Why use the Bitwise-Shift operator for values in a C enum definition?

Maybe writing the values in hexadecimal (or binary) helps :-)

enum {  kCGDisplayBeginConfigurationFlag  = (1 << 0), /* 0b0000000000000001 */  kCGDisplayMovedFlag               = (1 << 1), /* 0b0000000000000010 */  kCGDisplaySetMainFlag             = (1 << 2), /* 0b0000000000000100 */  kCGDisplaySetModeFlag             = (1 << 3), /* 0b0000000000001000 */  kCGDisplayAddFlag                 = (1 << 4), /* 0b0000000000010000 */  kCGDisplayRemoveFlag              = (1 << 5), /* 0b0000000000100000 */  kCGDisplayEnabledFlag             = (1 << 8), /* 0b0000000100000000 */  kCGDisplayDisabledFlag            = (1 << 9), /* 0b0000001000000000 */  kCGDisplayMirrorFlag              = (1 << 10),/* 0b0000010000000000 */  kCGDisplayUnMirrorFlag            = (1 << 11),/* 0b0000100000000000 */  kCGDisplayDesktopShapeChangedFlag = (1 << 12) /* 0b0001000000000000 */};

Now you can add them (or "or" them) and get different values

kCGDisplayAddFlag | kCGDisplayDisabledFlag /* 0b0000001000010000 */

This way you can add multiple flags together to create a "set" of flags and can then use & to find out whether any given flag is in such a set.

You couldn't do that if it simply used incrementing numbers.

Example:

int flags = kCGDisplayMovedFlag | kCGDisplaySetMainFlag; // 6if(flags & kCGDisplayMovedFlag) {} // trueif(flags & kCGDisplaySetModeFlag) {} // not true

New in C# 7 is finally adding binary literals, so you can just write it as this:

enum MyEnum{    kCGDisplayBeginConfigurationFlag  = 0b0000000000000001;    kCGDisplayMovedFlag               = 0b0000000000000010;    kCGDisplaySetMainFlag             = 0b0000000000000100;    kCGDisplaySetModeFlag             = 0b0000000000001000;    kCGDisplayAddFlag                 = 0b0000000000010000;    kCGDisplayRemoveFlag              = 0b0000000000100000;    kCGDisplayEnabledFlag             = 0b0000000001000000;    kCGDisplayDisabledFlag            = 0b0000000010000000;    kCGDisplayMirrorFlag              = 0b0000000100000000;    kCGDisplayUnMirrorFlag            = 0b0000001000000000;    kCGDisplayDesktopShapeChangedFlag = 0b0000010000000000;};

And if you want to make things even neater, you use this: _ which is also new to C# 7, which allows you to put spaces in numbers to make things more readable, like so:

enum MyEnum{    kCGDisplayBeginConfigurationFlag  = 0b_0000_0000_0000_0001;    kCGDisplayMovedFlag               = 0b_0000_0000_0000_0010;    kCGDisplaySetMainFlag             = 0b_0000_0000_0000_0100;    kCGDisplaySetModeFlag             = 0b_0000_0000_0000_1000;    kCGDisplayAddFlag                 = 0b_0000_0000_0001_0000;    kCGDisplayRemoveFlag              = 0b_0000_0000_0010_0000;    kCGDisplayEnabledFlag             = 0b_0000_0000_0100_0000;    kCGDisplayDisabledFlag            = 0b_0000_0000_1000_0000;    kCGDisplayMirrorFlag              = 0b_0000_0001_0000_0000;    kCGDisplayUnMirrorFlag            = 0b_0000_0010_0000_0000;    kCGDisplayDesktopShapeChangedFlag = 0b_0000_0100_0000_0000;};

Makes it so much easier to keep track of the numbers.