Passing objects to another class in Perl

Question

What happens when i do something like this:

$stallion = Horse->new(gender => "male");
$molly = Mule->new(gender => "female");
$colt = $molly->breed($stallion);

I cant comprehend what is going on on that code?

1. What happens when that stallion reference is passed to another instantiation of class?
2. And in that code is breed another method in another class?

3. From that example, are there 2 different classes? (Horse class with a new method or constructor, Mule class with another new constructor).

   $colt = $molly->breed($stallion);

From that code, my understanding is that $molly is just the same as the Mule class which have other method than new - breed. Am i right?

Answer 1

From that example, are there 2 different classes? (Horse class with a new method or constructor, Mule class with another new constructor).

Quite correct.

Anytime you see an expression of STRING_LITERAL->METHOD_NAME(), that STRING_LITERAL is almost guaranteed to be a class name.

And when you see STRING_LITERAL->new(), unless the person who wrote the code decided to be too smart for their own good, this would be a call to that class' constructor, returning an object of that class. _{(However, the "new" being a constructor is merely a voluntary convention; there's nothing in Perl enforcing that - someone too smart/dumb COULD have defined "new()" sub to return a value of 2+3 - you have no guarantee aside from looking at the code and testing)}

After that example, you get, hopefully:

• $stallion - holding a value returned from Horse class constructor - will have as a value an object of class Horse.

• $molly - holding a value returned from Mule class constructor - will have as a value an object of class Mule.

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What happens when that stallion reference is passed to another instantiation of class?

• What happens in $molly->breed($stallion); call - as was answered in yesterday's question, this is an object way of calling a method, so what you do is:

  • Create a list of parameters (in this case, $stallion)
  • Prepend the object to it, resulting in ($molly, $stallion)
  • Pass that list to breed() subroutine, which must of course be defined in the class that $molly object belongs to (e.g. be a method in Mule class)
  • Within breed() subroutine, special list variable @_ will be aliased to elements of this parameter list ($molly, $stallion).
  • What happens then depends 100% entirely on how the code for "breed" sub is written

And in that code is breed another method in another class?

As noted in a bullet point above, yes, if $molly is an object of a certain class (Mule), an expression $molly->METHOD_NAME() means calling a method of that name that is known (defined or inherited from parent class) in the class of which $molly is an object of.