Testing whether specific bits are set in a DWORD flag

Question

I have a DWORD variable & I want to test if specific bits are set in it. I have my code below but I am not sure if I am transferring the bits from the win32 data type KBDLLHOOKSTRUCT to my lparam datatype correctly?

See MSDN that documents the DWORD flag variable: http://msdn.microsoft.com/en-us/library/ms644967(v=vs.85).aspx

union KeyState 
{
    LPARAM lparam;      

    struct     
    {         
        unsigned nRepeatCount : 16;         
        unsigned nScanCode    : 8;         
        unsigned nExtended    : 1;         
        unsigned nReserved    : 4;         
        unsigned nContext     : 1;         
        unsigned nPrev        : 1;         
        unsigned nTrans       : 1;     
    }; 
}; 


KBDLLHOOKSTRUCT keyInfo = *((KBDLLHOOKSTRUCT*)lParam);
KeyState myParam;
myParam.nRepeatCount    = 1;
myParam.nScanCode       = keyInfo.scanCode;
myParam.nExtended       = keyInfo.flags && LLKHF_EXTENDED; // maybe it should be keyInfo.flags & LLKHF_EXTENDED or keyInfo.flags >> LLKHF_EXTENDED
myParam.nReserved       = 0;        
myParam.nContext        = keyInfo.flags && LLKHF_ALTDOWN;     
myParam.nPrev           = 0; // can store the last key pressed as virtual key/code, then check against this one, if its the same then set this to 1 else do 0   
myParam.nTrans          = keyInfo.flags && LLKHF_UP;


// Or maybe I shd do this to transfer bits...
myParam.nRepeatCount    = 1;
myParam.nScanCode       = keyInfo.scanCode;
myParam.nExtended       = keyInfo.flags & 0x01;
myParam.nReserved       = (keyInfo.flags >> 0x01) & (1<<3)-1;      
myParam.nContext        = keyInfo.flags & 0x05;     
myParam.nPrev           = 0;       // can store the last key pressed as virtual key/code, then check against this one, if its the same then set this to 1 else do 0   
myParam.nTrans          = keyInfo.flags & 0x07;

Answer 1

CheckBits(DWORD var, DWORD mask)
{
    DWORD setbits=var&mask; //Find all bits are set
    DWORD diffbits=setbits^mask; //Find all set bits that differ from mask
    return diffbits!=0; //Retrun True if all specific bits in variable are set
}

Answer 2

If you want to merge two bits, you would use | ( bitwise OR ) operator:

myParam.nExtended = keyInfo.flags | LLKHF_EXTENDED;

myParam.nExtended = keyInfo.flags | 0x01;

To check if bit was set, you would use & ( bitwise AND ) operator:

if(myParam.nExtended & LLKHF_EXTENDED)...

Answer 3

Rather than

myParam.nExtended = keyInfo.flags && LLKHF_EXTENDED

you need

myParam.nExtended = (keyInfo.flags & LLKHF_EXTENDED) != 0;

It's & not && because you want a bitwise and not a logical and . And the !=0 ensures the answer is either 0 or 1 (rather than 0 or some-other-nonzero-value) so it can be represented in your one-bit bitfield.