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<H1> 4.8 Allocators</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> [The evaluation of an <FONT FACE="Arial, Helvetica">allocator</FONT>
creates an object and yields an access value that designates the object.
<A NAME="I2831"></A><A NAME="I2832"></A><A NAME="I2833"></A>]</DIV>
<H4 ALIGN=CENTER>Syntax</H4>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="SyntaxIndented"><FONT FACE="Arial, Helvetica">allocator<A NAME="I2834"></A>
::= </FONT><BR>
<B>new</B> <A NAME="I2835"></A><FONT FACE="Arial, Helvetica">subtype_indication</FONT> | <B>new</B> <A NAME="I2836"></A><FONT FACE="Arial, Helvetica">qualified_expression</FONT></DIV>
<H4 ALIGN=CENTER>Name Resolution Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>3/1</FONT></DIV>
<DIV Class="Normal"> {<I><A HREF="defect1.html#8652/0010">8652/0010</A></I>}
<A NAME="I2837"></A>The expected type for an <FONT FACE="Arial, Helvetica">allocator</FONT>
shall be a single access-to-object type <U>with</U><S>whose</S> designated
type <U><I>D</I> such that either <I>D</I></U> covers the type determined
by the <FONT FACE="Arial, Helvetica">subtype_mark</FONT> of the <FONT FACE="Arial, Helvetica">subtype_indication</FONT>
or <FONT FACE="Arial, Helvetica">qualified_expression</FONT><U>, or the
expected type is anonymous and the determined type is <I>D</I>'Class</U>.
</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Discussion: </B>See <A HREF="AA-8-6.html">8.6</A>,
``<A HREF="AA-8-6.html">The Context of Overload Resolution</A>'' for
the meaning of ``shall be a single ... type whose ...'' </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3.a.1/1</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>{<I><A HREF="defect1.html#8652/0010">8652/0010</A></I>}
<U>An allocator is allowed as a controlling parameter of a dispatching
call (see <A HREF="AA-3-9-2.html">3.9.2</A>).</U> </FONT></DIV>
<H4 ALIGN=CENTER>Legality Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="Normal"> <A NAME="I2838"></A>An <I>initialized</I> allocator
is an <FONT FACE="Arial, Helvetica">allocator</FONT> with a <FONT FACE="Arial, Helvetica">qualified_expression</FONT>.
<A NAME="I2839"></A>An <I>uninitialized</I> allocator is one with a <FONT FACE="Arial, Helvetica">subtype_indication</FONT>.
In the <FONT FACE="Arial, Helvetica">subtype_indication</FONT> of an
uninitialized allocator, a <FONT FACE="Arial, Helvetica">constraint</FONT>
is permitted only if the <FONT FACE="Arial, Helvetica">subtype_mark</FONT>
denotes an [unconstrained] composite subtype; if there is no <FONT FACE="Arial, Helvetica">constraint</FONT>,
then the <FONT FACE="Arial, Helvetica">subtype_mark</FONT> shall denote
a definite subtype. <A NAME="I2840"></A></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>For example,
... <B>new</B> S'Class ... (with no initialization expression) is illegal,
but ... <B>new</B> S'Class'(X) ... is legal, and takes its tag and constraints
from the initial value X. (Note that the former case cannot have a constraint.)
</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="Normal"> If the type of the <FONT FACE="Arial, Helvetica">allocator</FONT>
is an access-to-constant type, the <FONT FACE="Arial, Helvetica">allocator</FONT>
shall be an initialized allocator. If the designated type is limited,
the <FONT FACE="Arial, Helvetica">allocator</FONT> shall be an uninitialized
allocator. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>For an access-to-constant
type whose designated type is limited, <FONT FACE="Arial, Helvetica">allocator</FONT>s
are illegal. The Access attribute is legal for such a type, however.
</FONT></DIV>
<H4 ALIGN=CENTER>Static Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="Normal"> If the designated type of the type of the <FONT FACE="Arial, Helvetica">allocator</FONT>
is elementary, then the subtype of the created object is the designated
subtype. If the designated type is composite, then the created object
is always constrained; if the designated subtype is constrained, then
it provides the constraint of the created object; otherwise, the object
is constrained by its initial value [(even if the designated subtype
is unconstrained with defaults)]. <A NAME="I2841"></A></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Discussion: </B>See AI83-00331.
</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>All objects created
by an allocator are aliased, and all aliased composite objects need to
be constrained so that access subtypes work reasonably. </FONT></DIV>
<H4 ALIGN=CENTER>Dynamic Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="Normal"> <A NAME="I2842"></A>For the evaluation of an <FONT FACE="Arial, Helvetica">allocator</FONT>,
the elaboration of the <FONT FACE="Arial, Helvetica">subtype_indication</FONT>
or the evaluation of the <FONT FACE="Arial, Helvetica">qualified_expression</FONT>
is performed first. <A NAME="I2843"></A><A NAME="I2844"></A>For the evaluation
of an initialized allocator, an object of the designated type is created
and the value of the <FONT FACE="Arial, Helvetica">qualified_expression</FONT>
is converted to the designated subtype and assigned to the object. <A NAME="I2845"></A></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>7.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>The conversion
might raise Constraint_Error. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<DIV Class="Normal"> <A NAME="I2846"></A>For the evaluation of an uninitialized
allocator: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<UL Class="Bulleted" Style="margin-bottom: 0.3em"><LI TYPE=DISC><A NAME="I2847"></A>If the designated type is elementary,
an object of the designated subtype is created and any implicit initial
value is assigned;</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>10/1</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>{<I><A HREF="defect1.html#8652/0002">8652/0002</A></I>}
<A NAME="I2848"></A>If the designated type is composite, an object of
the designated type is created with tag, if any, determined by the <FONT FACE="Arial, Helvetica">subtype_mark</FONT>
of the <FONT FACE="Arial, Helvetica">subtype_indication</FONT>; any per-object
constraints on subcomponents are elaborated <U>(see <A HREF="AA-3-8.html">3.8</A>)
</U>and any implicit initial values for the subcomponents of the object
are obtained as determined by the <FONT FACE="Arial, Helvetica">subtype_indication</FONT>
and assigned to the corresponding subcomponents. <A NAME="I2849"></A><A NAME="I2850"></A><A NAME="I2851"></A><A NAME="I2852"></A>A
check is made that the value of the object belongs to the designated
subtype. <A NAME="I2853"></A>Constraint_Error is raised if this check
fails. This check and the initialization of the object are performed
in an arbitrary order. </LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>10.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Discussion: </B>AI83-00150.
</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11</FONT></DIV>
<DIV Class="Normal"> [If the created object contains any tasks, they
are activated (see <A HREF="AA-9-2.html">9.2</A>).] Finally, an access
value that designates the created object is returned. </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>24 Allocators cannot create
objects of an abstract type. See <A HREF="AA-3-9-3.html">3.9.3</A>.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>13</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>25 If any part of the created
object is controlled, the initialization includes calls on corresponding
Initialize or Adjust procedures. See <A HREF="AA-7-6.html">7.6</A>.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>14</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>26 As explained in <A HREF="AA-13-11.html">13.11</A>,
``<A HREF="AA-13-11.html">Storage Management</A>'', the storage for an
object allocated by an <FONT FACE="Arial, Helvetica">allocator</FONT>
comes from a storage pool (possibly user defined). <A NAME="I2854"></A>The
exception Storage_Error is raised by an <FONT FACE="Arial, Helvetica">allocator</FONT>
if there is not enough storage. Instances of Unchecked_Deallocation may
be used to explicitly reclaim storage.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>15</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>27 Implementations are permitted,
but not required, to provide garbage collection (see <A HREF="AA-13-11-3.html">13.11.3</A>).
</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>15.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>Note that
in an <FONT FACE="Arial, Helvetica">allocator</FONT>, the exception Constraint_Error
can be raised by the evaluation of the <FONT FACE="Arial, Helvetica">qualified_expression</FONT>,
by the elaboration of the <FONT FACE="Arial, Helvetica">subtype_indication</FONT>,
or by the initialization. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>15.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Discussion: </B>By default,
the implementation provides the storage pool. The user may exercise more
control over storage management by associating a user-defined pool with
an access type. </FONT></DIV>
<H4 ALIGN=CENTER>Examples</H4>
<DIV Class="Paranum"><FONT SIZE=-2>16</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> <I>Examples of
allocators:</I> </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>17</FONT></DIV>
<DIV Class="Examples"><TT><B>new</B> Cell'(0, <B>null</B>, <B>null</B>) <I>-- initialized explicitly, see <A HREF="AA-3-10-1.html">3.10.1</A></I><BR>
<B>new</B> Cell'(Value => 0, Succ => <B>null</B>, Pred => <B>null</B>) <I>-- initialized explicitly</I><BR>
<B>new</B> Cell <I>-- not initialized</I></TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>18</FONT></DIV>
<DIV Class="Examples"><TT><B>new</B> Matrix(1 .. 10, 1 .. 20) <I>-- the bounds only are given</I><BR>
<B>new</B> Matrix'(1 .. 10 => (1 .. 20 => 0.0)) <I>-- initialized explicitly</I></TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>19</FONT></DIV>
<DIV Class="Examples"><TT><B>new</B> Buffer(100) <I>-- the discriminant only is given</I><BR>
<B>new</B> Buffer'(Size => 80, Pos => 0, Value => (1 .. 80 => 'A')) <I>-- initialized explicitly</I></TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>20</FONT></DIV>
<DIV Class="Examples"><TT>Expr_Ptr'(<B>new</B> Literal) <I>-- allocator for access-to-class-wide type, see <A HREF="AA-3-9-1.html">3.9.1</A></I><BR>
Expr_Ptr'(<B>new</B> Literal'(Expression <B>with</B> 3.5)) <I>-- initialized explicitly</I></TT></DIV>
<H4 ALIGN=CENTER>Incompatibilities With Ada 83</H4>
<DIV Class="Paranum"><FONT SIZE=-2>20.a/1</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><A NAME="I2855"></A>The <FONT FACE="Arial, Helvetica">subtype_indication</FONT>
of an uninitialized allocator may not have an explicit <FONT FACE="Arial, Helvetica">constraint</FONT>
if the designated type is an access type. In Ada 83, this was permitted
even though the <FONT FACE="Arial, Helvetica">constraint</FONT> had no
<U>e</U><S>a</S>ffect on the subtype of the created object. </FONT></DIV>
<H4 ALIGN=CENTER>Extensions to Ada 83</H4>
<DIV Class="Paranum"><FONT SIZE=-2>20.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><A NAME="I2856"></A>Allocators
creating objects of type <I>T</I> are now overloaded on access types
designating <I>T'</I>Class and all class-wide types that cover <I>T</I>.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>20.c</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>Implicit array subtype conversion
(sliding) is now performed as part of an initialized allocator. </FONT></DIV>
<H4 ALIGN=CENTER>Wording Changes from Ada 83</H4>
<DIV Class="Paranum"><FONT SIZE=-2>20.d</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>We have used a new organization,
inspired by the ACID document, that makes it clearer what is the subtype
of the created object, and what subtype conversions take place.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>20.e</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>Discussion of storage management
issues, such as garbage collection and the raising of Storage_Error,
has been moved to <A HREF="AA-13-11.html">13.11</A>, ``<A HREF="AA-13-11.html">Storage
Management</A>''. </FONT></DIV>
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