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<H1> 8.5.1 Object Renaming Declarations</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> [An <FONT FACE="Arial, Helvetica">object_renaming_declaration</FONT>
is used to rename an object.] </DIV>
<H4 ALIGN=CENTER>Syntax</H4>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">object_renaming_declaration<A NAME="I3401"></A>
::= </FONT><A NAME="I3402"></A><FONT FACE="Arial, Helvetica">defining_identifier</FONT> : <A NAME="I3403"></A><FONT FACE="Arial, Helvetica">subtype_mark</FONT> <B>renames</B> <I>object_</I><A NAME="I3404"></A><FONT FACE="Arial, Helvetica">name</FONT>;</DIV>
<H4 ALIGN=CENTER>Name Resolution Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="Normal"> The type of the <I>object_</I><FONT FACE="Arial, Helvetica">name</FONT>
shall resolve to the type determined by the <FONT FACE="Arial, Helvetica">subtype_mark</FONT>.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3.a</FONT></DIV>
<DIV Class="Annotations" Style="margin-bottom: 0.4em"><FONT SIZE=-1><B>Reason:
</B>A previous version of Ada 9X used the usual ``expected type'' wording:<BR>
``The expected type for the <I>object_</I><FONT FACE="Arial, Helvetica">name</FONT>
is that determined by the <FONT FACE="Arial, Helvetica">subtype_mark</FONT>.''<BR>
We changed it so that this would be illegal: </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3.b</FONT></DIV>
<DIV Class="SmallExamples"><TT>X: T;<BR>
Y: T'Class <B>renames</B> X; --<I> Illegal!</I></TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3.c</FONT></DIV>
<DIV Class="Annotations" Style="margin-bottom: 0.4em"><FONT SIZE=-1>When
the above was legal, it was unclear whether Y was of type T or T'Class.
Note that we still allow this: </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3.d</FONT></DIV>
<DIV Class="SmallExamples"><TT>Z: T'Class := ...;<BR>
W: T <B>renames</B> F(Z);</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3.e</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>where F is a function with a controlling
parameter and result. This is admittedly a bit odd.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3.f</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>Note that the matching rule for
generic formal parameters of mode <B>in out</B> was changed to keep it
consistent with the rule for renaming. That makes the rule different
for <B>in</B> vs. <B>in out</B>.</FONT></DIV>
<H4 ALIGN=CENTER>Legality Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="Normal"> The renamed entity shall be an object.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5/1</FONT></DIV>
<DIV Class="Normal"> {<I><A HREF="defect1.html#8652/0017">8652/0017</A></I>}
The renamed entity shall not be a subcomponent that depends on discriminants
of a variable whose nominal subtype is unconstrained, unless this subtype
is indefinite, or the variable is aliased. A <FONT FACE="Arial, Helvetica">slice</FONT>
of an array shall not be renamed if this restriction disallows renaming
of the array. <U>In addition to the places where Legality Rules normally
apply, these rules apply also in the private part of an instance of a
generic unit. These rules also apply for a renaming that appears in the
body of a generic unit, with the additional requirement that even if
the nominal subtype of the variable is indefinite, its type shall not
be a descendant of an untagged generic formal derived type.</U> </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>This prevents renaming
of subcomponents that might disappear, which might leave dangling references.
Similar restrictions exist for the Access attribute.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5.a.1/1</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>{<I><A HREF="defect1.html#8652/0017">8652/0017</A></I>}
<U>The "recheck on instantiation" and "assume-the-worst
in the body" restrictions on generics are necessary to avoid renaming
of components which could disappear even when the nominal subtype would
prevent the problem:</U></FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5.a.2/1</FONT></DIV>
<DIV Class="SmallExamples" Style="margin-bottom: 0.4em"><TT><U><B>type</B> T1 (D1 : Boolean) <B>is</B><BR>
<B>record</B><BR>
<B>case</B> D1 <B>is</B><BR>
<B>when</B> False =><BR>
C1 : Integer;<BR>
<B>when</B> True =><BR>
<B>null</B>;<BR>
<B>end</B> <B>case</B>;<BR>
<B>end</B> <B>record</B>;</U></TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5.a.3/1</FONT></DIV>
<DIV Class="SmallExamples"><TT><U><B>generic</B><BR>
<B>type</B> F <B>is</B> <B>new</B> T1;<BR>
X : <B>in out</B> F;<BR>
<B>package</B> G <B>is</B><BR>
C1_Ren : Integer <B>renames</B> X.C1;<BR>
<B>end</B> G;</U></TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5.a.4/1</FONT></DIV>
<DIV Class="SmallExamples"><TT><U><B>type</B> T2 (D2 : Boolean := True) <B>is</B> <B>new</B> T1 (D1 => D2);<BR>
<BR>
Y : T2;<BR>
<BR>
<B>package</B> I <B>is new</B> G (T2, Y);<BR>
<BR>
Y := (D1 => True); -- Oops! What happened to I.C1_Ren?</U></TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Implementation Note: </B>Note
that if an implementation chooses to deallocate-then-reallocate on <FONT FACE="Arial, Helvetica">assignment_statement</FONT>s
assigning to unconstrained definite objects, then it cannot represent
renamings and access values as simple addresses, because the above rule
does not apply to all components of such an object. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5.c</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>If it is
a generic formal object, then the assume-the-best or assume-the-worst
rules are applied as appropriate. </FONT></DIV>
<H4 ALIGN=CENTER>Static Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="Normal"> An <FONT FACE="Arial, Helvetica">object_renaming_declaration</FONT>
declares a new view [of the renamed object] whose properties are identical
to those of the renamed view. [Thus, the properties of the renamed object
are not affected by the <FONT FACE="Arial, Helvetica">renaming_declaration</FONT>.
In particular, its value and whether or not it is a constant are unaffected;
similarly, the constraints that apply to an object are not affected by
renaming (any constraint implied by the <FONT FACE="Arial, Helvetica">subtype_mark</FONT>
of the <FONT FACE="Arial, Helvetica">object_renaming_declaration</FONT>
is ignored).] </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Discussion: </B>Because the
constraints are ignored, it is a good idea to use the nominal subtype
of the renamed object when writing an <FONT FACE="Arial, Helvetica">object_renaming_declaration</FONT>.
</FONT></DIV>
<H4 ALIGN=CENTER>Examples</H4>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> <I>Example of renaming
an object:</I> </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<DIV Class="Examples"><TT><B>declare</B><BR>
L : Person <B>renames</B> Leftmost_Person; --<I> see <A HREF="AA-3-10-1.html">3.10.1</A></I><BR>
<B>begin</B><BR>
L.Age := L.Age + 1;<BR>
<B>end</B>;</TT></DIV>
<H4 ALIGN=CENTER>Wording Changes from Ada 83</H4>
<DIV Class="Paranum"><FONT SIZE=-2>8.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>The phrase ``subtype ... as defined
in a corresponding object declaration, component declaration, or component
subtype indication,'' from RM83-8.5(5), is incorrect in Ada 95; therefore
we removed it. It is incorrect in the case of an object with an indefinite
unconstrained nominal subtype. </FONT></DIV>
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