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From: olson@bootsie.UUCP (Eric Olson)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Think C 4.0 questions
Message-ID: <10@bootsie.UUCP>
Date: 2 Feb 90 18:54:39 GMT
References: <10682@bsu-cs.bsu.edu>
Reply-To: olson@bootsie.UUCP (Eric Olson)
Organization: Lexington Software Design, Lexington, MA
Lines: 69

In article <10682@bsu-cs.bsu.edu> mithomas@bsu-cs.bsu.edu (Michael Thomas Niehaus) writes:
>I have several questions that I hope someone out there can answer:
>
>1.  If you declare an array in an object class definition, is it safe to
>    use that array in a call to, say, an FSWrite routine?  I have created
>    a 1K buffer and when it fills up I want to call FSWrite to write the
>    whole chunk out.  Is this safe, or do I have to copy the whole array?
>    (Or can I lock it down?)

If you declared the array like so:

struct MyClass : Superclass {
	char	boofy[1024];
	}

then you'll want to lock the instance of the class any time you pass
this array to a routine that could move memory.  Lock the instance of
the object by doing a HLock(this).  Don't forget to unlock it!

A slightly better style might be to declare the buffer as a handle:

struct MyClass : Superclass {
	Handle	boofy;
	}

then in your initialization method for MyClass, do:

	boofy = NewHandle(1024L);

and when you want to access the buffer, do:

	HLock(boofy);
	RoutineThatMovesMem(*boofy);
	HUnlock(boofy);

You should also consider using HGetState and HSetState instead of
HUnlock, since it makes it possible for a subclass to lock the
handle and know that it won't be unlocked due to inherited method calls.

>2.  If you declare a superclass and then two or more subclasses, is it
>    possible to create an object of one subclass and assign it to an object
>    variable of the superclass?  I would like to have an array of objects of
>    different types (but of similar structures).

For example, if SpecialBoof is a subclass of BoofyClass, you can do:

	BoofyClass *	aBoof;
	SpecialBoof *	aSpecBoof;

	aBoof = aSpecBoof;

but not:

	aSpecBoof = aBoof;

This is because the struct for BoofyClass is just a subset of the
struct for SpecialBoof.  Note that objects always run their own
overridden versions of methods, even if they have been assigned
to a pointer of the superclass' type (e.g., coercing an object's
pointer does not change what methods it uses).

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