Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>There are two ways to approach this:</p> <h2>1. Package variables</h2> <p>Not each animal instance has an UA, but the UA is part of being an Animal. You can initialize the UA on startup. The animals can access the UA via an accessor, or a variable:</p> <pre><code>package Animal; my $ua = LWP::UserAgent-&gt;new(); sub ua { $ua } </code></pre> <p>and then</p> <pre><code>my $ua = $self-&gt;ua(); </code></pre> <p>or</p> <pre><code>package Animal; our $ua = LWP::UserAgent-&gt;new(); </code></pre> <p>and then</p> <pre><code>my $ua = $Animal::ua; </code></pre> <p>Technical terms for this are <em>class variable</em>, or in languages like Java <em>static method</em> or <em>static variable</em>.</p> <p>Java translations:</p> <pre><code>class Animal { private static LWP.UserAgent ua = new LWP.UserAgent(); public static LWP.UserAgent getUa() { return ua } ... } </code></pre> <p>and</p> <pre><code>class Animal { public static LWP.UserAgent ua = new LWP.UserAgent(); ... } </code></pre> <h2>2. <code>state</code> variables</h2> <p>In newer Perls (>= v5.10?) you can declare variables with <code>state</code>. They declare lexical variables. The variable is initialized once during the lifetime of the program, not every time the statement is executed (as it would be with <code>my</code>).</p> <pre><code>use feature 'state'; package Animal; sub new { my ($class, %args) = @_; state $ua = LWP::UserAgent-&gt;new; bless { ua =&gt; $ua, %args } =&gt; $class; } </code></pre> <p>and then</p> <pre><code>my $ua = $self-&gt;{ua}; </code></pre> <p>It may be useful to use a <code>do</code> block to perform wider intialization</p> <pre><code>state $ua = do { my $ua = LWP::UserAgent-&gt;new; ...; $ua; }; </code></pre> <p>or to offload initialization into a sub: <code>state $ua = make_ua();</code></p> <p>If you have to target earlier perls, you can enclose the <code>new</code> sub inside a seperate scope, and declare the variable there as a lexical for similar effect:</p> <pre><code>package Animal; { my $ua = LWP::UserAgent-&gt;new; sub new { my ($class, %args) = @_; bless { ua =&gt; $ua, %args } =&gt; $class; } } </code></pre> <p>The only difference here is <em>when</em> the UA is initialized. (And, of course, the number of irritating braces.) The intialization can be deffered, as <a href="https://stackoverflow.com/users/40285/len-jaffe">Len Jaffe</a> mentioned:</p> <pre><code>package Animal; { my $ua; # just declare scope here sub new { my ($class, %args) = @_; $ua ||= LWP::UserAgent-&gt;new; # $ua is false until initialized bless { ua =&gt; $ua, %args } =&gt; $class; } } </code></pre> <p>Because objects are references, all animals will get the same UA.</p> <p>Especially in C/C++, this is also called a <em>static variable</em>. The <code>state</code> modifier could be seen as a non-OO singleton constructor.</p>
    singulars
    1. This table or related slice is empty.
    1. POCan Perl handle this OOP concept - Explained in Animal Terms
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USCommunity
    1. USamon
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    1. COThere's also the Singleton pattern implementation in which the constructor calls LWP::UA->new() but 'caches' the reference in class variable: $ua ||= LWP::UA->new()
      singulars
      1. PO
      1. USLen Jaffe
    2. CO@LenJaffe Yes, one could go full OO and implement the actual singleton pattern. But, using `state` is the *perlish* way to go (I'd say).
      singulars
      1. PO
      1. USamon
    3. COWhy would the class variable be undef after having been instantiated by the first constructor?
      singulars
      1. PO
      1. USLen Jaffe
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload