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<H1> 13.4 Enumeration Representation Clauses</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> [An <FONT FACE="Arial, Helvetica">enumeration_representation_clause</FONT>
specifies the internal codes for enumeration literals.] </DIV>
<H4 ALIGN=CENTER>Syntax</H4>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">enumeration_representation_clause<A NAME="I4460"></A>
::= </FONT><BR>
<B>for</B> <I>first_subtype_</I><A NAME="I4461"></A><FONT FACE="Arial, Helvetica">local_name</FONT> <B>use</B> <A NAME="I4462"></A><FONT FACE="Arial, Helvetica">enumeration_aggregate</FONT>;</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">enumeration_aggregate<A NAME="I4463"></A>
::= </FONT><A NAME="I4464"></A><FONT FACE="Arial, Helvetica">array_aggregate</FONT></DIV>
<H4 ALIGN=CENTER>Name Resolution Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="Normal"> <A NAME="I4465"></A>The <FONT FACE="Arial, Helvetica">enumeration_aggregate</FONT>
shall be written as a one-dimensional <FONT FACE="Arial, Helvetica">array_aggregate</FONT>,
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. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>The ``full
coverage rules'' for <FONT FACE="Arial, Helvetica">aggregate</FONT>s
applies. An <B>others</B> is not allowed -- there is no applicable index
constraint in this context. </FONT></DIV>
<H4 ALIGN=CENTER>Legality Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="Normal"> The <I>first_subtype_</I><FONT FACE="Arial, Helvetica">local_name</FONT>
of an <FONT FACE="Arial, Helvetica">enumeration_representation_clause</FONT>
shall denote an enumeration subtype. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>As for all
type-related representation items, the <FONT FACE="Arial, Helvetica">local_name</FONT>
is required to denote a first subtype. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="Normal"> The expressions given in the <FONT FACE="Arial, Helvetica">array_aggregate</FONT>
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. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>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. </FONT></DIV>
<H4 ALIGN=CENTER>Static Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="Normal"> <A NAME="I4466"></A><A NAME="I4467"></A>An <FONT FACE="Arial, Helvetica">enumeration_representation_clause</FONT>
specifies the <I>coding</I> aspect of representation. <A NAME="I4468"></A>The
coding consists of the <I>internal code</I> for each enumeration literal,
that is, the integral value used internally to represent each literal.</DIV>
<H4 ALIGN=CENTER>Implementation Requirements</H4>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<DIV Class="Normal"> 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. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>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. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Discussion: </B>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!). </FONT></DIV>
<H4 ALIGN=CENTER>Implementation Advice</H4>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> <A NAME="I4469"></A>The
recommended level of support for <FONT FACE="Arial, Helvetica">enumeration_representation_clause</FONT>s
is: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>An implementation should support at least the internal
codes in the range System.Min_Int..System.Max_Int. An implementation
need not support <FONT FACE="Arial, Helvetica">enumeration_representation_clause</FONT>s
for boolean types. </LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>10.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>The implementation
may support numbers outside the above range, such as numbers greater
than System.Max_Int. See AI83-00564. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>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. </FONT></DIV>
<DIV Class="NotesHeader"><FONT SIZE=-1>NOTES</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11/1</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>11 {<I><A HREF="defect1.html#8652/0009">8652/0009</A></I>}
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 <FONT FACE="Arial, Helvetica"><U>enumeration_</U>representation_clause</FONT>.
For example, Pos always returns the position number, <I>not</I> the internal
integer code that might have been specified in a <FONT FACE="Arial, Helvetica"><U>enumeration_</U>representation_clause</FONT>.
</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11.a</FONT></DIV>
<DIV Class="Annotations" Style="margin-bottom: 0.4em"><FONT SIZE=-1><B>Discussion:
</B>Suppose the enumeration type in question is derived: </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11.b</FONT></DIV>
<DIV Class="SmallExamples"><TT><B>type</B> T1 <B>is</B> (Red, Green, Blue);<BR>
<B>subtype</B> S1 <B>is</B> T1 <B>range</B> Red .. Green;<BR>
<B>type</B> S2 <B>is</B> <B>new</B> S1;<BR>
<B>for</B> S2 <B>use</B> (Red => 10, Green => 20, Blue => 30);</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11.c/1</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>{<I><A HREF="defect1.html#8652/0009">8652/0009</A></I>}
The <FONT FACE="Arial, Helvetica"><U>enumeration_</U>representation_clause</FONT>
has to specify values for all enumerals, even ones that are not in S2
(such as Blue). The Base attribute can be used to get at these values.
For example: </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11.d</FONT></DIV>
<DIV Class="SmallExamples" Style="margin-bottom: 0.4em"><TT><B>for</B> I <B>in</B> S2'Base <B>loop</B><BR>
... --<I> When I equals Blue, the internal code is 30.</I><BR>
<B>end</B> <B>loop</B>;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11.e</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>We considered allowing or requiring
``<B>for</B> S2'Base <B>use</B> ...'' in cases like this, but it didn't
seem worth the trouble. </FONT></DIV>
<H4 ALIGN=CENTER>Examples</H4>
<DIV Class="Paranum"><FONT SIZE=-2>12</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> <I>Example of an
enumeration representation clause:</I> </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>13</FONT></DIV>
<DIV Class="Examples"><TT><B>type</B> Mix_Code <B>is</B> (ADD, SUB, MUL, LDA, STA, STZ);</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>14</FONT></DIV>
<DIV Class="Examples"><TT><B>for</B> Mix_Code <B>use</B><BR>
(ADD => 1, SUB => 2, MUL => 3, LDA => 8, STA => 24, STZ =>33);</TT></DIV>
<H4 ALIGN=CENTER>Extensions to Ada 83</H4>
<DIV Class="Paranum"><FONT SIZE=-2>14.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><A NAME="I4470"></A>As in other
similar contexts, Ada 95 allows expressions of any integer type, not
just expressions of type <I>universal_integer</I>, for the component
expressions in the <FONT FACE="Arial, Helvetica">enumeration_aggregate</FONT>.
The preference rules for the predefined operators of <I>root_integer</I>
eliminate any ambiguity.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>14.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>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.
</FONT></DIV>
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