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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: I/O streaming with custom data transport
Newsgroups: comp.lang.ada
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References: <ejv4u6$qvu$1@cernne03.cern.ch>
Date: Tue, 21 Nov 2006 20:02:04 +0100
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Date: 2006-11-21T20:01:58+01:00
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On Tue, 21 Nov 2006 16:11:02 +0100, Maciej Sobczak wrote:

> In C++ the IOStreams library allows to vary the behaviour of the stream 
> by decoupling formatting from data buffering and transport to the 
> physical device, which are in turn strategies for the stream object. 
> This means that having millions of functions like this:
> 
> void foo(ostream &s)
> {
>      s << "Hello";
>      s << someObject;
>      s << someOtherObject;
>      // ...
> }
> 
> (and *there are* millions of functions like this)

Alas, because it is a poor design based on ad-hoc polymorphism. All these
millions are overloaded. They should be overridden, but that were
impossible due to lack of multiple dispatch.

> I'm looking for something like this in Ada.

In Ada it is exactly same. Consider:

   Put (S : in out My_Root_Stream; X : String);
   Put (S : in out My_Root_Stream; X : SomeObject);
   ...

An alternative design (still non-MD) is:

   Put (S : in out Root_Stream'Class; X : String);
   Put (S : in out Root_Stream'Class; X : SomeObject);
   ...

This achieves what you want. You can implement I/O on an object type like
String using some common class-wide functionality of streams and then
re-use it over all possible streams.

> The basic I/O facilities in the standard library don't seem to provide 
> anything like this.
> I hoped that Ada.Streams allows this by subclassing Root_Stream_Type and 
> providing some overriding operations, but unfortunately I cannot even 
> find the specification of Root_Stream_Type (looks like there isn't any 
> and this type is just a name placeholder in ARM).

As I said it is basically same. But Ada also offers a sort of hard-wired
double dispatch through S'Read/Write, S'Input/Output attributes. See ARM
13.13.2.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de