x++ and ++x great confusion

[koopey]

HI guys. Can do simple statements involving x++ and ++x in multiple lines, but this expression in single cout statement is giving very weird result.

  int i=5;
    cout<<i++<<-i<<++i<<-i<<i;///output is 6-67-57 

more than others, that '5' is very weird. could anyone explain what goes on in step by step basis?

[Peter87]

You can't modify and read the same variable inside the same expression like that. The C++ standard just says this is undefined, which means anything could happen.

[coder777]

The order of evaluation is undefined:

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

So i++ / ++i / and what not may happen before any of the function calls in no specified order.

[dhayden]

It's worse than that. You can't have more than one side effect changing a variable in one expression. The results are undefined.