Hacker News Reader

Comment by dhosek | Hacker News Reader

I don’t have gcc available so I can’t test it, but I wonder what it does with

     int a = 5;
     int b = a++;

if it gives b==5 in this circumstance (which I would say is the correct value), then it seems that giving 13 for a++ + ++a is a bug in the compiler. I kind of feel like giving 6 as an answer would also be a bug in the compiler since postfix-++ should return the old value and then increment.

View on HN

Your code:

    int a = 5;
    int b = a++;

has well defined behavior. The first line initializes a to 5. The second initializes b to 5 and sets a to 6. (The language doesn't specify the order of the two operations of assigning a value to be and incrementing a, but in this case it doesn't matter.)

Giving 13 for a++ + ++a is not a bug in the compiler. It's a bug in the code.

The correct answer to "what does a++ + ++a do" is "it gets rejected in code review and replaced with code that expresses the actual intent.

The trick here is that the original expression contains undefined behavior. Your example does not.

Reply on news.ycombinator.com