Frequently used symbol “|” in Objective-C

Question

self.autoresizingMask = UIViewAutoresizingFlexibleWidth|UIViewAutoresizingFlexibleWidth;

The source code is shown above. Wonder what the symbol \"|

Answer 1

In short: that is a bitwise OR operation.

It is tipically used in generating bitmasks.

With this operation you can combine flags into on binary number.

For example: possible flags for UIViewAutoresizing are:

enum {
   UIViewAutoresizingNone                 = 0,           // = 0b 0000 0000 =  0
   UIViewAutoresizingFlexibleLeftMargin   = 1 << 0,      // = 0b 0000 0001 =  1
   UIViewAutoresizingFlexibleWidth        = 1 << 1,      // = 0b 0000 0010 =  2
   UIViewAutoresizingFlexibleRightMargin  = 1 << 2,      // = 0b 0000 0100 =  4
   UIViewAutoresizingFlexibleTopMargin    = 1 << 3,      // = 0b 0000 1000 =  8
   UIViewAutoresizingFlexibleHeight       = 1 << 4,      // = 0b 0001 0000 = 16
   UIViewAutoresizingFlexibleBottomMargin = 1 << 5       // = 0b 0010 0000 = 32
};
typedef NSUInteger UIViewAutoresizing;

Statement:

self.autoresizingMask = UIViewAutoresizingFlexibleWidth|UIViewAutoresizingFlexibleWidth;

is esentially the same as:

self.autoresizingMask = UIViewAutoresizingFlexibleWidth;

(since both operands are the same).

If you would ask about:

self.autoresizingMask = UIViewAutoresizingFlexibleWidth|UIViewAutoresizingFlexibleHeight;

this would set self.autoresizingMask to:

(1<<1)|(1<<4)=(0b 0000 0010)|(0b 0001 0000)=0b 0001 0010 = 9

Bitwise OR is not to be confused by Logical OR used with simple true/false algebra.

There is some relation between the two (bitwise or can be understood as a logical or between the bits on the same position) but that is about it.

Answer 2

The | character denotes an inclusive-or bitwise operation. which operates under the premise with matching the bitstrings of two objects.

if you have a bitstring 1101 and another 1001 the inclusive or of the two would produce 1011. Basically if the current bit is the same in both strings then a 1 is outputted in its place otherwise a 0 is.

Answer 3

| is the bitwise OR operator in C (and therefore in Objective-C).

See http://en.m.wikipedia.org/wiki/Bitwise_operations_in_C

In the context you asked about, it's being used to combine two flag values.

Answer 4

That is a bitwise OR operation, maybe this can help you : Bitwise operation