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 MATLAB. For example:

enum Fruit { Apple, Pear, Orange }

The MATLAB mapping for Fruit is shown below:

classdef Fruit < uint8
    enumeration
        Apple (0)
        Pear (1)
        Orange (2)
    end
    methods(Static)
        function r = ice_getValue(v)
            switch v
                case 0
                    r = Example.Fruit.Apple;
                case 1
                    r = Example.Fruit.Pear;
                case 2
                    r = Example.Fruit.Orange;
                otherwise
                    throw(Ice.MarshalException(...
                        sprintf('enumerator value %d is out of range', v)));
            end
        end
    end
end

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

f1 = Fruit.Apple;
f2 = Fruit.Orange;

if f1 == Fruit.Apple % Compare with constant
    % ...
end

if f1 == f2          % Compare two enums
    % ...
end

switch f2            % Switch on enum
    case Fruit.Apple
        % ...
    case Fruit.Pear
        % ...
    case Fruit.Orange
        % ...
end

You can obtain the ordinal value of an enumerator using the uint8 function:

val = uint8(Fruit.Pear);
assert(val == 1);

To convert an integer into its equivalent enumerator, call the ice_getValue function:

f = Fruit.ice_getValue(2);
assert(f == Fruit.Orange);

This function throws an exception if the given integer does not match any of the enumerators.

The Fruit definition above shows the ordinal values assigned by default to the enumerators. Suppose we modify the definition to include a custom enumerator value:

enum Fruit { Apple, Pear = 3, Orange }

The generated code changes accordingly:

classdef Fruit < uint8
    enumeration
        Apple (0)
        Pear (3)
        Orange (4)
    end
    ...
end

See Also

• Constants and Literals