11.4.3 Example of Exception Handling
Examples
{
AI12-0440-1}
Exception handling can be used to separate the detection of an error
from the response to that error:
File_Not_Found : exception;
procedure Open(F : in out File_Handle; Name : String);
-- raises File_Not_Found if named file does not exist
End_Of_File : exception;
procedure Read(F : in out File_Handle; Data : out Data_Type);
-- raises End_Of_File if the file is not open
Reason: {
AI12-0178-1}
The first ... provides a place for Close to be declared, and the second
... provides a place for File_Handle to be completed.
Reason: {
AI12-0178-1}
This ... provides a place for File_Exists and the body of Close to be
declared.
{
AI95-00433-01}
{
AI12-0178-1}
procedure Open(F :
in out File_Handle; Name : String)
is
begin
if File_Exists(Name)
then
...
else
raise File_Not_Found
with "File not found: " & Name & ".";
end if;
end Open;
procedure Read(F : in out File_Handle; Data : out Data_Type) is
begin
if F.Current_Position <= F.Last_Position then
...
else
raise End_Of_File;
end if;
end Read;
...
end File_System;
{
AI12-0178-1}
with Ada.Text_IO;
with Ada.Exceptions;
with File_System;
use File_System;
use Ada;
procedure Main
is
Verbosity_Desired : Boolean := ...;
begin
... --
call operations in File_System
exception
when End_Of_File =>
Close(Some_File);
when Not_Found_Error : File_Not_Found =>
Text_IO.Put_Line(Exceptions.Exception_Message(Not_Found_Error));
when The_Error :
others =>
Text_IO.Put_Line("Unknown error:");
if Verbosity_Desired
then
Text_IO.Put_Line(Exceptions.Exception_Information(The_Error));
else
Text_IO.Put_Line(Exceptions.Exception_Name(The_Error));
Text_IO.Put_Line(Exceptions.Exception_Message(The_Error));
end if;
raise;
end Main;
{
AI12-0440-1}
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 can have different handlers, even though
the exceptional situations arise from the same basic causes.
Wording Changes from Ada 83
{
AI05-0299-1}
The sections labeled “Exceptions Raised During ...” are subsumed
by this subclause, and by parts of Clause
9.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
