ROUNDUP? what does it do? in C++

Can someone explai开发者_如何转开发n to me what this does?

#define ROUNDUP(n,width) (((n) + (width) - 1) & ~unsigned((width) - 1))

Providing width is an even power of 2 (so 2,4,8,16,32 etc), it will return a number equal to or greater than n, which is a multiple of width, and which is the smallest value meeting that criteria.

So width = 16; 5->16, 7->16, 15->16, 16->16, 17->32, 18->32 etc.

EDIT I started out on providing an explanation of why this works as it does, as I sense that's really what the OP wants, but it turned into a rather convoluted story. If the OP is still confused, I'd suggest working through a few simple examples, say width = 16, n=15,16,17. Remember that & = bitwise AND, ~ = bitwise complement, and to use binary representation exclusively as you work through the examples.

It rounds n up to the next 'width' - but I think width needs to be a power of 2.

For example width == 8, n = 5:

(5 + 8 - 1) & ~(7) = 12 & ~7 = 8

So 5 rounds to 8. Anything 1 - 8 rounds to 8. 9 to 16 rounds to 16. Etc. (0 rounds to 0)

It defines a macro called ROUNDUP which takes two parameters, n and width, and returns the value (n + width - 1) & ~unsigned(width - 1).

:)

Try this if you think you know what it does:

std::string s("WTF");
std::complex<double> c(-11,5);
ROUNDUP(s, c);

It won't work in C because of the unsigned. Here is what is does, as long as width is confined to powers of 2:

 n width ROUNDUP(n,width)
----------------
 0   4    0
 1   4    4
 2   4    4
 3   4    4
 4   4    4
 5   4    8
 6   4    8
 7   4    8
 8   4    8
 9   4    12
10   4    12
11   4    12
12   4    12
13   4    16
14   4    16
15   4    16
16   4    16
17   4    20
18   4    20
19   4    20