I'm having trouble understanding why the output is 4... Can anyone explain? To my understanding,
this means i = i + i + 1, so i=3...

i += ++i
The increment comes first so:
First increment i by one i = 2
i = i + i = 2 + 2 = 4

This is an awful thing to do btw. Why are you doing this???

It's a sample exam to prepare for the final...Thanks for your answer!

The behavior of this program is undefined (in other words, it's an error of the same kind as accessing an array out of bounds or dereferencing a null pointer). There is no explanation.

See http://en.cppreference.com/w/cpp/language/eval_order or any other material on expression evaluation rules in C++ (or C, same thing here)

PS: actually, I believe it is well-defined under C++11's slightly stricter rules.. but it's still a very bad idea.

I get the output of 3 on IBM xlC, and 4 from GNU g++

like mats said, it's the same as


and because it's prefix, it's done first, hence both terms on the right are 2.

If i saw anything like this in any of the code i'm working on i'd find the developer and set them on fire.

i = (++i) + i;

Does not look like

i+=++i;

I understand for this specific code... but do you have any tips if the code changes slightly? How to I go from the initial code to your simplified code?

Basically, if I see ++i in a compound statement, I place it before the other things?