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<H1> 11.4.2 Example of Exception Handling</H1>
<H4 ALIGN=CENTER>Examples</H4>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> Exception handling
may be used to separate the detection of an error from the response to
that error: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="Examples"><TT><B>with</B> Ada.Exceptions;<BR>
<B>use</B> Ada;<BR>
<B>package</B> File_System <B>is</B><BR>
<B>type</B> File_Handle <B>is</B> <B>limited</B> <B>private</B>;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="Examples"><TT> File_Not_Found : <B>exception</B>;<BR>
<B>procedure</B> Open(F : <B>in</B> <B>out</B> File_Handle; Name : String);<BR>
--<I> raises File_Not_Found if named file does not exist</I></TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="Examples"><TT> End_Of_File : <B>exception</B>;<BR>
<B>procedure</B> Read(F : <B>in</B> <B>out</B> File_Handle; Data : <B>out</B> Data_Type);<BR>
--<I> raises End_Of_File if the file is not open</I></TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="Examples"><TT> ...<BR>
<B>end</B> File_System;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="Examples"><TT><B>package</B> <B>body</B> File_System <B>is</B><BR>
<B>procedure</B> Open(F : <B>in</B> <B>out</B> File_Handle; Name : String) <B>is</B><BR>
<B>begin</B><BR>
<B>if</B> File_Exists(Name) <B>then</B><BR>
...<BR>
<B>else</B><BR>
Exceptions.Raise_Exception(File_Not_Found'Identity,<BR>
"File not found: " & Name & ".");<BR>
<B>end</B> <B>if</B>;<BR>
<B>end</B> Open;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="Examples"><TT> <B>procedure</B> Read(F : <B>in</B> <B>out</B> File_Handle; Data : <B>out</B> Data_Type) <B>is</B><BR>
<B>begin</B><BR>
<B>if</B> F.Current_Position <= F.Last_Position <B>then</B><BR>
...<BR>
<B>else</B><BR>
<B>raise</B> End_Of_File;<BR>
<B>end</B> <B>if</B>;<BR>
<B>end</B> Read;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<DIV Class="Examples"><TT> ...</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<DIV Class="Examples"><TT><B>end</B> File_System;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<DIV Class="Examples"><TT><B>with</B> Ada.Text_IO;<BR>
<B>with</B> Ada.Exceptions;<BR>
<B>with</B> File_System; <B>use</B> File_System;<BR>
<B>use</B> Ada;<BR>
<B>procedure</B> Main <B>is</B><BR>
<B>begin</B><BR>
... --<I> call operations in File_System</I><BR>
<B>exception</B><BR>
<B>when</B> End_Of_File =><BR>
Close(Some_File);<BR>
<B>when</B> Not_Found_Error : File_Not_Found =><BR>
Text_IO.Put_Line(Exceptions.Exception_Message(Not_Found_Error));<BR>
<B>when</B> The_Error : <B>others</B> =><BR>
Text_IO.Put_Line("Unknown error:");<BR>
<B>if</B> Verbosity_Desired <B>then</B><BR>
Text_IO.Put_Line(Exceptions.Exception_Information(The_Error));<BR>
<B>else</B><BR>
Text_IO.Put_Line(Exceptions.Exception_Name(The_Error));<BR>
Text_IO.Put_Line(Exceptions.Exception_Message(The_Error));<BR>
<B>end</B> <B>if</B>;<BR>
<B>raise</B>;<BR>
<B>end</B> Main;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11</FONT></DIV>
<DIV Class="Normal"> In the above example, the File_System package
contains information about detecting certain exceptional situations,
but it does not specify how to handle those situations. Procedure Main
specifies how to handle them; other clients of File_System might have
different handlers, even though the exceptional situations arise from
the same basic causes. </DIV>
<H4 ALIGN=CENTER>Wording Changes from Ada 83</H4>
<DIV Class="Paranum"><FONT SIZE=-2>11.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>The sections labeled ``Exceptions
Raised During ...'' are subsumed by this clause, and by parts of Section
9. </FONT></DIV>
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