What does typedef enum syntax like '1 << 0' mean? [duplicate]

Question

I'm somewhat familiar with the typedef enum syntax of C and C++. I'm now programming in Objective-C and came across the syntax in the following example. I'm not sure if the syntax is Objective-C specific or not. But, my question is in the following code snippet, what does syntax like 1 << 0 mean?

typedef enum {
   CMAttitudeReferenceFrameXArbitraryZVertical = 1 << 0,
   CMAttitudeReferenceFrameXArbitraryCorrectedZVertical = 1 << 1,
   CMAttitudeReferenceFrameXMagneticNorthZVertical = 1 << 2,
   CMAttitudeReferenceFrameXTrueNorthZVertical = 1 << 3
} CMAttitudeReferenceFrame;

Answer 1

This is common to the C-family of languages, and works identically in C, C++, and Objective-C. Unlike Java, Pascal, and similar languages, a C enum is not limited to the values named for it; it actually is an integral type of a size that can represent all the named values, and one can set a variable of the enum type to an arithmetic expression in the enum members. Typically, one uses bit shifts to make the values powers of 2, and one uses bit-wise logical operations to combine values.

typedef enum {
   read    = 1 << 2,  // 4
   write   = 1 << 1,  // 2
   execute = 1 << 0   // 1
} permission;  // A miniature version of UNIX file-permission masks

Again, the bit-shift operations are all from C.

You can now write:

permission all = read | write | execute;

You could even include that line in the permission declaration itself:

typedef enum {
   read    = 1 << 2,  // 4
   write   = 1 << 1,  // 2
   execute = 1 << 0,  // 1
   all     = read | write | execute // 7
} permission;  // Version 2

How do you turn execute on for a file?

filePermission |= execute;

Note that this is dangerous:

filePermission += execute;

This will change something with value all to 8, which makes no sense.

Answer 2

<< is called the left shift operator.

http://www.techotopia.com/index.php/Objective-C_Operators_and_Expressions#Bitwise_Left_Shift

Long story short 1 << 0 = 1, 1 << 1 = 2, 1 << 2 = 4 and 1 << 3 = 8.