Confusion with C ~ operator (bitwise Not) and comparing char variables

Question

Using \"ordinary C\", I wish to compare two 8 bit bytes to determine if the second is the bitwise complement of the first. For example if Byte1 is binary 00001111 (15 in de

Answer 1

The ~ operator causes its operands to be promoted to int before being complemented. ~15 is not 240 but some other value, depending on the size of int.

Just use if (X + Y == 255) and it should work.

Answer 2

Because integral promotion causes the math on the right side to be done as int. If you assigned the result back to a char like unsigned char Z = ~X those upper bits would be truncated off again and Y == Z.