Re: Realtime cost of call by value

[Mark P. Esplin]

        For what it is worth, I like the C++ syntax with a "&" best then 
"byref" 
next.

                                      -Mark

On Monday 03 November 2003 9:37 am, Geraint Paul Bevan wrote:
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> John W. Eaton wrote:
> | Given the above definition of F, you could expect
> |
> |   F (X);     # OK, pass X by reference.  If F changes A, X also changes.
> |   F (1);     # ERROR, reference to constant.
> |   F (X+1);   # ERROR, reference to temporary.
> |   F (X(1));  # ERROR, reference to temporary.
> |
> |
> |
> | 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).
> |
> | 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;
> |
> |
> | What I'm most interested in now is what the syntax and semantics
> | should be, not precisely how it will have to be implemented.
> |
> | Comments?
> |
> | jwe
>
> I like the idea of creating references to arguments in function calls,
> however I'm not over keen on an error being produced for statements such
> as F(1) if the argument of F has been declared as a reference. It might
> be better to override the reference declaration and implicitly just pass
> a temporary value (perhaps with a warning to the user). I can't really
> see how any nasty errors would arise as a result of this; if the caller
> doesn't  make a variable available, presumably it would not care whether
> or not it is passed by value or by reference, i.e. "F(&rand(20,20))" and
> "F(rand(20,20))" should probably do the same thing.
>
> I don't understand why F(X(1)) should cause an error in your example.
> Why should a function not be able to alter the element of an array? I
> can think of examples where it might be desirable to pass some elements
> of an array to a function to be processed while ensuring that other
> elements are conserved:
>
> index = [1:10]'; data = rand(10,1); m = [index, data];
> F(&m(:,2)) # should allow column 2 to be altered but not column 1
>
>
> I like the idea of creating references to objects other than function
> arguments even more. In particular, I would like to be able to create
> references to substructures in large data structures and have the
> underlying data altered:
>
> for &my_substructure = some.large.data.structure.
> manipulate(my_substructure)
>
>
> As for the notation, the ampersand might be a bit confusing for people
> who haven't come across C++ before. Maybe a binary operator such as '->'
> would be more intuitive ("A -> B" means "A points to B"). At first I
> thought '=>' might be better as it doesn't clash with the C++ use of
> '->', but it would probably end up getting confused with '>='.
>
>
> - --
> Geraint Bevan
> http://www.mech.gla.ac.uk/~gbevan
>
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