Effective enums use

I read a book where the author does this:

public enum Move
{ 开发者_开发问答 
    normal            = 0,
    swim              = 1 << 0,
    crawl             = 1 << 1,
    creep             = 1 << 3,
    jump              = 1 << 3,
    fly               = 1 << 4,
    grapple           = 1 << 5,
    goes_through_door = 1 << 6
}  

Why would you do that, why not just let them have default values like 0,1,2,3...

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This is a enum where the values are usable with bitwise operators. It allows for an enum value to be a combination of multiple values. For example

Move crawlAndSwim = Move.crawl | Move.swim;

Bitwise operators can then be used to check for the presence or absence of certain values

if (0 != (aMoveValue & Move.crawl)) {
  // It's crawling
}

if (0 != (aMoveValue & Move.swim)) {
  // It's swiming
}

Wikipedia has a nice article on this subject which may be of interest to you

• http://en.wikipedia.org/wiki/Bit_field

Note: As casperOne pointed out, this type of enum definition should be annotated with FlagsAttribute to denote it's a flags / bitfield vs a normal one.

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Its so that you can combine them with bit-wise operators. I'm not sure that makes total sense with Move, but it makes sense with other things.

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It has to do with masks. This way you can represent if it can crawl and swim by a=(crawl | swim). Then you can check the individuals by a&swim.

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Because you can use an enum type as flags.

That means you can combine some enum values of the definition into one variable.

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<< is a (edit)left-shift operator.

He does this so he can combine multiple values of Move using bitwise operators, though the names are a bit of a bad example.

Lets say I have

public enum VehicleType{
  Car = 1<<0,
  Van = 1<<1,
  Truck = 1<<2
}

Now I want to make a method Depot.RetrieveVehicles(VehicleType type);

Because the values are set out in this fashion, I can do

Depot.RetrieveVehicles(VehicleType.Car | VehicleType.Van); 

To retrieve vehicles that are either a Car or a Van.

So Car (001) | Van (010) gives you 011 in binary.

Just to clear it up a little, I could also do this:

public enum VehicleType{
  Car = 1, //001
  Van = 2, //010
  Truck = 4//100
}

You cannot do this easily if you had just used 0,1,2. You would have to call that method twice.