Un-defined behavior when using hex values without Casting - c++

Question

This is the strangest problem I have ever encountered. I am programming a chess engine and I have a function that returns a rank mask. For example, Consider this 64-bit bitboard.
00000000
00000000
00000000
00000000
00000100
00000000
00000000
00000000
The 1 in the middle is a piece.
Now my function should return
00000000
00000000
00000000
00000000
11111111
00000000
00000000
00000000
Very simple, just return a mask of the rank. Here is my code
uint64_t get_rank_mask(int sq)
{
return (0xFF<<(sq-(sq%8)));
}
Here is what the function returns when I pass 36, or the square e5
00000000
00000000
00000000
00000000
00000000
00000000
00000000
11111111
Hmm, strange it is. Let me make a small change in the code.
uint64_t get_rank_mask(int sq)
{
uint64_t mask = 0xFF;
mask<<=(sq-(sq%8));
return mask;
}
Here is the return value now
00000000
00000000
00000000
11111111
00000000
00000000
00000000
00000000
And magically, that is the intended result. I don't even know the difference between the two code samples. Can anyone explain this behaviour?

Answer 1

0xFF is an int. Assuming an int has 32 bits, shifting this value left by 32 will invoke undefined behavior.
In the second version, you are storing 0xFF in an unsigned integral type. This can be shifted any number of bits, and that operation is well defined. In your case, you are storing it in an unsigned int with 64 bits, and so shifting left by 32 gives the result you want.