Chaining Bool values give opposite result to expected

Question

Unthinkingly I wrote some code to check that all the values of a struct were set to 0. To accomplish this I used:

bool IsValid() {
    return !(0 == year ==          


        

Answer 1

The behaviour shouldn't be seen as odd. The grammar rules for == (and most but not all binary operators) specify left to right grouping so your original expression is equivalent to:

!((((((0 == year) == month) == day) == hour) == minute) == second)

Note that when compared to an integer type a bool expression with value true will promote to 1 and with value false will promote to 0. (In C the result of the equality operator is an int in any case with a value or either 1 or 0.)

This means that, for example, ((0 == year) == month) will be true if year is zero and month is one or if year is non-zero but month is zero and false otherwise.