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Description
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The Raise keyword
creates an exception object that is passed to the
Delphi exception handler.
You would only raise an exception in literally
exceptional circumstances. This is partly because
of the resource and performance overheads incurred,
but also because there are neater ways for
application error handling, such as return codes
from functions.
Version
1
On its own, Raise is used
inside the Except clause of a
Try statement. It simply re-raises
the current exception for handling at a higher
level in the application.
Version
2
Uses an new exception object to report an
exception. Normally, you would use an
Exception object, or an inherited
Exception object, but you are not restricted to do
so. The exception address is that of the raise
statement.
You can create the object at the time of the
raise:
Raise Exception.Create('Error
happened');
Version
3 As for version 2, but overriding
the address value of the exception.
In all cases, when the Raise call is
made, code execution jumps to the Delphi exception
handler - it either terminates the program, or uses
the current Try statement to
handle it.
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Notes
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Warning use only when
appropriate.
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Related commands
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Except
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Starts the error trapping clause of a
Try statement
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Finally
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Starts the unconditional code section of a
Try statement
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On
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Defines exception handling in a Try
Except clause
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Try
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Starts code that has error trapping
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Example code : Use of
Raise in a function
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var
fred, jim : string;
begin
// Set up
some sample names
fred := 'Good name';
jim :=
'Badname ';
// Try to
swap these names
try
ShowMessage(fred+' swapped
= '+SwapNames(fred));
ShowMessage(jim+' swapped =
'+SwapNames(jim));
except
On E : Exception do
ShowMessage(E.Message);
end;
end;
// Swaps first and
second names in the passed name
string
// Raises an
exception if the name is
invalid
function TForm1.SwapNames(name: string):
string;
var
blankPos : Integer;
i :
Integer;
nameLen : Integer;
begin
// Clear
the result string to indicate no success
yet
Result := '';
// Find
the position of the last name
blankPos := Pos(' ', name);
// If
found, and position is short of the name
end
// then
we are OK so far
nameLen := Length(name);
if (blankPos > 0) and (blankPos <
nameLen)
then
begin
// Find the start of the second
name
i := blankPos + 1;
repeat
// If last name start found, swap
first and last names
if name[i]
<> ' '
then Result :=
Copy(name, i, nameLen-i+1) + ' ' +
Copy(name, 1, blankPos-1)
else
Inc(i);
until (i > nameLen) or
(Length(Result) > 0);
end;
//
Couldn't swap first and second names
?
if Length(Result) = 0
then Raise
Exception.CreateFmt('Invalid name : ''%s''',
[name]);
end;
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Show full unit code
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Good name swapped = name
Good
Invalid name : 'Badname '
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Example code : Raise a
new exception type
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var
age : Integer;
exc : EBadAge;
begin
// Simple
code that raises a new exception
type
age := 23;
if age < 30
then
begin
// Create a new EBadAge exception
object
exc :=
EBadAge.CreateFmt('Bad age : %d',[age]);
// Now raise this new exception
object
Raise exc;
end;
end;
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Show full unit code
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A Delphi error dialog is
shown with message :
Bad age : 23
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