Apple regularly use bit masks in the frameworks for allowing multiple values to be stored in a single variable (e.g. autoresizing masks):

view.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;

or passing options in a method parameter:

[NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingMutableLeaves | NSJSONReadingMutableContainers error:&error];

Using a | (bitwise OR) for combining values is easy to remember, but there are a few other bitwise operators that always slip my mind. Fortunately, I found this great post by Vipan Singla that describes them and gives examples of each. It’s quite long so here’s a summary.

| (Bitwise OR) is used for setting or turning on a flag.

view.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;
// or to add to existing flags
view.autoresizingMask |= UIViewAutoresizingFlexibleWidth;

& (Bitwise AND) is used to check the state of a flag.

if (view.autoresizingMask & UIViewAutoresizingFlexibleWidth == UIViewAutoresizingFlexibleWidth) {
    // UIViewAutoresizingFlexibleWidth is enabled
}

^ (Bitwise XOR) is used to toggle a flag in a mask (turn in on if it was off, or off if it was on).

view.autoresizingMask = view.autoresizingMask ^ UIViewAutoresizingFlexibleWidth;
// or
view.autoresizingMask ^= UIViewAutoresizingFlexibleWidth

~ (Bitwise NOT) is used for unsetting or turning off a flag (when combined with &).

view.autoresizingMask = view.autoresizingMask & ~UIViewAutoresizingFlexibleWidth;
// or
view.autoresizingMask &= ~UIViewAutoresizingFlexibleWidth

And finally, here’s how to define a bit mask flags:

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

// or using the NS_OPTIONS macro

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