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<H1> 12.8 Example of a Generic Package</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> The following example provides a possible formulation
of stacks by means of a generic package. The size of each stack and the
type of the stack elements are provided as generic formal parameters.
</DIV>
<H4 ALIGN=CENTER>Examples</H4>
<DIV Class="Paranum"><FONT SIZE=-2>2/1</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> <FONT SIZE=-1><I>This
paragraph was deleted.</I></FONT><S> </S></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="Examples"><TT><B>generic</B><BR>
Size : Positive;<BR>
<B>type</B> Item <B>is</B> <B>private</B>;<BR>
<B>package</B> Stack <B>is</B><BR>
<B>procedure</B> Push(E : <B>in</B> Item);<BR>
<B>procedure</B> Pop (E : <B>out</B> Item);<BR>
Overflow, Underflow : <B>exception</B>;<BR>
<B>end</B> Stack;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="Examples"><TT><B>package</B> <B>body</B> Stack <B>is</B></TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="Examples"><TT> <B>type</B> Table <B>is</B> <B>array</B> (Positive <B>range</B> <>) <B>of</B> Item;<BR>
Space : Table(1 .. Size);<BR>
Index : Natural := 0;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="Examples"><TT> <B>procedure</B> Push(E : <B>in</B> Item) <B>is</B><BR>
<B>begin</B><BR>
<B>if</B> Index >= Size <B>then</B><BR>
<B>raise</B> Overflow;<BR>
<B>end</B> <B>if</B>;<BR>
Index := Index + 1;<BR>
Space(Index) := E;<BR>
<B>end</B> Push;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="Examples"><TT> <B>procedure</B> Pop(E : <B>out</B> Item) <B>is</B><BR>
<B>begin</B><BR>
<B>if</B> Index = 0 <B>then</B><BR>
<B>raise</B> Underflow;<BR>
<B>end</B> <B>if</B>;<BR>
E := Space(Index);<BR>
Index := Index - 1;<BR>
<B>end</B> Pop;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<DIV Class="Examples"><TT><B>end</B> Stack;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<DIV Class="Wide" Style="margin-bottom: 0.4em"> Instances of this
generic package can be obtained as follows: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<DIV Class="Examples"><TT><B>package</B> Stack_Int <B>is</B> <B>new</B> Stack(Size => 200, Item => Integer);<BR>
<B>package</B> Stack_Bool <B>is</B> <B>new</B> Stack(100, Boolean);</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11</FONT></DIV>
<DIV Class="Wide" Style="margin-bottom: 0.4em"> Thereafter, the procedures
of the instantiated packages can be called as follows: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12</FONT></DIV>
<DIV Class="Examples"><TT>Stack_Int.Push(N);<BR>
Stack_Bool.Push(True);</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>13</FONT></DIV>
<DIV Class="Wide" Style="margin-bottom: 0.4em"> Alternatively, a
generic formulation of the type Stack can be given as follows (package
body omitted): </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>14</FONT></DIV>
<DIV Class="Examples"><TT><B>generic</B><BR>
<B>type</B> Item <B>is</B> <B>private</B>;<BR>
<B>package</B> On_Stacks <B>is</B><BR>
<B>type</B> Stack(Size : Positive) <B>is</B> <B>limited</B> <B>private</B>;<BR>
<B>procedure</B> Push(S : <B>in</B> <B>out</B> Stack; E : <B>in</B> Item);<BR>
<B>procedure</B> Pop (S : <B>in</B> <B>out</B> Stack; E : <B>out</B> Item);<BR>
Overflow, Underflow : <B>exception</B>;<BR>
<B>private</B><BR>
<B>type</B> Table <B>is</B> <B>array</B> (Positive <B>range</B> <>) <B>of</B> Item;<BR>
<B>type</B> Stack(Size : Positive) <B>is</B><BR>
<B>record</B><BR>
Space : Table(1 .. Size);<BR>
Index : Natural := 0;<BR>
<B>end</B> <B>record</B>;<BR>
<B>end</B> On_Stacks;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>15</FONT></DIV>
<DIV Class="Wide" Style="margin-bottom: 0.4em"> In order to use such
a package, an instance has to be created and thereafter stacks of the
corresponding type can be declared: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>16</FONT></DIV>
<DIV Class="Examples"><TT><B>declare</B><BR>
<B>package</B> Stack_Real <B>is</B> <B>new</B> On_Stacks(Real); <B>use</B> Stack_Real;<BR>
S : Stack(100);<BR>
<B>begin</B><BR>
...<BR>
Push(S, 2.54);<BR>
...<BR>
<B>end</B>;</TT></DIV>
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