Could someone explain me the output of this segment of c++ code?

Question

int j=5;
cout<<++j<<j++<<j; // 757 How?
cout<<endl;
j=5;
cout<<++j; // 6 Then why
cout <<j++; // 6 not here
cout <<j; // 7 (757)?

Answer 1

To add to Immortal's answer, undefined behavior actually isn't just compiler dependant, it may result in literally any outcome. On the other hand, unspecified behavior is compiler dependant, but not enforced by the standard. In this case, the expression "cout << ++j << j++ << j;" has undefined behavior in pre C++17, and unspecified in C++17.

The reason behind this kind of mistake is a common misconception that C++ evaluates lines of code from left to right, one operator at a time. This isn't true. You can read more on this topic here: http://en.cppreference.com/w/cpp/language/eval_order

An here's a link to previous similar question, that contains some links to previous questions, and so on:

https://www.sololearn.com/Discuss/1077016/?ref=app