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<H1> 7.2 Package Bodies</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> [In contrast to the entities declared in the visible
part of a package, the entities declared in the <FONT FACE="Arial, Helvetica">package_body</FONT>
are visible only within the <FONT FACE="Arial, Helvetica">package_body</FONT>
itself. As a consequence, a package with a <FONT FACE="Arial, Helvetica">package_body</FONT>
can be used for the construction of a group of related subprograms in
which the logical operations available to clients are clearly isolated
from the internal entities.] </DIV>
<H4 ALIGN=CENTER>Syntax</H4>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">package_body<A NAME="I3205"></A>
::= </FONT><BR>
<B>package</B> <B>body</B> <A NAME="I3206"></A><FONT FACE="Arial, Helvetica">defining_program_unit_name</FONT> <B>is</B><BR>
<A NAME="I3207"></A><FONT FACE="Arial, Helvetica">declarative_part</FONT><BR>
[<B>begin</B><BR>
<A NAME="I3208"></A><FONT FACE="Arial, Helvetica">handled_sequence_of_statements</FONT>]<BR>
<B>end</B> [[<A NAME="I3209"></A><FONT FACE="Arial, Helvetica">parent_unit_name</FONT>.]<A NAME="I3210"></A><FONT FACE="Arial, Helvetica">identifier</FONT>];</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="SyntaxIndented">If an <FONT FACE="Arial, Helvetica">identifier</FONT>
or <FONT FACE="Arial, Helvetica">parent_unit_name</FONT>.<FONT FACE="Arial, Helvetica">identifier</FONT>
appears at the end of a <FONT FACE="Arial, Helvetica">package_body</FONT>,
then this sequence of lexical elements shall repeat the <FONT FACE="Arial, Helvetica">defining_program_unit_name</FONT>.
</DIV>
<H4 ALIGN=CENTER>Legality Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="Normal"> A <FONT FACE="Arial, Helvetica">package_body</FONT>
shall be the completion of a previous <FONT FACE="Arial, Helvetica">package_declaration</FONT>
or <FONT FACE="Arial, Helvetica">generic_package_declaration</FONT>.
A library <FONT FACE="Arial, Helvetica">package_declaration</FONT> or
library <FONT FACE="Arial, Helvetica">generic_package_declaration</FONT>
shall not have a body unless it requires a body[; <B>pragma</B> Elaborate_Body
can be used to require a <FONT FACE="Arial, Helvetica">library_unit_declaration</FONT>
to have a body (see <A HREF="AA-10-2-1.html">10.2.1</A>) if it would
not otherwise require one]. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>The first
part of the rule forbids a <FONT FACE="Arial, Helvetica">package_body</FONT>
from standing alone -- it has to belong to some previous <FONT FACE="Arial, Helvetica">package_declaration</FONT>
or <FONT FACE="Arial, Helvetica">generic_package_declaration</FONT>.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>A nonlibrary <FONT FACE="Arial, Helvetica">package_declaration</FONT>
or nonlibrary <FONT FACE="Arial, Helvetica">generic_package_declaration</FONT>
that does not require a completion may have a corresponding body anyway.
</FONT></DIV>
<H4 ALIGN=CENTER>Static Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="Normal"> In any <FONT FACE="Arial, Helvetica">package_body</FONT>
without <FONT FACE="Arial, Helvetica">statement</FONT>s there is an implicit
<FONT FACE="Arial, Helvetica">null_statement</FONT>. For any <FONT FACE="Arial, Helvetica">package_declaration</FONT>
without an explicit completion, there is an implicit <FONT FACE="Arial, Helvetica">package_body</FONT>
containing a single <FONT FACE="Arial, Helvetica">null_statement</FONT>.
For a noninstance, nonlibrary package, this body occurs at the end of
the <FONT FACE="Arial, Helvetica">declarative_part</FONT> of the innermost
enclosing program unit or <FONT FACE="Arial, Helvetica">block_statement</FONT>;
if there are several such packages, the order of the implicit <FONT FACE="Arial, Helvetica">package_bodies</FONT>
is unspecified. <A NAME="I3211"></A>[(For an instance, the implicit <FONT FACE="Arial, Helvetica">package_body</FONT>
occurs at the place of the instantiation (see <A HREF="AA-12-3.html">12.3</A>).
For a library package, the place is partially determined by the elaboration
dependences (see Section 10).)] </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Discussion: </B>Thus, for example,
we can refer to something happening just after the <B>begin</B> of a
<FONT FACE="Arial, Helvetica">package_body</FONT>, and we can refer to
the <FONT FACE="Arial, Helvetica">handled_sequence_of_statements</FONT>
of a <FONT FACE="Arial, Helvetica">package_body</FONT>, without worrying
about all the optional pieces. The place of the implicit body makes a
difference for tasks activated by the package. See also RM83-9.3(5).</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>The implicit body would be illegal
if explicit in the case of a library package that does not require (and
therefore does not allow) a body. This is a bit strange, but not harmful.
</FONT></DIV>
<H4 ALIGN=CENTER>Dynamic Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="Normal"> <A NAME="I3212"></A>For the elaboration of a nongeneric
<FONT FACE="Arial, Helvetica">package_body</FONT>, its <FONT FACE="Arial, Helvetica">declarative_part</FONT>
is first elaborated, and its <FONT FACE="Arial, Helvetica">handled_sequence_of_statements</FONT>
is then executed. </DIV>
<DIV Class="NotesHeader"><FONT SIZE=-1>NOTES</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>3 A variable declared in
the body of a package is only visible within this body and, consequently,
its value can only be changed within the <FONT FACE="Arial, Helvetica">package_body</FONT>.
In the absence of local tasks, the value of such a variable remains unchanged
between calls issued from outside the package to subprograms declared
in the visible part. The properties of such a variable are similar to
those of a ``static'' variable of C.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>4 The elaboration of the
body of a subprogram explicitly declared in the visible part of a package
is caused by the elaboration of the body of the package. Hence a call
of such a subprogram by an outside program unit raises the exception
Program_Error if the call takes place before the elaboration of the <FONT FACE="Arial, Helvetica">package_body</FONT>
(see <A HREF="AA-3-11.html">3.11</A>). </FONT></DIV>
<H4 ALIGN=CENTER>Examples</H4>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> <I>Example of a
package body (see <A HREF="AA-7-1.html">7.1</A>):</I> </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<DIV Class="Examples"><TT><B>package</B> <B>body</B> Rational_Numbers <B>is</B></TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11</FONT></DIV>
<DIV Class="Examples"><TT> <B>procedure</B> Same_Denominator (X,Y : <B>in</B> <B>out</B> Rational) <B>is</B><BR>
<B>begin</B><BR>
--<I> reduces X and Y to the same denominator:</I><BR>
...<BR>
<B>end</B> Same_Denominator;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12</FONT></DIV>
<DIV Class="Examples"><TT> <B>function</B> "="(X,Y : Rational) <B>return</B> Boolean <B>is</B><BR>
U : Rational := X;<BR>
V : Rational := Y;<BR>
<B>begin</B><BR>
Same_Denominator (U,V);<BR>
<B>return</B> U.Numerator = V.Numerator;<BR>
<B>end</B> "=";</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>13</FONT></DIV>
<DIV Class="Examples"><TT> <B>function</B> "/" (X,Y : Integer) <B>return</B> Rational <B>is</B><BR>
<B>begin</B><BR>
<B>if</B> Y > 0 <B>then</B><BR>
<B>return</B> (Numerator => X, Denominator => Y);<BR>
<B>else</B><BR>
<B>return</B> (Numerator => -X, Denominator => -Y);<BR>
<B>end</B> <B>if</B>;<BR>
<B>end</B> "/";</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>14</FONT></DIV>
<DIV Class="Examples"><TT> <B>function</B> "+" (X,Y : Rational) <B>return</B> Rational <B>is</B> ... <B>end</B> "+";<BR>
<B>function</B> "-" (X,Y : Rational) <B>return</B> Rational <B>is</B> ... <B>end</B> "-";<BR>
<B>function</B> "*" (X,Y : Rational) <B>return</B> Rational <B>is</B> ... <B>end</B> "*";<BR>
<B>function</B> "/" (X,Y : Rational) <B>return</B> Rational <B>is</B> ... <B>end</B> "/";</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>15</FONT></DIV>
<DIV Class="Examples"><TT><B>end</B> Rational_Numbers;</TT></DIV>
<H4 ALIGN=CENTER>Wording Changes from Ada 83</H4>
<DIV Class="Paranum"><FONT SIZE=-2>15.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>The syntax rule for <FONT FACE="Arial, Helvetica">package_body</FONT>
now uses the syntactic category <FONT FACE="Arial, Helvetica">handled_sequence_of_statements</FONT>.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>15.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>The <FONT FACE="Arial, Helvetica">declarative_part</FONT>
of a <FONT FACE="Arial, Helvetica">package_body</FONT> is now required;
that doesn't make any real difference, since a <FONT FACE="Arial, Helvetica">declarative_part</FONT>
can be empty.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>15.c</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>RM83 seems to have forgotten to
say that a <FONT FACE="Arial, Helvetica">package_body</FONT> can't stand
alone, without a previous declaration. We state that rule here.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>15.d</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>RM83 forgot to restrict the definition
of elaboration of <FONT FACE="Arial, Helvetica">package_bodies</FONT>
to nongeneric ones. We have corrected that omission.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>15.e</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>The rule about implicit bodies
(from RM83-9.3(5)) is moved here, since it is more generally applicable.
</FONT></DIV>
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