I'd like to work with 12 bits unsigned integer. Since I am working with array, it is of interest for me to have overflowing value, e.g., 0 - 1 = 4095.
I tried the following but I don't obtain the expected behaviour:
struct bit_field
{
unsigned int x: 12; // 12 bits
};
bit_field ii, jj, kk;
ii.x = 4096;
jj.x = 1;
kk.x = 0;
cout << ii.x;
cout << kk.x - jj.x;
Output:
>> 0 // ov as expected
>> -1 // expected 4095
This is how C/C++ is expected to work; you don't get arbitarily sized integers. your storage width declaration within the struct doesn't change that: the type your operators see is still unsigned int. It's just that you say "when I store this, it's 12 bits".
Because kk.x and kk.x are unsigned integers, their subtraction works just as defined for these: their subtraction is promoting values to signed integers.
Note that you're writing C++, so you can perfectly well write your own class that implements the mathematical operations you want and has cast operators for integer types.