Question

I know this is silly question but I don’t know which step I’m missing to count so can’t understand why the output is that of this code.

int i=2;
int c;
c = 2 * - ++ i << 1;
cout<< c;

I have trouble to understanding this line in this code:

c = 2 * - ++ i <<1;

I’m getting result -12. But I’m unable to get it how is precedence of operator is working here?

Answer 1

Have a look at the C++ Operator Precedence table.

1. The ++i is being evaluated, yielding 3.
2. The unary - is being evaluated, yielding -3.
3. The multiplication is being done¹, yielding -6.
4. The bit shift is evaluated (shifting left by 1 is effectively multiplying by two) yielding -12.
5. The result -12 is being assigned to the variable c.

If you used parentheses to see what operator precedence was doing, you’d get

c = ((2 * (-(++i))) << 1);

Plus that expression is a bit misleading due to the weird spacing between operators. It would be better to write it c = 2 * -++i << 1;

¹ Note that this is not the unary *, which dereferences a pointer. This is the multiplication operator, which is a binary operator.