Re: What databases have taught me

From: Robert Martin <unclebob_at_objectmentor.com>
Date: Sun, 2 Jul 2006 18:50:42 -0500
Message-ID: <2006070218504217709-unclebob@objectmentorcom>

On 2006-06-30 20:44:16 -0500, Chris Smith <cdsmith_at_twu.net> said:

> Robert Martin <unclebob_at_objectmentor.com> wrote:

>> I've looked over the thread you referenced above.  You asked whether 
>> 'this' and 'self' were kind of like fields in OO languages.  I refuted 
>> this by saying that, at least in C++, 'this' is not like a field, 
>> because it's value depends on the method being called.  If the method 
>> of a base class is called then 'this' will point to the base class and 
>> not to the class of the object itself.  (This only matters in cases of 
>> multiple inheritance).  You then asked me for the definition of 
>> "field", and I did not answer that (yet).

>
> I suppose they say "fools rush in...", so I'll prove that about myself
> now. :) (By which I mean, PMFJI.)
>
> I don't find it particularly compelling to distinguish this/self from
> instance fields in that way, especially since the distinction appeals
> primarily to implementation rather than the actual meaning of what's
> going on. I could very easily implement C++ in such a way that 'this'
> remains constant regardless of its static type in terms of the
> inheritance hierarchy. The only thing I'd lose is a bit of performance
> in metghod dispatch.

You'd also lose a bit of performance in accessing fields. What's more, I'm not sure the language would be C++ any longer; though I'm not sure of that.
>
>

>> So, in OO, a field is more often known as an instance variable.

>
> Wouldn't it be quite the opposite? I've never heard a Smalltalker go
> on about instance variables.

I actually learned the term (in 1985) from the Smalltalk-80 book which created the distinction between instance-variable and class-variables. The C++ vocabulary is "member variable".

-- 
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Received on Sun Jul 02 2006 - 18:50:42 CDT