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Path: utzoo!attcan!uunet!snorkelwacker!usc!zaphod.mps.ohio-state.edu!mips!gumby!murphy
From: murphy@mips.COM (Mike Murphy)
Newsgroups: comp.lang.ada
Subject: Re: Initialization
Message-ID: <39390@mips.mips.COM>
Date: 14 Jun 90 19:37:57 GMT
References: <1957@sparko.gwu.edu>
Sender: news@mips.COM
Reply-To: murphy@mips.COM (Mike Murphy)
Organization: MIPS Computer Systems, Sunnyvale, CA
List-Id: <comp.lang.ada>

In article <1957@sparko.gwu.edu> mfeldman@seas.gwu.edu (Michael Feldman) writes:
>I don't remember having seen any discussion on the group about why other
>types cannot be default-initialized, e.g.
>   type Little is range -10 .. 10 := 0;
>or
>   type Vector is array (1..10) of float := (1..10 => 0.0);
>or even
>   type Vector is array (integer range <>) of float := (others => 0.0);

I question how useful type initialization is given that we can easily
initialize variables, but I agree that it is an irregularity to only
allow it for record types.  When I first read this question I thought,
yea, that should be easy to do.  But then I thought of some complications.

To take your above example, suppose we later said:
	subtype PosLittle is Little range 1..10;
	p : PosLittle;
What is p initialized to?
If we implicitly initialize p to 0 we should raise constraint_error;
is that acceptable?

A related issue is initializing unconstrained arrays; either we limit
the initialization to the others clause, or we have situations like:
	type Vector is array (integer range <>) of float := 
		(0 => 0.0, others => 1.0);
	v : Vector(1..10);
What happens here?  Either we could raise constraint_error because there
is an initialization to a component outside the range of v, or we could
treat it like a variant part of a record that is not used and just ignore
that part of the initialization.  Which brings up the fact that type 
initialization is not free; it often requires building an implicit
procedure that the compiler calls to dynamically decide how to initialize
the object.

A last case is initializing access types; is anything other than "null"
a legal initial value, e.g.
	type astring is access string := new string'("void");
	a : astring(1..3);	-- constraint_error?

-- Mike Murphy
-- UUCP: sun!decwrl!mips!murphy or murphy@mips.com