Inheriting from an abstract base class is dictating an object's implementation. To me, there's a world of difference between saying "even though this mock object inherits absolutely no data or behavior from this class hierarchy, it's a member of this class hierarchy" and "this mock object has the same fingerprint as objects of this class and can be used in their place".

A mock object is not a DBI object. It's not a CGI object. It can act like either though.

I think there's a broader principle here -- substitutability. Inheritance is not the only means by which objects of two different classes can be substitutable. There's also composition and delegation (the "has a" relationship) and equivalence of behavior (the "acts like a" relationship). Making an abstract class would work, but it doesn't express the nature of the substitutability.

I'd like to avoid forcing composition, delegation, and equivalence relationships into the inheritance scheme. If my substitutable object does not fit within the class hierarchy conceptually, why should I have to lie to the compiler and all future maintenance programmers and say that it does?

(I'm very much in agreement with your comments on Java, though. Pity that a good concept sucked up the proper terminology so much that people think I like that approach.)

In reply to Re: Re: Class::Interface -- isa() Considered Harmful by chromatic
in thread Class::Interface -- isa() Considered Harmful by chromatic

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.