Question

Hi another silly simple question. I have noticed that in certain typedefs in Apple’s frameworks use the symbols "<<" can anyone tell me what that means?:

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

Edit: Alright so I now understand how and why you would use the left bit-shift, my next question is how would I test to see if the the value had a certain trait using and if/then statement or a switch/case method?

Answer 1

This is a way to create constants that would be easy to mix. For example you can have an API to order an ice cream and you can choose any of vanilla, chocolate and strawberry flavours. You could use booleans, but that’s a bit heavy:

- (void) iceCreamWithVanilla: (BOOL) v chocolate: (BOOL) ch strawerry: (BOOL) st;

A nice trick to solve this is using numbers, where you can mix the flavours using simple adding. Let’s say 1 for vanilla, 2 for chocolate and 4 for strawberry:

- (void) iceCreamWithFlavours: (NSUInteger) flavours;

Now if the number has its rightmost bit set, it’s got vanilla flavour in it, another bit stands for chocolate and the third bit from right is strawberry. For example vanilla + chocolate would be 1+2=3 decimal (011 in binary).

The bitshift operator x << y takes the left number (x) and shifts its bits y times. It’s a good tool to create numeric constants:

1 << 0 = 001 // vanilla
1 << 1 = 010 // chocolate
1 << 2 = 100 // strawberry

Voila! Now when you want a view with flexible left margin and flexible right margin, you can mix the flags using bitwise or: FlexibleRightMargin | FlexibleLeftMargin → 1<<2 | 1<<0 → 100 | 001 → 101. On the receiving end the method can mask the interesting bit using logical and:

// 101 & 100 = 100 or 4 decimal, which boolifies as YES
BOOL flexiRight = givenNumber & FlexibleRightMargin;

Hope that helps.