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<H1> 13.11.2 Unchecked Storage Deallocation</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> [<A NAME="I4667"></A> <A NAME="I4668"></A><A NAME="I4669"></A><A NAME="I4670"></A><A NAME="I4671"></A>Unchecked
storage deallocation of an object designated by a value of an access
type is achieved by a call to an instance of the generic procedure Unchecked_Deallocation.]
</DIV>
<H4 ALIGN=CENTER>Static Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> The following language-defined
generic library procedure exists: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="Examples"><TT><B>generic</B><BR>
<B>type</B> Object(<>) <B>is</B> <B>limited</B> <B>private</B>;<BR>
<B>type</B> Name <B>is</B> <B>access</B> Object;<BR>
<A NAME="I4672"></A><A NAME="I4673"></A><A NAME="I4674"></A><B>procedure</B> Ada.Unchecked_Deallocation(X : <B>in</B> <B>out</B> Name);<BR>
<B>pragma</B> Convention(Intrinsic, Ada.Unchecked_Deallocation);<BR>
<B>pragma</B> Preelaborate(Ada.Unchecked_Deallocation);</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>The <FONT FACE="Arial, Helvetica">pragma</FONT>
Convention implies that the attribute Access is not allowed for instances
of Unchecked_Deallocation. </FONT></DIV>
<H4 ALIGN=CENTER>Dynamic Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> Given an instance
of Unchecked_Deallocation declared as follows: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="Examples"><TT><B>procedure</B> Free <B>is</B><BR>
<B>new</B> Ada.Unchecked_Deallocation(<BR>
<I>object_subtype_name</I>, <I>access_to_variable_subtype_name</I>);</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="Wide" Style="margin-bottom: 0.4em"> Procedure Free has
the following effect: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DL Class="Enumerated"><DT> 1.<DD Class="Enumerated">
After executing Free(X), the value of X is <B>null</B>.</DL>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<DL Class="Enumerated"><DT> 2.<DD Class="Enumerated">
Free(X), when X is already equal to <B>null</B>, has no effect.</DL>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<DL Class="Enumerated"><DT> 3.<DD Class="Enumerated">
Free(X), when X is not equal to <B>null</B> first performs finalization,
as described in <A HREF="AA-7-6.html">7.6</A>. It then deallocates the
storage occupied by the object designated by X. If the storage pool is
a user-defined object, then the storage is deallocated by calling Deallocate,
passing <I>access_to_variable_subtype_name</I>'Storage_Pool as the Pool
parameter. Storage_Address is the value returned in the Storage_Address
parameter of the corresponding Allocate call. Size_In_Storage_Elements
and Alignment are the same values passed to the corresponding Allocate
call. There is one exception: if the object being freed contains tasks,
the object might not be deallocated. </DL>
<DIV Class="Paranum"><FONT SIZE=-2>9.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>Free calls
only the specified Deallocate procedure to do deallocation. For any given
object deallocation, the number of calls to Free (usually one) will be
equal to the number of Allocate calls it took to allocate the object.
We do not define the relative order of multiple calls used to deallocate
the same object -- that is, if the <FONT FACE="Arial, Helvetica">allocator</FONT>
allocated two pieces <I>x</I> and <I>y</I>, then Free might deallocate
<I>x</I> and then <I>y</I>, or it might deallocate <I>y</I> and then
<I>x</I>. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<DIV Class="Normal"> <A NAME="I4675"></A><A NAME="I4676"></A>After
Free(X), the object designated by X, and any subcomponents thereof, no
longer exist; their storage can be reused for other purposes. </DIV>
<H4 ALIGN=CENTER>Bounded (Run-Time) Errors</H4>
<DIV Class="Paranum"><FONT SIZE=-2>11</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> <A NAME="I4677"></A>It
is a bounded error to free a discriminated, unterminated task object.
The possible consequences are: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>This is an error
because the task might refer to its discriminants, and the discriminants
might be deallocated by freeing the task object. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>No exception is raised.</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>13</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC><A NAME="I4678"></A><A NAME="I4679"></A>Program_Error or
Tasking_Error is raised at the point of the deallocation.</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>14</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC><A NAME="I4680"></A><A NAME="I4681"></A>Program_Error or
Tasking_Error is raised in the task the next time it references any of
the discriminants. </LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>14.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Implementation Note: </B>This
last case presumes an implementation where the task references its discriminants
indirectly, and the pointer is nulled out when the task object is deallocated.
</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>15</FONT></DIV>
<DIV Class="Normal"> In the first two cases, the storage for the discriminants
(and for any enclosing object if it is designated by an access discriminant
of the task) is not reclaimed prior to task termination. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>15.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>The storage
might never be reclaimed. </FONT></DIV>
<H4 ALIGN=CENTER>Erroneous Execution</H4>
<DIV Class="Paranum"><FONT SIZE=-2>16</FONT></DIV>
<DIV Class="Normal"> <A NAME="I4682"></A><A NAME="I4683"></A>Evaluating
a name that denotes a nonexistent object is erroneous. The execution
of a call to an instance of Unchecked_Deallocation is erroneous if the
object was created other than by an <FONT FACE="Arial, Helvetica">allocator</FONT>
for an access type whose pool is Name'Storage_Pool. </DIV>
<H4 ALIGN=CENTER>Implementation Advice</H4>
<DIV Class="Paranum"><FONT SIZE=-2>17</FONT></DIV>
<DIV Class="Normal"> For a standard storage pool, Free should actually
reclaim the storage. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>17.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>This is not
a testable property, since we do not how much storage is used by a given
pool element, nor whether fragmentation can occur. </FONT></DIV>
<DIV Class="NotesHeader"><FONT SIZE=-1>NOTES</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>18</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>26 The rules here that refer
to Free apply to any instance of Unchecked_Deallocation.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>19</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>27 Unchecked_Deallocation
cannot be instantiated for an access-to-constant type. This is implied
by the rules of <A HREF="AA-12-5-4.html">12.5.4</A>. </FONT></DIV>
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