How to do Bit masking in C++?

[IkarusDowned]

So, i want to mask a certain number of bits in, say, an integer. How do i do that using another integer variable? Like, i want to mask the number of bits in int bit_mask and i was to mask the value int masked_number. I haven't found a good explaination or anything on how to do that. please help!

 

[DaveSimmons]

// use xor instead of and to clear bits instead of keep bits

int x = v & 0x00f ; // mask to only inlcude lower 4

int x = v & (1 << k) ; // mask just single bit k,, k = 0-31 for int

int x = v & ((1 << n)-1) ; // retain lower n bits, n = 1-32 NOT 0-1

 

[CTho9305]

// use xor instead of and to clear bits instead of keep bits

That's only if you know for sure the bit is already set. The easiest way is just to and with the bitwise inverse...

//clear lowest bit
int x = v & ~0x1;

//set lowest bit
int x = v | 0x1;

 

[DaveSimmons]

Originally posted by: CTho9305
That's only if you know for sure the bit is already set. The easiest way is just to and with the bitwise inverse...

d'oh! correct. AND to keep, OR to set, AND of inverse to clear.

to clear bits

int x = v & ~ (the bits to clear)

such as

int x = v & ~ (1 << k) ; // clear bit k where k = 0 - 31