Re: for S'Image use Func??

["Yannick Duchêne (Hibou57)" <yannick_duchene@yahoo.fr>]

Le Mon, 10 May 2010 22:56:11 +0200, Maciej Sobczak  
<see.my.homepage@gmail.com> a écrit:
> Coming back to I/O - what I miss in Ada is the equivalent of fread in
> C - that is, an operation that reads *up to* the given number of
> bytes. Or maybe there is something that I didn't notice?
It's there : look at [ARM 2005 13.13.1] which defines the package  
Ada.Streams, which in turn contains the following :

    type Root_Stream_Type is abstract tagged limited private;
    ...
    type Stream_Element is mod implementation-defined;
    ...

    procedure Read
      (Stream : in out Root_Stream_Type;
       Item   : out Stream_Element_Array;
       Last   : out Stream_Element_Offset)
       is abstract;

    procedure Write
      (Stream : in out Root_Stream_Type;
       Item   : in Stream_Element_Array)
       is abstract;


So, these are abstract. Now the question is : “where are the concret  
implementation” ? The answer is “in derived types”.

Here is one, as a quick and one of the most useful example (see [ARM 2005  
1.12.2]) :


    with Ada.Streams;
    package Ada.Text_IO.Text_Streams is
       type Stream_Access is access all Streams.Root_Stream_Type'Class;
       function Stream (File : in File_Type) return Stream_Access;
    end Ada.Text_IO.Text_Streams;

This re-define Stream_Access (why this access type is redefined here, is  
another story previously discussed with an answer from Randy), and define  
a function which returns a Stream_Access from a File_Type. This File_Type  
is the one defined in the parent package, that is, Ada.Text_IO.

In Ada.Text_IO, you have (see [ARM 2005 A.10.1]) some other interesting  
definitions, which will close the list of required stuff :


    function Standard_Input  return File_Access;
    function Standard_Output return File_Access;
    function Standard_Error  return File_Access;

Summary : in Ada.Text_IO, you have Standard_XXXX or other File_Type. You  
can use this (using an XXX.all to get a File_Type from a File_Access) as  
an argument to the Ada.Text_IO.Text_Streams.Stream function, which will  
return you a Stream_Access which will dispatch to an implementation of   
Ada.Streams.Read and Ada.Streams.Write.

Well, to be honest, this is not exactly the same as with C, as  
Ada.Streams.Stream_Element is implementation defined (which is good,  
because this is indeed platform dependent), so you will need an Assert  
pragma somewhere is your application, which ensures  
Ada.Streams.Stream_Element is 8 bits or at least 8 bits (this is, in most  
of case), and then convert from Ada.Streams.Stream_Element to your  
Byte_Type or anything your application defined as such.

Sorry for being long, I just wanted to give all the details. However, this  
is not so much complicated as all those explanations seems to suggest.

-- 
No-no, this isn't an oops ...or I hope (TM) - Don't blame me... I'm just  
not lucky