Re: Access type to member procedure of instance (Object Oriented programming in Ada)

[Brian Drummond <brian@shapes.demon.co.uk>]

On Mon, 19 Nov 2012 01:59:42 -0800, ake.ragnar.dahlgren wrote:

> Not sure if this has already been discussed but: Is it possible to
> define a pointer to a member procedure of an instance?

Yes.

with Some_Package;

procedure Main is
   Message : Some_Package.Message_Type;

   Method_Ref : access procedure (Message : Some_Package.Message_Type) 
		:= Some_Package.Print'Access;
begin
   Method_Ref.all(Message);
end Main;

However this is not the same question as:

> Is it possible to create a parameterless access type to the
> Message.Print procedure? The following application refuses to compile:

Message.Print is not parameterless, it is identical to Print(Message) 
so ... no. 

(This is not specifically Ada, in any OO language I have ever seen, the 
receiver is always the first parameter. I don't know of any language that 
replicates every method for every instance of a class, but I suppose it's 
theoretically possible!. Unless "Message" is a singleton, it would be 
horribly inefficient)

You can certainly define a pointer to the member procedure, but it still 
expects a parameter, and you still have to supply that.

You can certainly wrap it in a parameterless procedure (binding Message 
to Print) and create a parameterless access to that... 
with Some_Package;

procedure Main is
   Message : Some_Package.Message_Type;

procedure PrintMessage is
begin
   Message.Print;
end PrintMessage;
   
   Method_Ref : access procedure := PrintMessage'Access;
begin
   Method_Ref.all;
end Main;

but I have a feeling you are trying to accomplish something else here and 
I can't see what. 

Hopefully someone else has a better idea...

- Brian