Pushing hash ref onto array ref

Bod has asked for the wisdom of the Perl Monks concerning the following question:

A module I am creating has a blessed hash ref as is common. One of the hashes is a reference to an array of anonymous hashes created like so:

sub new {
    my $class = shift;
    my %attrs = @_;
    
    my @products = ({
        'id'            => 0,
        'name'          => 'Test',
        'description'   => 'Some test data',
        'qty'           => 1,
        'price'         => 1000,
    });
    $attrs{'trolley'} = \@products;
    
    return bless \%attrs, $class;
}
[download]

Later on I want to push another anonymous hash onto @products.

What is the best way to do this?
Both of these push lines appear to work identically in testing

sub add_product {
    my ($self, $product_data) = @_;
    # create $new_product hasfref
    push $self->{'trolley'}, $new_product;
    push @{$self->{'trolley'}}, $new_product;
}
[download]

However, I suspect there is some subtle difference between the two which might trip me up in the future!

Is there a practical difference and is there a 'best' option to use?

Comment on Pushing hash ref onto array ref Select or Download Code

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Re: Pushing hash ref onto array ref by AnomalousMonk (Archbishop) on Apr 11, 2021 at 23:23 UTC
The "subtle difference" is that push-ing to an array reference was an experimental feature (~~added circa-5.10 IIRC~~ \| added with Perl 5.14, removed as of Perl 5.24) that is now removed. For this reason, I would avoid it even if your particular version of Perl still supports it. (Update: If you intend ever to publish your module, I would absolutely avoid it!) Win8 Strawberry 5.30.3.1 (64) Sun 04/11/2021 19:15:44 C:\@Work\Perl\monks >perl use strict; use warnings; use Data::Dump qw(dd); my %attrs = qw(foo 1 bar 2);; my @products = ({ 'id' => 0, 'name' => 'Test', 'description' => 'Some test data', 'qty' => 1, 'price' => 1000, }); $attrs{'trolley'} = \@products; my $self = \%attrs; my $new_product = { qw(id 1 name Widget) }; push $self->{'trolley'}, $new_product; # push @{ $self->{'trolley'} }, $new_product; dd \%attrs; ^Z Experimental push on scalar is now forbidden at - line 22, near "$new_ +product;" Execution of - aborted due to compilation errors. [download] Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re^2: Pushing hash ref onto array ref by Fletch (Bishop) on Apr 12, 2021 at 05:08 UTC
I'd use `push @{ $hash->{'key'} }, $new` out of habit personally, but if you've got a new enough perl (post 5.24.0 according to perlref; you can use back to 5.20 with a feature declaration) you could use the new postfix deref syntax `push $hash->{'key'}->@, $new` instead. Edit:* Clarified version requirement. The cake is a lie. The cake is a lie. The cake is a lie.	[reply] [d/l] [select]
Re^3: Pushing hash ref onto array ref by kcott (Archbishop) on Apr 12, 2021 at 10:58 UTC
I've been making a point of using "perlref: Postfix Dereference Syntax". It removes the need for nested braces and brackets (e.g. `%{$aref->[$i]}` and `@{$href->{$key}}`) which can be confusing. It also reads linearly; i.e. `ref->deref_to` (e.g. `$ref->%`) vs. `deref_to{ref}`. For very simple dereferencing, I often find myself using the older (original) syntax (e.g. `$$scalarref`). This is no doubt out of years of habit and muscle memory. I still think `$scalarref->$` is clearer and I try to remember to use that; it also adds consistency to my code. I also find "perlref: Postfix Reference Slicing" to be a lot clearer; especially when the initial reference is a complex variable itself. Of course, that all requires a sufficiently new version of Perl. For instance, I can't use that for $work where I'm constrained to `5.16`; for personal code, where I'm using the latest Perl (currently `5.32`) that's not an issue. — Ken	[reply] [d/l] [select]
Re^4: Pushing hash ref onto array ref by Bod (Parson) on Apr 12, 2021 at 11:51 UTC
Re^2: Pushing hash ref onto array ref by Bod (Parson) on Apr 12, 2021 at 11:54 UTC
(Update: If you intend ever to publish your module, I would absolutely avoid it!) Yes...this module is probably of sufficient utility to be heading for CPAN so expect a few more questions about it and an RFC when I get that far...	[reply]
Re: Pushing hash ref onto array ref by Marshall (Canon) on Apr 11, 2021 at 23:25 UTC
Well, in my testing, for line "`push $self->{'trolley'}, $new_product;`", I get "experimental push on a scalar now forbidden". So I would not do that! Your second version, `push @{$self->{'trolley'}}, $new_product;` appears to be "the answer". update: I was using Perl 5.24. When I see "experimental", I will "play with" the feature. But not put it into production code. In this case, even if #1 had worked, I would use #2 because the code is more clear.	[reply] [d/l] [select]
Re^2: Pushing hash ref onto array ref by Bod (Parson) on Apr 12, 2021 at 11:47 UTC
I was doing my testing on the server where the code is going to be running which uses v5.16.3 which explains why you get warnings I don't. I should have tried it locally where I have v5.32.1 but it didn't even occur to be that there might be a versioning issue!	[reply]