each call to a comparison function, and also between any call to a comparison function and any movement of the objects passed as arguments to that call (7.20.5).