Why is the Output of `cout

Undefined Output of cout << a << a

In the code snippet:

int a = 0;
cout << a++ << a;

it is commonly assumed that the behavior is equivalent to:

cout << (a++) << a;

However, due to the lack of sequence points between function argument evaluations, the order of execution is not guaranteed. The compiler may evaluate a before or after std::operator<<<>(std::cout, a ).

Therefore, the correct interpretation is:

cout << ((a++) << a);

This means that the result is undefined, as the value of a after the increment is used in the second call to operator<<<>.

C 17 Amendment

In C 17, the rules have been modified such that:

E1 << E2

is evaluated as:

std::operator<<<>(std::operator<<<>(E1, E2), E3)

with all side effects of E1 sequenced before those of E2. This ensures that the code fragment now produces the expected output of "01".

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