Re: Pass by reference

[dan.r.mcleran@seagate.com (Dan McLeran)]

> 
> > I am investigating using Ada for a project at work 
> 
> Excellent choice :).
> 
> > and I have a question I hope someone can answer. If I am
> > interpreting the RM section 6.2 correctly, an array of elementary
> > types does not meet the criteria of a by-reference type.
> 
> A 'by-reference type' is simply one that is always passed by
> reference, never by copy. 

That's what I thought.

> 
> > But, when looking at the disassembly, it seems that passing an array
> > of elementary types to a function as an in-mode parameter is passing
> > by reference. Does this mean that an array of elementary types is an
> > unspecfied type?
> 
> The compiler is always free to choose either copy or reference for
> passing parameters.

This statement seems to contradict the RM and what you've said above.
The way I read the RM, a by-reference type is passed by reference. No
choice. A by-value type is passed by value. Again, no choice. The RM
seems to indicate that the only time a compiler is free to choose is
for the 'unspecified' types.

> 
> I'm not clear what you mean by "unspecified type". Hmm, perhaps you
> are refering to 6.2(11): 
> 
>     For parameters of other types, it is unspecified whether the
>     parameter is passed by copy or by reference.

Exactly.

> 
> Yes, an array of elementary types is an "other type" in this sense; it
> may be passed either by reference or copy. The compiler will choose
> whichever mechanism it feels is "best".

That's what I assumed when reading the RM. I'll play around with small
arrays and see if the compiler decides by-value is better.

> 
> > A 2nd part to my question is: Does Ada automatically pass tagged types
> > by reference no matter what mode the parameter is specfied (in, in
> > out, or out)? The language RM 6.2 seems to suggest this is so. 
> 
> Yes.
> 
> > If so, does this mean that there is no way to pass a tagged type by
> > value? 
> 
> Yes.
> 
> > I believe C# works this way without programmer intervention.
> 
> Don't know.

> A couple of larger questions; 
> 
> Why do you care whether a type is passed by copy or reference? 
> 
> If you are worried about time or space efficiency, specify the
> appropriate option to the compiler, and forget about it, until
> measurements of your final application show a problem.
> 
> Why would you want to pass a tagged type by copy?
> 
> You might want to read the Ada Rationale, and the annotated Ada
> Reference manual, on this topic. See
> http://www.adaic.com/standards/ada95.html