13.4 Enumeration Representation Clauses
Syntax
Name Resolution Rules
The
enumeration_aggregate
shall be written as a one-dimensional
array_aggregate,
for which the index subtype is the unconstrained subtype of the enumeration
type, and each component expression is expected to be of any integer
type.
Legality Rules
Each component of the
array_aggregate
shall be given by an
expression
rather than a <>. The
expressions
given in the
array_aggregate
shall be static, and shall specify distinct integer codes for each value
of the enumeration type; the associated integer codes shall satisfy the
predefined ordering relation of the type.
Static Semantics
An
enumeration_representation_clause
specifies the
coding aspect of representation.
The
coding consists of the
internal code for each enumeration literal,
that is, the integral value used internally to represent each literal.
Implementation Requirements
For nonboolean enumeration types, if the coding is
not specified for the type, then for each value of the type, the internal
code shall be equal to its position number.
Implementation Advice
An implementation should support at least the internal
codes in the range System.Min_Int..System.Max_Int. An implementation
need not support
enumeration_representation_clauses
for boolean types.
10 Unchecked_Conversion may be used to
query the internal codes used for an enumeration type. The attributes
of the type, such as Succ, Pred, and Pos, are unaffected by the
enumeration_representation_clause.
For example, Pos always returns the position number,
not the internal
integer code that might have been specified in an
enumeration_representation_clause}.
Examples
Example of an enumeration
representation clause:
type Mix_Code is (ADD, SUB, MUL, LDA, STA, STZ);
for Mix_Code use
(ADD => 1, SUB => 2, MUL => 3, LDA => 8, STA => 24, STZ =>33);
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