Re: Return by reference

[Robert A Duff <bobduff@world.std.com>]

Brian Rogoff <bpr@shell5.ba.best.com> writes:

> I understand why, for a return-by-reference type T, the function 
> 
> function Some_Func( ... ) return T is 
>     Local_Var : T
> begin
>     ...
>     return Local_Var;
> end Some_Func;
> 
> causes Program_Error to be raised at run time, but I'm a bit 
> perplexed as to why there is a problem for 
> 
> function Some_Func( Param : T; ... ) return T is
> begin
>     ...
>     return Param;
> end Some_Func;
> 
> Could someone explain why the latter case is problematic? 

We wanted the checks to be done at compile time.

So you may well ask, why does this one raise Program_Error?  It's
because in a shared-body generic, we don't know enough to do the check
at compile time.  However, most compilers use macro-expanded generics,
so they can check the above at compile time, and just generate code to
raise P_E if the check fails, and, one hopes, give a message at compile
time.

If the above were allowed, then somebody could do this:

    function Mumble return T is
        Local: T;
    begin
        return Some_Func(Local); -- Wrong.
    end Mumble;

To detect that error would require keeping too much dope around at run
time.

On the other hand, if you want true run-time checks, you can use access
parameters.  Some extra dope is normally passed with an access
parameter.

- Bob