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<H1> 13.6 Change of Representation</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> [<A NAME="I4522"></A> <A NAME="I4523"></A>A <FONT FACE="Arial, Helvetica">type_conversion</FONT>
(see <A HREF="AA-4-6.html">4.6</A>) can be used to convert between two
different representations of the same array or record. To convert an
array from one representation to another, two array types need to be
declared with matching component subtypes, and convertible index types.
If one type has packing specified and the other does not, then explicit
conversion can be used to pack or unpack an array.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="Normal"> To convert a record from one representation to
another, two record types with a common ancestor type need to be declared,
with no inherited subprograms. Distinct representations can then be specified
for the record types, and explicit conversion between the types can be
used to effect a change in representation.] </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>2.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>This technique
does not work if the first type is an untagged type with user-defined
primitive subprograms. It does not work at all for tagged types. </FONT></DIV>
<H4 ALIGN=CENTER>Examples</H4>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> <I>Example of change
of representation:</I> </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="Examples"><TT>--<I> Packed_Descriptor and Descriptor are two different types</I><BR>
--<I> with identical characteristics, apart from their</I><BR>
--<I> representation</I></TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="Examples"><TT><B>type</B> Descriptor <B>is</B><BR>
<B>record</B><BR>
--<I> components of a descriptor</I><BR>
<B>end</B> <B>record</B>;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="Examples"><TT><B>type</B> Packed_Descriptor <B>is</B> <B>new</B> Descriptor;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="Examples"><TT><B>for</B> Packed_Descriptor <B>use</B><BR>
<B>record</B><BR>
--<I> component clauses for some or for all components</I><BR>
<B>end</B> <B>record</B>;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<DIV Class="Examples"><TT><I>-- Change of representation can now be accomplished by explicit type conversions:</I></TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<DIV Class="Examples"><TT>D : Descriptor;<BR>
P : Packed_Descriptor;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<DIV Class="Examples"><TT>P := Packed_Descriptor(D); --<I> pack D</I><BR>
D := Descriptor(P); --<I> unpack P</I></TT></DIV>
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