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<H1> 9.4 Protected Units and Protected Objects</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> <A NAME="I3527"></A><A NAME="I3528"></A><A NAME="I3529"></A><A NAME="I3530"></A>A
<I>protected object</I> provides coordinated access to shared data, through
calls on its visible <I>protected operations</I>, which can be <I>protected
subprograms</I> or <I>protected entries</I>. <A NAME="I3531"></A><A NAME="I3532"></A><A NAME="I3533"></A>A
<I>protected unit</I> is declared by a <I>protected declaration</I>,
which has a corresponding <FONT FACE="Arial, Helvetica">protected_body</FONT>.
A protected declaration may be a <FONT FACE="Arial, Helvetica">protected_type_declaration</FONT>,
in which case it declares a named protected type; alternatively, it may
be a <FONT FACE="Arial, Helvetica">single_protected_declaration</FONT>,
in which case it defines an anonymous protected type, as well as declaring
a named protected object of that type. <A NAME="I3534"></A></DIV>
<H4 ALIGN=CENTER>Syntax</H4>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">protected_type_declaration<A NAME="I3535"></A>
::= </FONT><BR>
<B>protected</B> <B>type</B> <A NAME="I3536"></A><FONT FACE="Arial, Helvetica">defining_identifier</FONT> [<A NAME="I3537"></A><FONT FACE="Arial, Helvetica">known_discriminant_part</FONT>] <B>is</B> <A NAME="I3538"></A><FONT FACE="Arial, Helvetica">protected_definition</FONT>;</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">single_protected_declaration<A NAME="I3539"></A>
::= </FONT><BR>
<B>protected</B> <A NAME="I3540"></A><FONT FACE="Arial, Helvetica">defining_identifier</FONT> <B>is</B> <A NAME="I3541"></A><FONT FACE="Arial, Helvetica">protected_definition</FONT>;</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">protected_definition<A NAME="I3542"></A>
::= </FONT><BR>
{ <A NAME="I3543"></A><FONT FACE="Arial, Helvetica">protected_operation_declaration</FONT> }<BR>
[ <B>private</B><BR>
{ <A NAME="I3544"></A><FONT FACE="Arial, Helvetica">protected_element_declaration</FONT> } ]<BR>
<B>end</B> [<I>protected_</I><A NAME="I3545"></A><FONT FACE="Arial, Helvetica">identifier</FONT>]</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5/1</FONT></DIV>
<DIV Class="SyntaxIndented">{<I><A HREF="defect1.html#8652/0009">8652/0009</A></I>}
<FONT FACE="Arial, Helvetica">protected_operation_declaration<A NAME="I3546"></A>
::= </FONT><A NAME="I3547"></A><FONT FACE="Arial, Helvetica">subprogram_declaration</FONT><BR>
| <A NAME="I3548"></A><FONT FACE="Arial, Helvetica">entry_declaration</FONT><BR>
| <U><A NAME="I3549"></A><FONT FACE="Arial, Helvetica">aspect_clause</FONT></U><S><A NAME="I3550"></A><FONT FACE="Arial, Helvetica">representation_clause</FONT></S></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">protected_element_declaration<A NAME="I3551"></A>
::= </FONT><A NAME="I3552"></A><FONT FACE="Arial, Helvetica">protected_operation_declaration</FONT><BR>
| <A NAME="I3553"></A><FONT FACE="Arial, Helvetica">component_declaration</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>We allow the operations
and components to be mixed because that's how other things work (for
example, package declarations). We have relaxed the ordering rules for
the items inside <FONT FACE="Arial, Helvetica">declarative_part</FONT>s
and <FONT FACE="Arial, Helvetica">task_definition</FONT>s as well. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">protected_body<A NAME="I3554"></A>
::= </FONT><BR>
<B>protected</B> <B>body</B> <A NAME="I3555"></A><FONT FACE="Arial, Helvetica">defining_identifier</FONT> <B>is</B><BR>
{ <A NAME="I3556"></A><FONT FACE="Arial, Helvetica">protected_operation_item</FONT> }<BR>
<B>end</B> [<I>protected_</I><A NAME="I3557"></A><FONT FACE="Arial, Helvetica">identifier</FONT>];</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8/1</FONT></DIV>
<DIV Class="SyntaxIndented">{<I><A HREF="defect1.html#8652/0009">8652/0009</A></I>}
<FONT FACE="Arial, Helvetica">protected_operation_item<A NAME="I3558"></A>
::= </FONT><A NAME="I3559"></A><FONT FACE="Arial, Helvetica">subprogram_declaration</FONT><BR>
| <A NAME="I3560"></A><FONT FACE="Arial, Helvetica">subprogram_body</FONT><BR>
| <A NAME="I3561"></A><FONT FACE="Arial, Helvetica">entry_body</FONT><BR>
| <U><A NAME="I3562"></A><FONT FACE="Arial, Helvetica">aspect_clause</FONT></U><S><A NAME="I3563"></A><FONT FACE="Arial, Helvetica">representation_clause</FONT></S></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<DIV Class="SyntaxIndented">If a <I>protected_</I><FONT FACE="Arial, Helvetica">identifier</FONT>
appears at the end of a <FONT FACE="Arial, Helvetica">protected_definition</FONT>
or <FONT FACE="Arial, Helvetica">protected_body</FONT>, it shall repeat
the <FONT FACE="Arial, Helvetica">defining_identifier</FONT>. </DIV>
<H4 ALIGN=CENTER>Legality Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<DIV Class="Normal"> <A NAME="I3564"></A>A protected declaration requires
a completion[, which shall be a <FONT FACE="Arial, Helvetica">protected_body</FONT>,]
and every <FONT FACE="Arial, Helvetica">protected_body</FONT> shall be
the completion of some protected declaration. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>To be honest: </B>The completion
can be a <FONT FACE="Arial, Helvetica">pragma</FONT> Import, if the implementation
supports it. </FONT></DIV>
<H4 ALIGN=CENTER>Static Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>11</FONT></DIV>
<DIV Class="Normal"> A <FONT FACE="Arial, Helvetica">protected_definition</FONT>
defines a protected type and its first subtype. <A NAME="I3565"></A>The
list of <FONT FACE="Arial, Helvetica">protected_operation_declaration</FONT>s
of a <FONT FACE="Arial, Helvetica">protected_definition</FONT>, together
with the <FONT FACE="Arial, Helvetica">known_discriminant_part</FONT>,
if any, is called the visible part of the protected unit. [<A NAME="I3566"></A>
The optional list of <FONT FACE="Arial, Helvetica">protected_element_declaration</FONT>s
after the reserved word <B>private</B> is called the private part of
the protected unit.] </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Proof: </B>Private part is
defined in Section 8. </FONT></DIV>
<H4 ALIGN=CENTER>Dynamic Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>12</FONT></DIV>
<DIV Class="Normal"> [<A NAME="I3567"></A> The elaboration of a protected
declaration elaborates the <FONT FACE="Arial, Helvetica">protected_definition</FONT>.
<A NAME="I3568"></A>The elaboration of a <FONT FACE="Arial, Helvetica">single_protected_declaration</FONT>
also creates an object of an (anonymous) protected type.] </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Proof: </B>This is redundant
with the general rules for the elaboration of a <FONT FACE="Arial, Helvetica">full_type_declaration</FONT>
and an <FONT FACE="Arial, Helvetica">object_declaration</FONT>. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>13</FONT></DIV>
<DIV Class="Normal"> <A NAME="I3569"></A>[The elaboration of a <FONT FACE="Arial, Helvetica">protected_definition</FONT>
creates the protected type and its first subtype;] it also includes the
elaboration of the <FONT FACE="Arial, Helvetica">component_declaration</FONT>s
and <FONT FACE="Arial, Helvetica">protected_operation_declaration</FONT>s
in the given order.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>14</FONT></DIV>
<DIV Class="Normal"> [<A NAME="I3570"></A> As part of the initialization
of a protected object, any per-object constraints (see <A HREF="AA-3-8.html">3.8</A>)
are elaborated.] </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>14.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Discussion: </B>We do not mention
pragmas since each pragma has its own elaboration rules. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>15</FONT></DIV>
<DIV Class="Normal"> <A NAME="I3571"></A>The elaboration of a <FONT FACE="Arial, Helvetica">protected_body</FONT>
has no other effect than to establish that protected operations of the
type can from then on be called without failing the Elaboration_Check.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>16</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> The content of
an object of a given protected type includes: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>17</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>The values of the components of the protected object, including
(implicitly) an entry queue for each entry declared for the protected
object; </LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>17.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>"For
each entry" implies one queue for each single entry, plus one for
each entry of each entry family. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>18</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC><A NAME="I3572"></A>A representation of the state of the
execution resource <I>associated</I> with the protected object (one such
resource is associated with each protected object). </LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>19</FONT></DIV>
<DIV Class="Normal"> [The execution resource associated with a protected
object has to be acquired to read or update any components of the protected
object; it can be acquired (as part of a protected action -- see <A HREF="AA-9-5-1.html">9.5.1</A>)
either for concurrent read-only access, or for exclusive read-write access.]</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>20</FONT></DIV>
<DIV Class="Normal"> <A NAME="I3573"></A><A NAME="I3574"></A>As the
first step of the <I>finalization</I> of a protected object, each call
remaining on any entry queue of the object is removed from its queue
and Program_Error is raised at the place of the corresponding <FONT FACE="Arial, Helvetica">entry_call_statement</FONT>.
</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>20.a</FONT></DIV>
<DIV Class="Annotations" Style="margin-bottom: 0.4em"><FONT SIZE=-1><B>Reason:
</B>This is analogous to the raising of Tasking_Error in callers of a
task that completes before accepting the calls. This situation can only
occur due to a requeue (ignoring premature unchecked_deallocation), since
any task that has accessibility to a protected object is awaited before
finalizing the protected object. For example: </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>20.b</FONT></DIV>
<DIV Class="SmallExamples"><TT><B>procedure</B> Main <B>is</B><BR>
<B>task</B> T <B>is</B><BR>
<B>entry</B> E;<BR>
<B>end</B> T;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>20.c</FONT></DIV>
<DIV Class="SmallExamples"><TT> <B>task</B> <B>body</B> T <B>is</B><BR>
<B>protected</B> PO <B>is</B><BR>
<B>entry</B> Ee;<BR>
<B>end</B> PO;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>20.d</FONT></DIV>
<DIV Class="SmallExamples"><TT> <B>protected</B> <B>body</B> PO <B>is</B><BR>
<B>entry</B> Ee <B>when</B> False <B>is</B><BR>
<B>begin</B><BR>
<B>null</B>;<BR>
<B>end</B> Ee;<BR>
<B>end</B> PO;<BR>
<B>begin</B><BR>
<B>accept</B> E <B>do</B><BR>
<B>requeue</B> PO.Ee;<BR>
<B>end</B> E;<BR>
<B>end</B> T;<BR>
<B>begin</B><BR>
T.E;<BR>
<B>end</B> Main;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>20.e</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>The environment task is queued
on PO.EE when PO is finalized.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>20.f</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>In a real example, a server task
might park callers on a local protected object for some useful purpose,
so we didn't want to disallow this case. </FONT></DIV>
<DIV Class="NotesHeader"><FONT SIZE=-1>NOTES</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>21</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>13 Within the declaration
or body of a protected unit, the name of the protected unit denotes the
current instance of the unit (see <A HREF="AA-8-6.html">8.6</A>), rather
than the first subtype of the corresponding protected type (and thus
the name cannot be used as a <FONT FACE="Arial, Helvetica">subtype_mark</FONT>).
</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>21.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Discussion: </B>However, it
is possible to refer to some other subtype of the protected type within
its body, presuming such a subtype has been declared between the <FONT FACE="Arial, Helvetica">protected_type_declaration</FONT>
and the <FONT FACE="Arial, Helvetica">protected_body</FONT>. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>22</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>14 A <FONT FACE="Arial, Helvetica">selected_component</FONT>
can be used to denote a discriminant of a protected object (see <A HREF="AA-4-1-3.html">4.1.3</A>).
Within a protected unit, the name of a discriminant of the protected
type denotes the corresponding discriminant of the current instance of
the unit.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>23</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>15 A protected type is a
limited type (see <A HREF="AA-7-5.html">7.5</A>), and hence has neither
an assignment operation nor predefined equality operators.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>24</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>16 The bodies of the protected
operations given in the <FONT FACE="Arial, Helvetica">protected_body</FONT>
define the actions that take place upon calls to the protected operations.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>25</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>17 The declarations in the
private part are only visible within the private part and the body of
the protected unit. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>25.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B><FONT FACE="Arial, Helvetica">Component_declarations</FONT>
are disallowed in a <FONT FACE="Arial, Helvetica">protected_body</FONT>
because, for efficiency, we wish to allow the compiler to determine the
size of protected objects (when not dynamic); the compiler cannot necessarily
see the body. Furthermore, the semantics of initialization of such objects
would be problematic -- we do not wish to give protected objects complex
initialization semantics similar to task activation.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>25.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>The same applies to <FONT FACE="Arial, Helvetica">entry_declaration</FONT>s,
since an entry involves an implicit component -- the entry queue. </FONT></DIV>
<H4 ALIGN=CENTER>Examples</H4>
<DIV Class="Paranum"><FONT SIZE=-2>26</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> <I>Example of declaration
of protected type and corresponding body:</I> </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>27</FONT></DIV>
<DIV Class="Examples"><TT><B>protected</B> <B>type</B> Resource <B>is</B><BR>
<B>entry</B> Seize;<BR>
<B>procedure</B> Release;<BR>
<B>private</B><BR>
Busy : Boolean := False;<BR>
<B>end</B> Resource;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>28</FONT></DIV>
<DIV Class="Examples"><TT><B>protected</B> <B>body</B> Resource <B>is</B><BR>
<B>entry</B> Seize <B>when not</B> Busy <B>is</B><BR>
<B>begin</B><BR>
Busy := True;<BR>
<B>end</B> Seize;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>29</FONT></DIV>
<DIV Class="Examples"><TT> <B>procedure</B> Release <B>is</B><BR>
<B>begin</B><BR>
Busy := False;<BR>
<B>end</B> Release;<BR>
<B>end</B> Resource;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>30</FONT></DIV>
<DIV Class="Wide" Style="margin-bottom: 0.4em"> <I>Example of a single
protected declaration and corresponding body:</I> </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>31</FONT></DIV>
<DIV Class="Examples"><TT><B>protected</B> Shared_Array <B>is</B><BR>
<I>-- Index, Item, and Item_Array are global types</I><BR>
<B>function</B> Component (N : <B>in</B> Index) return Item;<BR>
<B>procedure</B> Set_Component(N : <B>in</B> Index; E : <B>in</B> Item);<BR>
<B>private</B><BR>
Table : Item_Array(Index) := (others => Null_Item);<BR>
<B>end</B> Shared_Array;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>32</FONT></DIV>
<DIV Class="Examples"><TT><B>protected</B> <B>body</B> Shared_Array <B>is</B><BR>
<B>function</B> Component(N : <B>in</B> Index) <B>return</B> Item <B>is</B><BR>
<B>begin</B><BR>
<B>return</B> Table(N);<BR>
<B>end</B> Component;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>33</FONT></DIV>
<DIV Class="Examples"><TT> <B>procedure</B> Set_Component(N : <B>in</B> Index; E : <B>in</B> Item) <B>is</B><BR>
<B>begin</B><BR>
Table(N) := E;<BR>
<B>end</B> Set_Component;<BR>
<B>end</B> Shared_Array;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>34</FONT></DIV>
<DIV Class="Wide" Style="margin-bottom: 0.4em"> <I>Examples of protected
objects:</I> </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>35</FONT></DIV>
<DIV Class="Examples"><TT>Control : Resource;<BR>
Flags : <B>array</B>(1 .. 100) <B>of</B> Resource;</TT></DIV>
<H4 ALIGN=CENTER>Extensions to Ada 83</H4>
<DIV Class="Paranum"><FONT SIZE=-2>35.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><A NAME="I3575"></A>This entire
clause is new; protected units do not exist in Ada 83. </FONT></DIV>
<HR>
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