Re: Binary and XML serialization of types

[adambeneschan@gmail.com]

On Thursday, January 23, 2014 10:53:23 AM UTC-8, han...@gmail.com wrote:
> Hello,
> 
> I am an Ada beginner who is working on a private project.
> 
> The project is to implement a protocol which either does binary or xml serialization of the different defined records. 
> 
> I have tried to separate all the "encoding" details from the type declaration in a sub package like this:
> 
> 
> 
> package A.Types is
> 
>    type Guid_Array is array(1 .. 8) of Unsigned_8;
>    type Guid is  record
>       Data1 : Unsigned_32;
>       Data2 : Unsigned_16;
>       Data3 : Unsigned_16;
>       Data4 : Guid_Array;
>    end record;
> 
> end A.Types;

I think you simply want to do something like this.  Put the declaration of Guid_Write and the "for" clause in A.Types.  Then, in the body of A.Types:

    with A.Types.BinaryEncoder;
    package body A.Types is

       -- other stuff as needed

       procedure Guid_Write( 
           Stream : access Ada.Streams.Root_Stream_Type'Class; Item : Guid )
               renames A.Types.BinaryEncoder.Guid_Write;
                   -- this is called a "renaming-as-body"

or this, which amounts to the same thing:

    with A.Types.BinaryEncoder;
    package body A.Types is

       -- other stuff as needed

       procedure Guid_Write( 
           Stream : access Ada.Streams.Root_Stream_Type'Class; Item : Guid ) is
       begin
           A.Types.BinaryEncoder.Guid_Write (Stream, Item);
       end Guid_Write;

(Note: I think the "renames" will work, but I haven't tested it.  The second one will definitely work.)

Now you declare and implement Guid_Write in A.Types.BinaryEncoder as you were trying to do.  (You don't actually need to give it the same name.  You can call your "implementation" procedure Guid_Write_Impl, or Any_Other_Name_You_Feel_Like.)

What's going on is that if some client package says "with A.Types" and uses the Guid_Write type, and uses Guid_Write'Write(...) or Guid_Write'Output(...), the client has to know that there's a Write routine that isn't the default.  That's why the "for Guid_Write'Write use ..." has to be in the visible part of A.Types, so that other clients are allowed to know about it.
 
                            -- Adam