问题
In PHP, if I run the following simple program
$number = 9;
var_dump( ~ $number );
my output is
int(-10)
This is confusing to me. I thought ~ was the bitwise NOT operator. So I was expecting something like.
if binary 9 is 00000000000000000000000000001001
then Bitwise NOT 9 11111111111111111111111111110110
Meaning ~9 should come out as some ludicrously large integer like 4,294,967,286.
What subtly of precedence, type coercion, or something else am I missing here?
回答1:
Your output is defaulting to a signed int - wrap it in decbin to get a binary representation.
Consider:
$number = 9;
var_dump( bindec(decbin(~ $number)) );
With two's compliment, the MSB of a signed binary number becomes 0-MSB, but every other bit retains its respective positive values.
So for argument's sake (an 8-bit example),
Binary 9: 0000 1001
Inverse: 1111 0110
This results in (-128) + 64 + 32 + 16 + 4 + 2 = -10, so PHP is calculating correctly, its just applying two's compliment to the MSB.
来源：https://stackoverflow.com/questions/18754198/confusing-php-bitwise-not-behavior