Enumerations

Enumeration Syntax and Semantics

A Slice enumerated type definition looks identical to C++:

module M
{
    enum Fruit { Apple, Pear, Orange }
}

This definition introduces a type named Fruit that becomes a new type in its own right. Slice guarantees that the values of enumerators increase from left to right, so Apple compares less than Pear in every language mapping. By default, the first enumerator has a value of zero, with sequentially increasing values for subsequent enumerators.

A Slice enum type introduces a new namespace scope, so the following is legal:

module M
{
    enum Fruit { Apple, Pear, Orange }
    enum ComputerBrands { Apple, Dell, HP, Lenovo }
}

The example below shows how to refer to an enumerator from a different scope:

module M
{
    enum Color { Red, Green, Blue }
}

module N
{
    struct Pixel
    {
        M::Color c = Blue;
    }
}

Slice does not permit empty enumerations.

In Ice releases prior to Ice 3.7, an enum type did not create a new namespace and its enumerators were in the same namespace as the enum type itself. With these releases, you had to select longer enumerator names to avoid a naming clash.

Custom Enumerator Values

Slice also permits you to assign custom values to enumerators:

const int PearValue = 7;
enum Fruit { Apple = 0, Pear = PearValue, Orange }

Custom values must be unique and non-negative, and may refer to Slice constants of integer types. If no custom value is specified for an enumerator, its value is one greater than the enumerator that immediately precedes it. In the example above, Orange has the value 8.

The maximum value for an enumerator value is the same as the maximum value for int, 2 ³¹ - 1.

Slice does not require custom enumerator values to be declared in increasing order:

enum Fruit { Apple = 5, Pear = 3, Orange = 1 }   // Legal

Note however that when there is an inconsistency between the declaration order and the numerical order of the enumerators, the behavior of comparison operations may vary between language mappings.

For an application that is still using version 1.0 of the Ice encoding, changing the definition of an enumerated type may break backward compatibility with existing applications. For more information, please refer to the encoding rules for enumerators.

Language Mapping

A Slice enumeration maps to the corresponding enumeration in Java. For example:

enum Fruit { Apple, Pear, Orange }

The Java mapping for Fruit is shown below:

public enum Fruit {
    Apple,
    Pear,
    Orange;

    public int value();

    public static Fruit valueOf(int v);

    // ...
}

Given the above definitions, we can use enumerated values as follows:

Fruit f1 = Fruit.Apple;
Fruit f2 = Fruit.Orange;

if (f1 == Fruit.Apple) { // Compare with constant
    // ...
}

if (f1 == f2) { // Compare two enums
    // ...
}

switch (f2) {   // Switch on enum
    case Fruit.Apple:
        // ...
        break;
    case Fruit.Pear:
        // ...
        break;
    case Fruit.Orange:
        // ...
        break;
}

The Java mapping includes two methods of interest. The value method returns the Slice value of an enumerator, which is not necessarily the same as its ordinal value. The valueOf method translates a Slice value into its corresponding enumerator, or returns null if no match is found.

In the Fruit definition above, the Slice value of each enumerator matches its ordinal value. This will not be true if we modify the definition to include a custom enumerator value:

enum Fruit { Apple, Pear = 3, Orange }

The table below shows the new relationship between ordinal value and Slice value:

Java enumerated types inherit implicitly from java.lang.Enum, which defines methods such as ordinal and compareTo that operate on the ordinal value of an enumerator, not its Slice value.

See Also

• Constants and Literals