Re: generics in Ada 83

[Jeffrey Carter <spam@spam.com>]

REH wrote:
> 
> Well I guess there is always a better way.  Here is my actual problem.
> My team maintains a set of middleware services for various
> applications.  We have a Client/Server package as part of this.  It
> defines a header type for transmitting messages across TCP/IP.  One
> application group has requested the ability to define their own header.
>  There are certain pieces of information our software needs to know
> from the header, such as the message length.  So I re-defined the
> package thus:
> 
> generic
>     type Header_Type is private;
> 
>     with function Get_Length(Header : in Header_Type) return Integer;
> 
>     with procedure Set_Length(Header : in out Header_Type;
>                               Length : in Integer);
> package Client_Server;
> 
> This works fine for using most of the services in the package.  But the
> thorn is the package provides a "synchronous send" feature that does a
> request/response type of transaction over a network.  This requires
> several more fields to be present in the header and accessed at various
> times.  Thus, I have added get and set subprograms for those.  But I
> don't want a client application that does not use this feature to have
> to define stubs for all these.  Which is where I am tried to find a way
> to "default" them.

Then you're in good shape. 1st, define your complete generic for using 
the synchronous send feature:

generic -- Client_Server_With_Synchronous_Send
    type Header_Type is private;

    -- many formal subprograms
package Client_Server_With_Synchronous_Send is
    procedure Op (Header : in out Header_Type);

    ...
end Client_Server_With_Synchronous_Send;

then define your shorter generic:

generic -- Client_Server
    type Header_Type is private;

    with function Get_Length(Header : in Header_Type) return Integer;

    with procedure Set_Length(Header : in out Header_Type;
                              Length : in Integer);
package Client_Server is
    -- presumably a smaller set of operations than
    -- Client_Server_With_Synchronous_Send
    procedure Op (Header : in out Header_Type);

    ...
end Client_Server;

Now, the body of Client_Server instantiates 
Client_Server_With_Synchronous_Send and implements its operations in 
terms of Client_Server_With_Synchronous_Send's operations:

with Client_Server_With_Synchronous_Send;
package body Client_Server is
    -- Define stub subprograms for instantiation here.

    package Synchronous is new Client_Server_With_Synchronous_Send
       (Header_Type => Header_Type,
        Get_Length  => Get_Length,
        Set_Length  => Set_Length, ...);

    procedure Op (Header : in out Header_Type) renames Synchronous.Op;

    ...
end Client_Server;

-- 
Jeffrey Carter
"Now go away or I shall taunt you a second time."
Monty Python and the Holy Grail
E-mail: jeffrey_r_carter-nr [commercial-at]
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