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Date: Tue, 10 Jul 2007 10:07:12 +0200
From: Georg Bauhaus <bauhaus.rm.tsoh@maps.futureapps.de>
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Newsgroups: comp.lang.ada
Subject: Re: Translating C++ class types into Ada tagged types?
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Date: 2007-07-10T10:03:36+02:00
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jimmaureenrogers@worldnet.att.net wrote:

> 
> Ada is object oriented, but does not provide constructors or
> destructors. The following code is similar to your example
> without constructors or destructors.

Just to add that Ada does have construction and destruction
(I am sure Jim Rogers knows them well), the language provides
a number of ways to make sure that initialization and
finalization takes place (and when it takes place!).
So "Cat Frisky(5)" can be expressed in Ada as well, just not
using constructors as in C++. 

One such facility well suited to polymorphic objects uses a
factory function such as

   function Make 
     (species: String; age: Natural) return Animal'class;
    -- an animal as requested in `species`, `age` years old

where Animal could be a superclass of Cat in C++ terms.
The 'class in Animal'class means that Make will return an object
from the Animal hierarchy. If you wanted just a Cat or more
specific heirs of Cat, say a cat that has fur of some color,
use

   function Make 
     (fur_color: Color; age: Natural) return Cat'class;
    -- some cat whose fur has the given `fur_color`,
    -- `age` years old

And, finally, a simple construction function for Cat specifically
and not heirs, but simply Cats could be

   function Make 
     (age: Natural) return Cat;
    -- a cat, `age` years old

(Unlike C++, Ada can use the type of the return value (Cat)
in order to decide what specific type of object to expect on
the LHS of ":=", for example.)

This isn't the only way to initialize objects, either
at run-time, or at compile time. In particular, you can
have default initialization etc..


 -- Georg