How does C++ do bitwise “or” operations on negative numbers?

Question

When I give to a variable such value: e = 17|-15; , I get -15 as an answer after compiling.I can\'t understand what arithmetic c++ uses. How does it perform a

Answer 1

The operator | is a "bitwise OR" operator, meaning that every bit in the target is computed as the OR-combination of the corresponding bits in the two operands. This means, that a bit in the result is 1 if any of the two bits in the numbers at the same positions are 1, otherwise 0.

Clearly, the result depends on the binary representation of the numbers which again depends on the platform.

Almost all platforms use the Two's complement, which can be thought as a circle of unsigned numbers, in which negative numbers are just in the opposite direction than positive numbers and "wrap around" the circle.

Unsigned integers:

Signed integers:

The calculation of your example is as follows.

 17:   00000000 00000000 00000000 00010001
-15:   11111111 11111111 11111111 11110001
------------------------------------------
-15:   11111111 11111111 11111111 11110001