Is condition evaluation optimized ? Is this code bad?

Question

1.Imagine condition if (obj.is_x() || obj.is_y() || obj.is_z())
Will obj.is_y() and obj.is_z() be called and evaluated if obj.is_x() returned true ?

2.Is this a bad idea(in general)? Does this code look bad ?

bool isbn13_prefix_valid (const string& prefix)
{
    unsigned num = stoi(prefix);
    if (num == 978 || num == 979) return 1;  //super common ones
        else if (   num >= 0 && num <= 5 || num == 7 || num >= 600 && num <= 649
                || num >= 80 && num <= 94 || num >= 950 && num <= 989
                || num >= 9900 && num <= 9989 || num >= 99900 && num <= 99999)
            return 1;
    return 0;
}

Answer 1

1. No, it will not, due to short-circuiting.

2. Yes, that code looks bad. Not because it's incorrect, but because you're stuffing an extremely long conditional into a single if statement. Try refactoring your code to make it cleaner.