How to force list context on passed args to built-in pack function?

[krystian.wojtas]

Hi

The program I'm working on is creating a database using the built-in pack function. It needs to add a feature which makes some statistics about data usage by particular arrays.

The best approach I think is to use the Aspect module and insert some additional code after invoking the pack method. But as far as I see, hijacking the built-in function is not working. Could you confirm this?

In this case I prepared a wrapper function which is like this:

sub pack_wrapper { pack(@_); }

But it doesn't work. It looks like the pack function forces the list @_ to be passed in scalar context. In this case, pack gets the size of the array instead of its content.

The pack function hasn't a fixed number of arguments, so I cannot pass it directly like this:

sub pack_wrapper { pack($_[0], $_[1]); }

Probably I can use meta-programming to generate the code which invokes pack with all arguments explicitly depending on the list size. But such a workaround feels really awful for me.

Is it possible to pass my list @_ in list context to the pack function in some perl clean way?

Thanks for your answer and sorry if my question is obvious for you, I have already spent some time and cannot find the right solution.

Best regards,
Krystian

 

[Peter J. Holzer]

In this case I prepared a wrapper function which is like this:

sub pack_wrapper { pack(@_); }

But it doesn't work. It looks like the pack function forces the list
@_ to be passed in scalar context.

Yes. pack expects a scalar and a list.

In this case, pack gets the size of
the array instead of its content.

The pack function hasn't a fixed number of arguments, so I cannot pass
it directly like this:

sub pack_wrapper { pack($_[0], $_[1]); }

You can, however, do pass the rest of the arguments like this:

sub pack_wrapper { pack($_[0], @_[1..$#_]); }

However, I would prefer the more readable variant

sub pack_wrapper {
my ($template, @list) = @_;
pack($template, @list);
}

hp

 

[Rainer Weikusat]

The program I'm working on is creating a database using the built-in
pack function. It needs to add a feature which makes some statistics
about data usage by particular arrays.

The best approach I think is to use the Aspect module and insert
some additional code after invoking the pack method. But as far as I
see, hijacking the built-in function is not working. Could you
confirm this?

In this case I prepared a wrapper function which is like this:

sub pack_wrapper { pack(@_); }

But it doesn't work.

That's because of the pack prototype. You could use

sub pack_wrapper { pack(shift, @_); }

instead.

 

[krystian.wojtas]

That's because of the pack prototype. You could use

sub pack_wrapper { pack(shift, @_); }



instead.

Thank you very much for your answer, it works perfectly now

 

[Peter J. Holzer]

Thank you very much for your answer, it works perfectly now

Do you understand why it works?

Will you still understand it in 6 months?

hp