13.4 Enumeration Representation Clauses
Name Resolution Rules
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
The “full coverage
rules” for aggregate
applies. An others
is not allowed — there is no applicable
index constraint in this context.
As for all type-related
representation items, the local_name
is required to denote a first subtype.
Each component of the array_aggregate
shall be given by an expression
rather than a <>.
The expressions 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.
Reason: Each value of the enumeration
type has to be given an internal code, even if the first subtype of the
enumeration type is constrained to only a subrange (this is only possible
if the enumeration type is a derived type). This “full coverage”
requirement is important because one may refer to Enum'Base'First and
Enum'Base'Last, which need to have defined representations.
specifies the coding
aspect of representation.
coding consists of the internal code
for each enumeration literal,
that is, the integral value used internally to represent each literal.
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.
Reason: This default representation is
already used by all known Ada compilers for nonboolean enumeration types.
Therefore, we make it a requirement so users can depend on it, rather
than feeling obliged to supply for every enumeration type an enumeration
representation clause that is equivalent to this default rule.
Discussion: For boolean types, it is
relatively common to use all ones for True, and all zeros for False,
since some hardware supports that directly. Of course, for a one-bit
Boolean object (like in a packed array), False is presumably zero and
True is presumably one (choosing the reverse would be extremely unfriendly!).
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_clause
for boolean types.
Ramification: The implementation may
support numbers outside the above range, such as numbers greater than
System.Max_Int. See AI83-00564.
Reason: The benefits of specifying the
internal coding of a boolean type do not outweigh the implementation
costs. Consider, for example, the implementation of the logical operators
on a packed array of booleans with strange internal codes. It's implementable,
but not worth it.
Suppose the enumeration type in question is derived:
type T1 is (Red, Green, Blue);
subtype S1 is T1 range Red .. Green;
type S2 is new S1;
for S2 use (Red => 10, Green => 20, Blue => 30);
for I in S2'Base loop
... -- When I equals Blue, the internal code is 30.
We considered allowing or requiring “for
S2'Base use ...” in cases like this, but it didn't seem
worth the trouble.
Example of an enumeration
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);
Extensions to Ada 83
As in other similar contexts,
Ada 95 allows expressions of any integer type, not just expressions of
, for the component expressions in the enumeration_aggregate
The preference rules for the predefined operators of root_integer
eliminate any ambiguity.
For portability, we now require that the default
coding for an enumeration type be the “obvious” coding using
position numbers. This is satisfied by all known implementations.
Wording Changes from Ada 95
Corrigendum: Updated to reflect that we
no longer have something called representation_clause.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe