Question

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Could anyone explain these undefined behaviors (i = i++ + ++i , i = i++, etc…)

For the code below:

main() {

int i = 1 ;

cout << i << ++i << i++ ;

}

Why do I get the output as 331 instead of what was expected i.e 122 ?

( Same is the case even if I use printf instead of cout ?)

Answer 1

<< is a function, namely something like ostream& operator<<(ostream& lhs, RhsType rhs).

cout << a;

is equivalent to

operator<<(cout, a);

The function returns lhs, that is return lhs, – so in the above examples cout is returned.

So your example

cout << i << ++i << i++ ;

is equivalent to

operator<<(operator<<(operator<<(cout, i), ++i), i++);

Correction C++ does not specify which order the increment operations are performed. It seems logical to you and me that the most nested would go first, but as far as the compiler is concerned it is free to execute the increment whenever it likes. It is the same behaviour as a function like myFunc(cout, i++, ++i, i) where the order in which the increments are evaluated is undefined. The only thing guaranteed is the functions are evaluated inside to outside.