Creating exception classes

[perlbaski]

I am trying to create exception class hierarchy. But I think I am not declaring them correctly as I am not able to refer to the exception objects.

use Exception::Class (
    'MyException',

    'Commands' => { isa => 'MyException' },

    'Timeout' => {
        isa => 'Commands',
        description =>
          'These exceptions are resulted from running the metrics comm
+ands'
    },
    'DBError' => {
        isa         => 'MyException',
        description => 'DB returned error'
      }

);
ThrowMyException();

sub ThrowMyException() {

    eval{MyException::Commands::Timeout->throw(error=>"This is error d
+ue to timeout");};


        my $err;

        if($err=Exception::Class->caught('MyException::Commands::Timeo
+ut')){
             #print Dumper($err);
             #print "err is of type ".ref($err);
             die $err->{"description"}.": ".$err->{"error"};
        }
        elsif ( $err = Exception::Class->caught("MetricsException") ) 
+{
            #print Dumper($err);
            die $err->error;
        }


}

Can somebody see what is wrong with the code?

[Athanasius]

The problems are not in the declaration, but in sub ThrowMyException. Here is a working version, with changes commented:

#! perl
use strict;
use warnings;

use Exception::Class
(
    'MyException',

    Commands =>
    {
        isa         => 'MyException',
    },

    Timeout  =>
    {
        isa         => 'Commands',
        description => 'This exception results from running the metric
+s commands',
    },

    DBError  =>
    {
        isa         => 'MyException',
        description => 'DB returned error',
    },
);

ThrowMyException();

print "No error caught\n";

sub ThrowMyException                # NO prototype
{
    eval {
             Timeout->throw         # NOT MyException::Commands::Timeo
+ut->
             (
                 error => "This error is due to a timeout"
             );
         };

    my $err;

    if ($err = Exception::Class->caught('Timeout'))
    {
        die $err->description .     # NOT $err->{"description"}
            ': ' .
            $err->error;            # NOT $err->{"error"}
    }
    elsif ($err = Exception::Class->caught('MetricsException'))
    {
        die $err->error;
    }
    else
    {
        $err = Exception::Class->caught();
        ref $err ? $err->rethrow : die $err;
    }
}

Output:

12:30 >perl 517_SoPW.pl
This exception results from running the metrics commands: This error i
+s due to a timeout at 517_SoPW.pl line 56.

12:33 >

Hope that helps,

[perlbaski]

Thanks, that worked..I was thinking classes are blessed hashes, so I was trying to access them that way, looks like Exception::Class module creates accessors to access the fields. Am I correct?

[Athanasius]

From the source code it appears that Exception::Class inherits from Exception::Class::Base. The documentation for Exception::Class::Base catalogues a number of class and/or object methods, among which are these:

MyException->description()
Returns the description for the given Exception::Class::Base subclass. ... This is also an object method.

$exception->error()
Returns the error/message associated with the exception.

So these accessors are, as it were, “built-in” to any class derived from Exception::Class.

Hope that helps,

[7stud]

use strict;
use warnings;
use 5.012;

use Exception::Class (
    'MyException',

    'CommandException' => { isa => 'MyException' },

    'TimeoutException' => {
        isa => 'CommandException',
        description =>
          'This exception resulted from running the metrics commands'
    },
    'DBException' => {
        isa         => 'MyException',
        description => 'DB returned error'
      }

);

&ThrowMyException();

sub ThrowMyException() {

    eval{ TimeoutException->throw(error=>"This is error due to timeout
+") };
    my $err;

    if ($err = Exception::Class->caught('TimeoutException') ) {
         #print Dumper($err);
         #print "err is of type ".ref($err);
         die $err->description . ": " . $err->error;
    }
    elsif ( $err = Exception::Class->caught("MetricsException") ) {
        #print Dumper($err);
        die $err->error;
    }


}

If you see class names like A::B::C, that means the class is named C and it can be found in the directory A/B/. The A::B:: part is a namespace name to prevent name conflicts, but it also indicates the directory structure where the class is located. When you write A::B::C in your program, perl searches all the directories in @INC for a directory named A that has a subdirectory named B, which contains a file called C.pm.

[tobyink]

"When you write A::B::C in your program, perl searches all the directories in @INC for a directory named A that has a subdirectory named B, which contains a file called C.pm."

No it doesn't. It only does that search if you write use A::B::C, no A::B::C or require A::B::C. Merely mentioning the A::B::C package (for example, to call a constructor) does not force Perl to attempt to load the module.

The following code does not attempt to load "This/File/Does/No/Exist/On/My/Disk.pm"...

use strict;
use warnings;

{
    package This::File::Does::No::Exist::On::My::Disk;
    require Math::BigInt;
    our @ISA = 'Math::BigInt';
}

my $number = This::File::Does::No::Exist::On::My::Disk->new(7);
print $number * 6, "\n";