Re: C Strings in Ada?

[murphy@mips.COM (Mike Murphy)]

In article <57288@bbn.BBN.COM> adoyle@vax.bbn.com (Allan Doyle) writes:
>When I am using the Ada "new" keyword, I know full well that I'm creating
>potential for garbage. I can control when I call it. I can write my own
>memory manager around it. I don't get that control when I need a construct
>like
>
>	declare
>		c_string : constant string := "foo" & ascii.nul;
>	begin
>		<stuff>
>	end;
>
>If I can't even trust Ada to reclaim the 4 or so bytes used by foo, what
>can I trust Ada to do????

Most Ada compilers will reclaim the space used by an object when it 
exits its scope.  I say "most" because this is not required by the 
semantics of the language, but any good compiler will have this feature, 
as it is needed for large software systems, and it is not too difficult
to implement.  Basically, you emit some code at the end of any blocks 
or statements that use temporary space, to reclaim that space.
This might mean a "free", or it could be something even cheaper 
(e.g. some compilers will put these kind of block-structured temporaries 
on a mark-release stack).  You can code up a little example like the above
to test what your compiler does.  This is not considered to be "real"
garbage-collection, which no Ada compiler I know of does, which requires
searching through memory for dangling pointers.  Instead this is just
an important optimization for a common situation in Ada.  The situation
is more common in Ada than in languages like C because we can have
function calls that return dynamic-sized objects, such as strings,
and these need temporary space.  The reclaiming of the space is helped
by the block-structured nature of its scope (as opposed to the
full-blown garbage collection problem).

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