What's the difference between (size_t)-1 and ~0?

What's the difference between (size_t)-1 and ~0?

Type and value differ.

(size_t)-1 is the same value as SIZE_MAX and has a type of size_t.

~0 is often -1 and has the type of int.

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Assigning both of those to a size_t will result in SIZE_MAX.

size_t a = (size_t)-1; 
size_t b = ~0;

In the 2nd case, -1 is assigned to a b and undergoes a conversion first, wrapping around the -1 to the maximum size_t value.

(size_t)-1 is of type size_t. It typically has a value of 2³²-1 or 2⁶⁴-1 (4294967295 or 18446744073709551615).

~0 is of type int, and has the value -1 on a 2's-complement system (i.e., just about everywhere).

Both are likely to have the same bit pattern -- if int and size_t are the same size, which they very commonly are not.

If you want the maximum value of type size_t, you can use the SIZE_MAX macro, defined in <stdint.h>. If you're using an older implementation (pre-C99) that doesn't provide SIZE_MAX, (size_t)-1 will work. I'm not sure why you'd want to write ~0 rather than -1 -- unless perhaps you're considering non-two's-complement systems.

Note that the previous answers assume a 2's complement machine (very likely to be the case these days, but not guaranteed).

If you had a sign-magnitude machine then -1 would have a sign bit and least significant bit set with all others clear, if you had a 1's complement machine then -1 would have all bits but the LSB set.

In all of these cases (including the common 2's complement machine), ~0 has all bits set.