Re: Realtime cost of call by value

[Per Persson]

On Saturday, November 1, 2003, at 02:53 AM, John W. Eaton wrote:
> I would be interested in a way to pass arguments by reference, but it
> must not be the default.  Here is a (half-baked) proposal to think
> about.

I for one would really appreciate an option to pass arguments by 
reference.
(Actually, the idea gives me the same feeling I had on a christmas 
morning as a child;-)

> Similar to C++, I think the notion of a reference argument should be a
> property of the function definition, not the code that calls the
> function.

Yes, definitely.

> So we would write something like
>
>   function F (reference A)
>     ...
>   endfunction

this is OK,

> or perhaps, if you prefer concise and cryptic
>
>   function F (&A)
>     ...
>   endfunction

nope, this is probably very alien to people who doesn't know C++.

> to declare that A is passed by reference for all calls to F rather
> than something like

[snip]

> I'm not sure what syntax we should use, but a unary & as above would
> be one possibility that would not be incompatible with Matlab syntax
> (at least currently).

FWIW, in Objective-C the keywords "byref" and "bycopy" are used to 
state how live objects passed between different threads or processes 
should behave. Again, this is a property of the method (function) that 
receives the message (call) rather the calling statement.
Even with pass by copy as the default, I think it might be of some use 
to have a keyword such as "bycopy", which would evaluate to a 
whitespace, that _could_ be used for clarity.

> We could also introduce a way to make one variable a reference to
> another apart from argument lists.  Something like
>
>   reference A = B;
>
> or perhaps
>
>   &A = B;
>
> would cause A and B to share the same value in memory.  If A changes,
> so does B, or if B changes, so does A.  If you did something like
>
>   C = B;
>   reference A = B;
>
> then changed A, only A and B would change, not C.

I'm not sure about this...
In defining function F(byref A) we state that A is passed by reference, 
but
with = interpreted as an assignment one could argue that the construct 
should be

A = byref B;

i.e. A is assigned a reference to B.
(I may be completely off here though...)

This part, however, I don't see as important as the option to 
explicitly state a function as taking a parameter by reference.

Just my 0.02€

/Per




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