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

Question

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What are bitwise shift (bit-shift) operators and how do they work?
(10 answers)
Closed 9 years ago.
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.

Answer 3

It looks like the typedef is representing a bit field value. 1 << n is 1 shifted left n bits. So each enum item represents a different bit setting. That particular bit set or clear would indicate something being one of two states. 1 shifted left by zero bits is 1.
If a datum is declared:
CMAttitudeReferenceFrame foo;
Then you can check any one of four independent states using the enum values, and foo is no bigger than an int. For example:
if ( foo & CMAttitudeReferenceFrameXArbitraryCorrectedZVertical ) {
// Do something here if this state is set
}