D Programming Language
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# Enums

    EnumDeclaration:
        enum Identifier EnumBody
        enum Identifier : EnumBaseType EnumBody
        AnonymousEnumDeclaration

    EnumBaseType:
        Type

    EnumBody:
        { EnumMembers }
        ;

    EnumMembers:
        EnumMember
        EnumMember ,
        EnumMember , EnumMembers

    EnumMember:
        Identifier
        Identifier = AssignExpression

    AnonymousEnumDeclaration:
        enum : EnumBaseType { EnumMembers }
        enum { EnumMembers }
        enum { AnonymousEnumMembers }

    AnonymousEnumMembers:
        AnonymousEnumMember
        AnonymousEnumMember ,
        AnonymousEnumMember , AnonymousEnumMembers

    AnonymousEnumMember:
        EnumMember
        Type Identifier = AssignExpression

Enum declarations are used to define a group of constants. They come in these forms:

1. Named enums, which have a name.
2. Anonymous enums, which do not have a name.
3. Manifest constants.

## Named Enums

Named enums are used to declare related constants and group them by giving them a unique type. The EnumMembers are declared in the scope of the named enum. The named enum declares a new type, and all the EnumMembers have that type.

This defines a new type X which has values X.A=0, X.B=1, X.C=2:

    enum X { A, B, C }  // named enum

If the EnumBaseType is not explicitly set, and the first EnumMember has an AssignExpression, it is set to the type of that AssignExpression. Otherwise, it defaults to type int.

Named enum members may not have individual Types.

A named enum member can be implicitly cast to its EnumBaseType, but EnumBaseType types cannot be implicitly cast to an enum type.

The value of an EnumMember is given by its AssignExpression. If there is no AssignExpression and it is the first EnumMember, its value is EnumBaseType.init.

If there is no AssignExpression and it is not the first EnumMember, it is given the value of the previous EnumMember+1. If the value of the previous EnumMember is EnumBaseType.max, it is an error. If the value of the previous EnumMember+1 is the same as the value of the previous EnumMember, it is an error. (This can happen with floating point types.)

All EnumMembers are in scope for the AssignExpressions.

    enum A = 3;
    enum B
    {
        A = A // error, circular reference
    }
    enum C
    {
        A = B,  // A = 4
        B = D,  // B = 4
        C = 3,  // C = 3
        D       // D = 4
    }
    enum E : C
    {
        E1 = C.D,
        E2      // error, C.D is C.max
    }

An empty enum body (For example enum E;) signifies an opaque enum - the enum members are unknown.

### Enum Default Initializer

The .init property of an enum type is the value of the first member of that enum. This is also the default initializer for the enum type.

    enum X { A=3, B, C }
    X x;      // x is initialized to 3

### Enum Properties

Enum properties only exist for named enums.

Named Enum Properties
.init | First enum member value
.min | Smallest value of enum
.max | Largest value of enum
.sizeof | Size of storage for an enumerated value

For example:

    enum X { A=3, B, C }
    X.min    // is X.A
    X.max    // is X.C
    X.sizeof // is same as int.sizeof

The EnumBaseType of named enums must support comparison in order to compute the .max and .min properties.

## Anonymous Enums

If the enum Identifier is not present, then the enum is an anonymous enum, and the EnumMembers are declared in the scope the EnumDeclaration appears in. No new type is created.

The EnumMembers can have different types. Those types are given by the first of:

1. The Type, if present. Types are not permitted when an EnumBaseType is present.
2. The EnumBaseType, if present.
3. The type of the AssignExpression, if present.
4. The type of the previous EnumMember, if present.
5. int

    enum { A, B, C }  // anonymous enum

Defines the constants A=0, B=1, C=2, all of type int.

Enums must have at least one member.

The value of an EnumMember is given by its AssignExpression. If there is no AssignExpression and it is the first EnumMember, its value is the .init property of the EnumMember's type.

If there is no AssignExpression and it is not the first EnumMember, it is given the value of the previous EnumMember+1. If the value of the previous EnumMember is the .max property if the previous EnumMember's type, it is an error. If the value of the previous EnumMember+1 is the same as the value of the previous EnumMember, it is an error. (This can happen with floating point types.)

All EnumMembers are in scope for the AssignExpressions.

    enum { A, B = 5+7, C, D = 8+C, E }

Sets A=0, B=12, C=13, D=21, and E=22, all of type int.

    enum : long { A = 3, B }

Sets A=3, B=4 all of type long.

    enum : string
    {
        A = "hello",
        B = "betty",
        C     // error, cannot add 1 to "betty"
    }

    enum
    {
        A = 1.2f,  // A is 1.2f of type float
        B,         // B is 2.2f of type float
        int C = 3, // C is 3 of type int
        D          // D is 4 of type int
    }

## Manifest Constants

If there is only one member of an anonymous enum, the { } can be omitted. Gramatically speaking, this is an AutoDeclaration.

    enum i = 4;      // i is 4 of type int
    enum long l = 3; // l is 3 of type long

Such declarations are not lvalues, meaning their address cannot be taken. They exist only in the memory of the compiler.

    enum size = __traits(classInstanceSize, Foo);  // evaluated at compile-time

Using manifest constants is an idiomatic D method to force compile-time evaluation of an expression.

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