Reading bits from the bin value - c++

Question

What is the order one has to read the bits from the bin value? Having for e.g. this official MS doc site regarding the lParam of the WM_CHAR message, they explain what bits have what meaning. Taking the bits 16-23 for the scan code value should I read the bits from right to left or vice versa?

Answer 1

The page you linked to uses LSB 0 bit numbering so you can extract bits 16-23 with
lParam & 0b00000000111111110000000000000000U
// | | | |
// bit 31 23 16 0
// MSB LSB
Note: The 0b prefix for binary numbers requires C++14. In C it's only available as an extension in some implementations.
You may also want to shift down the result with
(lParam & 0b00000000111111110000000000000000U) >> 16U
or simpler
(lParam >> 16U) & 0b11111111U // or (lParam >> 16U) & 0xFFU

Answer 2

two ways:
UINT bitsxtractBits(UINT val, int startBit, int nbits)
{
UINT mask = ((UINT)1 << nbits) - 1;
return (val & mask) >> startBit;
}
//note it will not work if you want to extract all bits (in this case 32).
//but in this case you do not need to extract them :)
and the usage to extract your bits:
bitsxtractBits(message, 16, 8)
or
union WM_CHAR_message
{
struct
{
UINT repatCount : 16;
UINT scanCode : 8;
UINT : 4;
UINT contextCode : 1;
UINT previousState : 1;
UINT transitionState : 1;
};
UINT raw;
};
LRESULT CALLBACK WndProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
{
union WM_CHAR_message msgu;
//C++ safe
memcpy(&msgu, &message, sizeof(msgu)); // will be optimized to the store instruction only
switch (message)
{
// ...
case WM_CHAR:
switch(msgu.scanCode)
{
//....
}
OnKeyPress(wParam);
break;
default:
return DefWindowProc(hwnd, message, wParam, lParam);
}
return 0;
}