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Archive for the ‘Language’ Category

Function pointers

A useful techniques is the ability to have pointer to functions. Their declaration is easy: write the declaration as it would be for the function, example:

int my_func(int var_a, int var_b);

and the following form make a pointer which pointed to my_func:

int (*my_func_p)(int, int);

Once you have got the pointer, you can assign the address of the function you declared by using just its name:

like an array, a function name is returned into an address when it is used in an expression. You can call the function using one of two form:

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Error handling in Perl

  • Using unless
    • The unless is the logical opposite to if. Statements can completely bypass the success status and only be executed if the expression returns false.
      • example: die “Error message\n”unless(chdir(“dir”))
      • the unless statement is best used when you want to raise an error only if the expression fails.
  • Using the conditional operator
    • For some short tests, you can use the conditional operator
      • example: print (exists($hash{$key}))
  • Using Warn function
    • Warn function just raises a warning, a message is printed to STDERR, but no further action is taken
      • example: chdir(“dir”) or warn “warn message”;
  • Using Die function
    • Die function works like warn, except that is also calls exit, within a normal script, this function has the effect of immediately terminating execution.
      • example: chdir(“dir”) or die “die message”;
  • Using Carp function
    • Carp function is the basic equivalent of warn and prints the message to STDERR without existing the script and printing the script name
      • example: carp “error message”; will result in “error message, at scriptname.pl line number”
  • Using Cluck function
    • Cluck function is a sort of supercharged carp, it follows the same basic principle but also prints a stack trace of all the modules that led to the function being called, including the information on the original script.
  • Using Croak function
    • Croak function is the equivalent of die. it reports the caller one level up. Like die, this function also exists the script after reporting the error to STDERR.

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The main() function’s double life :::

  • int main (int argc, char *argv[]);
  • int main (int argc, char **argv);

The global variables set by getopt() include:

  1. optarg ::: A pointer to the current option argument, if there is one
  2. optind ::: An index of the next argv pointer to process when getopt() is called again
  3. optopt ::: This is the last known option

If you don’t want getopt to print error message, you can set opterr to zero

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Tie tech in Perl

hide an object class in a simple variable::::::{ tie $scalar, ‘package’, ARGUMENTS… }

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The tie() function binds a variable to a class (package) that will provide the implementation for access methods for that variable. Once this magic has been performed, accessing a tied variable automatically triggers method calls in the proper class. The complexity of the class is hidden behind magic methods calls. The method names are in ALL CAPS, which is a convention that Perl uses to indicate that they’re called implicitly rather than explicitly–just like the BEGIN() and END() functions.

  1. Tying scalar:::
  2. Tying arrays:::
  3. Tying hashes:::
  4. Tying filehandle:::

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Arrays::::::

perl array tying

Hashes::::::

perl hash tie

FileHandles::::::

perl filehandle tying

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