Re: some questions re. Ada/GNAT from a C++/GCC user

[leschkes@ferret.cig.mot.com (Scott Leschke)]

wnewman@netcom.com (Bill Newman) writes:

>When I make two different instantiations of a generic package with the
>same arguments, I understand the compiler treats them formally as two
>different packages, which is OK with me.  However, I'd appreciate
>knowing the compiler wouldn't actually output two redundant copies of
>the corresponding (identical?) machine code, but instead share the
>code.  I saw somewhere that the compiler is given considerable freedom
>to share one instantiation between several arguments if it thinks it's
>appropriate, which is also OK with me.  However, I haven't seen any
>guarantee that the compiler won't output redundant copies for
>instantiations with identical arguments.  Is there such a guarantee?

My first question would be, why do you want redundant instantiations.
Note that Ada separates instantiation of a generic from use of the
resulting package/subunit.  The general rule of thumb to avoid code-bloat
due to duplicate instantiations is to do the instantiation at the library
level and 'with' the resulting package/subunit.

>Why doesn't Ada 95 allow declarations to be interspersed with ordinary
>statements as C++ does?  (Or does it?  _Ada as a Second Language_ is a
>big book!)  It seems to me that the C++ approach is a small but
>definite win.  Does it interact very badly somehow with all those
>guarantees on elaboration order?

You can use a block statement.  This is different than C++ in the sense
that objects declared within the block are only in existence within the
block and are finalized at the end.  A block can also have its own
exception handlers.  The syntactic form is (from page 500 of the LRM):

block statement ::=
  [block_statement_identifier:]
    [declare
       declarative_part]
    begin
       handled_sequence_of_statements
    end [block_identifier];

Note that the block identifier and declaration section are optional.


For example:

declare
   Obj : Pkg.SomeType;
begin
   Pkg.Operation (Object => Obj);
   -- Other stuff
exception
   when Pkg.Some_Exception =>

      Do_Something;
end;
-- 
Scott Leschke.........................email: leschkes@cig.mot.com
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