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<H1> 7.5 Limited Types</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> <A NAME="I3108"></A>A limited type is (a view
of) a type for which the assignment operation is not allowed. A nonlimited
type is a (view of a) type for which the assignment operation is allowed.
</DIV>
<H4 ALIGN=CENTER>Legality Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="Normal"> If a tagged record type has any limited components,
then the reserved word <B>limited</B> shall appear in its <FONT FACE="Arial, Helvetica">record_type_definition</FONT>.
</DIV>
<H4 ALIGN=CENTER>Static Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> <A NAME="I3109"></A>A
type is <I>limited</I> if it is a descendant of one of the following:
</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>a type with the reserved word <B>limited</B> in its definition;
</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>a task or protected type;</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>a composite type with a limited component. </LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="Normal"> <A NAME="I3110"></A>Otherwise, the type is nonlimited.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<DIV Class="Normal"> There are no predefined equality operators for
a limited type. </DIV>
<DIV Class="NotesHeader"><FONT SIZE=-1>NOTES</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<DIV Class="Notes" Style="margin-bottom: 0.4em"><FONT SIZE=-1>13 The
following are consequences of the rules for limited types: </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<UL Class="NotesBulleted"><FONT SIZE=-1><LI TYPE=DISC>An initialization expression is not allowed in an <FONT FACE="Arial, Helvetica">object_declaration</FONT>
if the type of the object is limited.</LI></FONT></UL>
<DIV Class="Paranum"><FONT SIZE=-2>11</FONT></DIV>
<UL Class="NotesBulleted"><FONT SIZE=-1><LI TYPE=DISC>A default expression is not allowed in a <FONT FACE="Arial, Helvetica">component_declaration</FONT>
if the type of the record component is limited.</LI></FONT></UL>
<DIV Class="Paranum"><FONT SIZE=-2>12</FONT></DIV>
<UL Class="NotesBulleted"><FONT SIZE=-1><LI TYPE=DISC>An initialized allocator is not allowed if the designated
type is limited.</LI></FONT></UL>
<DIV Class="Paranum"><FONT SIZE=-2>13</FONT></DIV>
<UL Class="NotesBulleted"><FONT SIZE=-1><LI TYPE=DISC>A generic formal parameter of mode <B>in</B> must not be
of a limited type. </LI></FONT></UL>
<DIV Class="Paranum"><FONT SIZE=-2>14</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>14 <FONT FACE="Arial, Helvetica">Aggregate</FONT>s
are not available for a limited composite type. Concatenation is not
available for a limited array type.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>15</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>15 The rules do not exclude
a <FONT FACE="Arial, Helvetica">default_expression</FONT> for a formal
parameter of a limited type; they do not exclude a deferred constant
of a limited type if the full declaration of the constant is of a nonlimited
type.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>16</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>16 <A NAME="I3111"></A><A NAME="I3112"></A><A NAME="I3113"></A>As
illustrated in <A HREF="RM-7-3-1.html">7.3.1</A>, an untagged limited
type can become nonlimited under certain circumstances. </FONT></DIV>
<H4 ALIGN=CENTER>Examples</H4>
<DIV Class="Paranum"><FONT SIZE=-2>17</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> <I>Example of a
package with a limited type:</I> </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>18</FONT></DIV>
<DIV Class="Examples"><TT><B>package</B> IO_Package <B>is</B><BR>
<B>type</B> File_Name <B>is</B> <B>limited</B> <B>private</B>;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>19</FONT></DIV>
<DIV Class="Examples"><TT> <B>procedure</B> Open (F : <B>in</B> <B>out</B> File_Name);<BR>
<B>procedure</B> Close(F : <B>in</B> <B>out</B> File_Name);<BR>
<B>procedure</B> Read (F : <B>in</B> File_Name; Item : <B>out</B> Integer);<BR>
<B>procedure</B> Write(F : <B>in</B> File_Name; Item : <B>in</B> Integer);<BR>
<B>private</B><BR>
<B>type</B> File_Name <B>is</B><BR>
<B>limited</B> <B>record</B><BR>
Internal_Name : Integer := 0;<BR>
<B>end</B> <B>record</B>;<BR>
<B>end</B> IO_Package;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>20</FONT></DIV>
<DIV Class="Examples"><TT><B>package</B> <B>body</B> IO_Package <B>is</B><BR>
Limit : <B>constant</B> := 200;<BR>
<B>type</B> File_Descriptor <B>is</B> <B>record</B> ... <B>end</B> <B>record</B>;<BR>
Directory : <B>array</B> (1 .. Limit) <B>of</B> File_Descriptor;<BR>
...<BR>
<B>procedure</B> Open (F : <B>in</B> <B>out</B> File_Name) <B>is</B> ... <B>end</B>;<BR>
<B>procedure</B> Close(F : <B>in</B> <B>out</B> File_Name) <B>is</B> ... <B>end</B>;<BR>
<B>procedure</B> Read (F : <B>in</B> File_Name; Item : <B>out</B> Integer) <B>is</B> ... <B>end</B>;<BR>
<B>procedure</B> Write(F : <B>in</B> File_Name; Item : <B>in</B> Integer) <B>is</B> ... <B>end</B>;<BR>
<B>begin</B><BR>
...<BR>
<B>end</B> IO_Package;</TT></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>17 <I>Notes on the example:</I>
In the example above, an outside subprogram making use of IO_Package
may obtain a file name by calling Open and later use it in calls to Read
and Write. Thus, outside the package, a file name obtained from Open
acts as a kind of password; its internal properties (such as containing
a numeric value) are not known and no other operations (such as addition
or comparison of internal names) can be performed on a file name. Most
importantly, clients of the package cannot make copies of objects of
type File_Name.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>22</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>This example is characteristic of any
case where complete control over the operations of a type is desired.
Such packages serve a dual purpose. They prevent a user from making use
of the internal structure of the type. They also implement the notion
of an encapsulated data type where the only operations on the type are
those given in the package specification.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>23</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>The fact that the full view of File_Name
is explicitly declared <B>limited</B> means that parameter passing and
function return will always be by reference (see <A HREF="RM-6-2.html">6.2</A>
and <A HREF="RM-6-5.html">6.5</A>). </FONT></DIV>
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