Question

In the Cocoa and Cocoa Touch frameworks, enums are used as constant. I understand how to use it except in one case, the case you can pass as a parameter multiple value with the | operator. Like in :

pageControl.autoresizingMask = (UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleTopMargin);

The enum is declared like that:

enum {
    UIViewAutoresizingNone                 = 0,
    UIViewAutoresizingFlexibleLeftMargin   = 1 << 0,
    UIViewAutoresizingFlexibleWidth        = 1 << 1,
    UIViewAutoresizingFlexibleRightMargin  = 1 << 2,
    UIViewAutoresizingFlexibleTopMargin    = 1 << 3,
    UIViewAutoresizingFlexibleHeight       = 1 << 4,
    UIViewAutoresizingFlexibleBottomMargin = 1 << 5
};
typedef NSUInteger UIViewAutoresizing;

How can I define myself this type of enum (i.e. what << means) and how can I check for multiples values when passed as a parameter?

Answer 1

<< is the bitshift operator. So 1 << 2 tells it to shift the bit two spaces over.

Example:

In binary the number 1 is:

0001

1 << 2 means to shift all the bits to the left 2 spaces, which results in this value:

0100

or 4.

So the values of each ENUM in your example is, 1, 2, 4, 8, 16, etc. They could have just as well set each enum to those values. But since they use that enum for multiple values, the binary values makes it more clear:

0001
0010
0100
1000

so they wrote using the bit shifts.

so if I OR (|) two of those values together, for example FlexibleLeftMargin (0001) and FlexibleWidth (0010), I would get the following value:

0011

So they use each bit as a flag so they know you have multiple values set.

You can now use the AND operator & to figure out if you have a specific value set.

0010 & 0011 = 0010

So you could do this to check if you have one of enums set:

myenum = (UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleRightMargin);
if((myenum & UIViewAutoresizingFlexibleLeftMargin) == UIViewAutoresizingFlexibleLeftMargin) {
  // myenum has UIViewAutoresizingFlexibleLeftMargin set!
}

Hopefully this makes sense.
For a more thurough explanation on bitwise operations read this: Wikipedia ~ Bit Operators or search around for “bit operators“