what does x+=++x mean??

Question

int x=9;
x+=++x;
Cout<<x;

//outputs 20

How did this happen? I know that x++=x+1 but what does ++x mean??

Answer 1

Or it may make demons fly out of your nose.

http://en.cppreference.com/w/cpp/language/eval_order

https://en.wikipedia.org/wiki/Sequence_point

https://en.wikipedia.org/wiki/Undefined_behavior

https://en.wikichip.org/wiki/nasal_demons