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<H1> 12.4 Formal Objects</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> [<A NAME="I4236"></A> <A NAME="I4237"></A>A generic
formal object can be used to pass a value or variable to a generic unit.]
</DIV>
<H4 ALIGN=CENTER>Language Design Principles</H4>
<DIV Class="Paranum"><FONT SIZE=-2>1.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>A generic formal object of mode
<B>in</B> is like a constant initialized to the value of the <FONT FACE="Arial, Helvetica">explicit_generic_actual_parameter</FONT>.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>1.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>A generic formal object of mode
<B>in out</B> is like a renaming of the <FONT FACE="Arial, Helvetica">explicit_generic_actual_parameter</FONT>.
</FONT></DIV>
<H4 ALIGN=CENTER>Syntax</H4>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">formal_object_declaration<A NAME="I4238"></A>
::= </FONT><BR>
<A NAME="I4239"></A><FONT FACE="Arial, Helvetica">defining_identifier_list</FONT> : <A NAME="I4240"></A><FONT FACE="Arial, Helvetica">mode</FONT> <A NAME="I4241"></A><FONT FACE="Arial, Helvetica">subtype_mark</FONT> [:= <A NAME="I4242"></A><FONT FACE="Arial, Helvetica">default_expression</FONT>];</DIV>
<H4 ALIGN=CENTER>Name Resolution Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="Normal"> <A NAME="I4243"></A>The expected type for the
<FONT FACE="Arial, Helvetica">default_expression</FONT>, if any, of a
formal object is the type of the formal object.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="Normal"> <A NAME="I4244"></A>For a generic formal object
of mode <B>in</B>, the expected type for the actual is the type of the
formal.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="Normal"> For a generic formal object of mode <B>in out</B>,
the type of the actual shall resolve to the type of the formal. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>See the corresponding
rule for <FONT FACE="Arial, Helvetica">object_renaming_declaration</FONT>s
for a discussion of the reason for this rule. </FONT></DIV>
<H4 ALIGN=CENTER>Legality Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="Normal"> If a generic formal object has a <FONT FACE="Arial, Helvetica">default_expression</FONT>,
then the mode shall be <B>in</B> [(either explicitly or by default)];
otherwise, its mode shall be either <B>in</B> or <B>in out</B>. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>Mode <B>out</B>
is not allowed for generic formal objects. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="Normal"> For a generic formal object of mode <B>in</B>,
the actual shall be an <FONT FACE="Arial, Helvetica">expression</FONT>.
For a generic formal object of mode <B>in out</B>, the actual shall be
a <FONT FACE="Arial, Helvetica">name</FONT> that denotes a variable for
which renaming is allowed (see <A HREF="AA-8-5-1.html">8.5.1</A>). </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>7.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>To be honest: </B>The part
of this that requires an <FONT FACE="Arial, Helvetica">expression</FONT>
or <FONT FACE="Arial, Helvetica">name</FONT> is a Name Resolution Rule,
but that's too pedantic to worry about. (The part about denoting a variable,
and renaming being allowed, is most certainly <I>not</I> a Name Resolution
Rule.) </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<DIV Class="Normal"> The type of a generic formal object of mode <B>in</B>
shall be nonlimited. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>Since a generic
formal object is like a constant of mode <B>in</B> initialized to the
value of the actual, a limited type would not make sense, since initializing
a constant is not allowed for a limited type. That is, generic formal
objects of mode <B>in</B> are passed by copy, and limited types are not
supposed to be copied. </FONT></DIV>
<H4 ALIGN=CENTER>Static Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<DIV Class="Normal"> A <FONT FACE="Arial, Helvetica">formal_object_declaration</FONT>
declares a generic formal object. The default mode is <B>in</B>. <A NAME="I4245"></A>For
a formal object of mode <B>in</B>, the nominal subtype is the one denoted
by the <FONT FACE="Arial, Helvetica">subtype_mark</FONT> in the declaration
of the formal. <A NAME="I4246"></A>For a formal object of mode <B>in
out</B>, its type is determined by the <FONT FACE="Arial, Helvetica">subtype_mark</FONT>
in the declaration; its nominal subtype is nonstatic, even if the <FONT FACE="Arial, Helvetica">subtype_mark</FONT>
denotes a static subtype.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<DIV Class="Normal"> <A NAME="I4247"></A>In an instance, a <FONT FACE="Arial, Helvetica">formal_object_declaration</FONT>
of mode <B>in</B> declares a new stand-alone constant object whose initialization
expression is the actual, whereas a <FONT FACE="Arial, Helvetica">formal_object_declaration</FONT>
of mode <B>in out</B> declares a view whose properties are identical
to those of the actual. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>These rules
imply that generic formal objects of mode <B>in</B> are passed by copy,
whereas generic formal objects of mode <B>in out</B> are passed by reference.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>Initialization and finalization
happen for the constant declared by a <FONT FACE="Arial, Helvetica">formal_object_declaration</FONT>
of mode <B>in</B> as for any constant; see <A HREF="AA-3-3-1.html">3.3.1</A>,
``<A HREF="AA-3-3-1.html">Object Declarations</A>'' and <A HREF="AA-7-6.html">7.6</A>,
``<A HREF="AA-7-6.html">User-Defined Assignment and Finalization</A>''.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10.c</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><A NAME="I4248"></A>In an instance,
the subtype of a generic formal object of mode <B>in</B> is as for the
equivalent constant. In an instance, the subtype of a generic formal
object of mode <B>in out</B> is the subtype of the corresponding generic
actual. </FONT></DIV>
<H4 ALIGN=CENTER>Dynamic Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>11</FONT></DIV>
<DIV Class="Normal"> <A NAME="I4249"></A><A NAME="I4250"></A>For the
evaluation of a <FONT FACE="Arial, Helvetica">generic_association</FONT>
for a formal object of mode <B>in</B>, a constant object is created,
the value of the actual parameter is converted to the nominal subtype
of the formal object, and assigned to the object[, including any value
adjustment -- see <A HREF="AA-7-6.html">7.6</A>]. <A NAME="I4251"></A></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>This includes
evaluating the actual and doing a subtype conversion, which might raise
an exception. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Discussion: </B>The rule for
evaluating a <FONT FACE="Arial, Helvetica">generic_association</FONT>
for a formal object of mode <B>in out</B> is covered by the general Dynamic
Semantics rule in <A HREF="AA-12-3.html">12.3</A>. </FONT></DIV>
<DIV Class="NotesHeader"><FONT SIZE=-1>NOTES</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>6 The constraints that apply
to a generic formal object of mode <B>in out</B> are those of the corresponding
generic actual parameter (not those implied by the <FONT FACE="Arial, Helvetica">subtype_mark</FONT>
that appears in the <FONT FACE="Arial, Helvetica">formal_object_declaration</FONT>).
Therefore, to avoid confusion, it is recommended that the name of a first
subtype be used for the declaration of such a formal object. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>Constraint
checks are done at instantiation time for formal objects of mode <B>in</B>,
but not for formal objects of mode <B>in out</B>. </FONT></DIV>
<H4 ALIGN=CENTER>Extensions to Ada 83</H4>
<DIV Class="Paranum"><FONT SIZE=-2>12.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><A NAME="I4252"></A>In Ada 83,
it is forbidden to pass a (nongeneric) formal parameter of mode <B>out</B>,
or a subcomponent thereof, to a generic formal object of mode <B>in out</B>.
This restriction is removed in Ada 95. </FONT></DIV>
<H4 ALIGN=CENTER>Wording Changes from Ada 83</H4>
<DIV Class="Paranum"><FONT SIZE=-2>12.c</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>We make ``<FONT FACE="Arial, Helvetica">mode</FONT>''
explicit in the syntax. RM83 refers to the mode without saying what it
is. This is also more uniform with the way (nongeneric) formal parameters
are defined.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12.d</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>We considered allowing mode <B>out</B>
in Ada 95, for uniformity with (nongeneric) formal parameters. The semantics
would be identical for modes <B>in out</B> and <B>out</B>. (Note that
generic formal objects of mode <B>in out</B> are passed by reference.
Note that for (nongeneric) formal parameters that are allowed to be passed
by reference, the semantics of <B>in out</B> and <B>out</B> is the same.
The difference might serve as documentation. The same would be true for
generic formal objects, if <B>out</B> were allowed, so it would be consistent.)
We decided not to make this change, because it does not produce any important
benefit, and any change has some cost. </FONT></DIV>
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