12th Chapter-Exception Handling and program debugging (i) (6)

12.2.2 generates a resource protection block

Delphi provides a reserved word finally, which is used to achieve resource protection:

{Allocating Resources}

Try

{Resource Usage}

Finally

{Releasing Resources}

End

The Try...finally...end form a resource protection block. Finally, the following statement is executed in any case, regardless of whether the program has an exception.

For the example in (12.2.1), the following code ensures the release of the allocated memory resources:

Var

Apointer:pointer;

Aint, Adiv:integer;

Begin

Adiv: = 0;

Getmem (Apointer, 1024);

Try

Aint: = ten div adiv;

Finally

Freemem (Apointer, 1024);

End

End

The following example is taken from section (6.4), which protects file resources in a copy of a file:

Procedure CopyFile (const FileName, destname:tfilename);

Var

Copybuffer:pointer;

TimeStamp, Bytescopied:longint;

Source, Dest:integer;

Destination:tfilename;

Const

Chunksize:longint = 8192;

Begin

Destination: = Expandfilename (Destname);

If hasattr (destination, fadirectory) then

Destination: = destination + ' + ' + extractfilename (FileName);

TimeStamp: = Fileage (FileName);

Getmem (Copybuffer, chunksize);

Try

Source: = FileOpen (FileName, fmsharedenywrite);

If Source < 0 Then

Raise Efopenerror.create (Fmtloadstr (Sfopenerror, [FileName]));

Try

Dest: = filecreate (destination);

If Dest < 0 Then

Raise Efcreateerror.create (Fmtloadstr (sfcreateerror, [destination]));

Try

Repeat

Bytescopied: = Fileread (Source, copybuffer^, chunksize);

If bytescopied > 0 Then

FileWrite (Dest, copybuffer^, bytescopied);

Until Bytescopied < chunksize;

Finally

FileClose (Dest);

End

Finally

FileClose (Source);

End

Finally

Freemem (Copybuffer, chunksize);

End

End

The specific interpretation of the procedure is shown in section (6.4).

In the case of exception protection, when an exception occurs, the system automatically pops up a message box to display the message for the exception. The exception class is automatically cleared after exiting the current module.