在C，C ++中由位移运算符（<<，>>）进行位移的位

Question

can we access the bits shifted by bit shifting operators(<<, >>) in C, C++? For example: 23>>1 can we access the last bit shifted(1 in this case)?

Answer 1

No, the shift operators only give the value after shifting. You'll need to do other bitwise operations to extract the bits that are shifted out of the value; for example:

unsigned all_lost  = value & ((1 << shift)-1);  // all bits to be removed by shift
unsigned last_lost = (value >> (shift-1)) & 1;  // last bit to be removed by shift
unsigned remaining = value >> shift;            // lose those bits

Answer 2

By using 23>>1 , the bit 0x01 is purged - you have no way of retrieving it after the bit shift.

That said, nothing's stopping you from checking for the bit before shifting:

int  value   = 23;
bool bit1    = value & 0x01;
int  shifted = value >> 1;

Answer 3

You can access the bits before shifting, eg

value  = 23;          // start with some value
lsbits = value & 1;   // extract the LSB
value >>= 1;          // shift

Answer 4

It worth signal that on MSVC compiler an intrinsic function exists: _bittest

that speeds up the operation.