Querying the status of a bit flag?

Question

I am trying to parse some files that have a bitwise flag column. There are 11 bits in this flag and I need to find out, for each row in the files, what is the value of the 5th bit (1-based).

Answer 1

if (flags & 0x10) ....

how did I know that mask (0x10)

here are 8 bits

0b00000000

here is the fifth one starting from one (from the right)

  87654321        
0b00010000

and in hex that is

0x10

Answer 2

May be overkill for small number of flags, but I find easier to manipulate bits using std::bitset.

First, "construct" a bitset of 11 bits from the flags.

std::bitset< 11 > flags_bitset( flags );

Then, "test" the fifth bit

if( flags_bitset.test( 4 ) {  // 4, because indexing is 0 based.
    /* something */
}

See: http://www.cplusplus.com/reference/stl/bitset/test/

For doing by hand, try

const uint32_t mask = 1U << 4;  // '1' in fifth bit and rest '0' 
if( flag & mask ) { .. }