Re: Extending A Generic Signature Package

[bobduff@world.std.com (Robert A Duff)]

In article <5gvgg0INNj32@snoopy.cis.ohio-state.edu>,
david scott gibson <dgibson@snoopy.cis.ohio-state.edu> wrote:
>Okay, I've primarily been using limited tagged types lately and forgot
>that this rule only applied to them.  Nevertheless, I assume compilers
>may return tagged types, especially big ones, by reference.

No, not unless the compiler can prove it makes no difference.  E.g.:

    Global: T'Class := ...;

    function F return T'Class is
    begin
        return Global;
    end F;
    
    X: T'Class := F;
    ...
    Global.Component := 3;

X and Global are not aliased.  A copy must be made at the return
statement.  The assignment to a component of Global does not modify X.
(The fact that the generated code might assume that F returns the
address of the result in register R0 is irrelevant -- if that's what's
going on, R0 will point to a *copy* of Global.)  If T were limited, then
F would return its result by reference, but then the above would be
illegal, due to the ":= F".

>...  I didn't
>mean to imply that objects passed by reference should be thought of as
>"pointers".  Ada seems to do a good job of making the mechanism
>transparent.  

Parameter passing mechanism is not transparent in Ada -- you can tell
the difference between by-ref and by-copy, and Ada allows either one
(compiler's choice) in some cases.  (I happen to think that's a flaw in
the language.  We've discussed it here before.)  For function return, on
the other hand, the language always requires by-ref or by-copy -- the
choice is not up to the compiler.

- Bob