What's the value of i++ + i++?

What's the value of i++ + i++??

It's undefined. Basically, in C and C++, if you read a variable twice in an expression where you also write it, the result is undefined. Don't do that. Another example is:

v[i] = i++;

Related example:

f(v[i], i++);

Here, the result is undefined because the order of evaluation of function arguments are undefined.