Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucsd!usc!apple!escher
From: escher@Apple.COM (Michael Crawford)
Newsgroups: comp.lang.c++,apple.general
Subject: Failed allocation in constructors?
Keywords: exception handling
Message-ID: <9075@goofy.Apple.COM>
Date: 10 Jul 90 23:07:39 GMT
Organization: Apple Computer Inc., Cupertino, CA
Lines: 78

Can anyone suggest strategies for handling failed memory allocation
in constructors?

Constructors may not have return values, and aren't even void.
I have seen some references to how there is not unified method, and
one just needs to hack it.

Example source code I have seen does not even address the issue, and
just assumes the construction succeeds.

The problem, as I see it, is that what one does upon memory failure
depends greatly on the situation: do you crash and burn, wait, advise
the user to close documents, or free a sentinel block (a large block
allocated at the start of a program to provide a reserve) and retry
the allocation?

Here is a the problem:

class foo {
	char *thePtr;
public:
	CauseACrash();
	foo();
	~foo();
}

void theFun()
{
	foo theFoo;	// foo(), the constructor, gets called

	foo.CauseACrash();

	// ~foo() gets called here, if we're still running!
}

foo::foo()
{
	thePtr = NewPtr( 123456789 );

	if ( thePtr == (char*)NULL ){
		// Whadda I do?
	}
}

foo::CauseACrash()
{
	int i;
	
	for ( i = 0; i < 123456789; i++ ){
		*i = 'Q';
	}
}

On a Mac, CauseACrash will scrawl all over the interrupt vector table; on
Unix, it should core dump (if page 0 is mapped out, as good Unices do).

Seems to me a member function ought to be able to assume that its instance
is valid, and one should not have to validate every instance one creates;
the fact that the object is dynamically allocated should be hidden to the
user.  This would mean that the failure should be handled within the
constructor.

I can conceive of some ways to do it, but I am a beginner here, and having
enough trouble figuring out the language.  Others must have invented this
wheel before, care to roll one by me?

I will summarize and post if you mail me suggestions, though this looks
like a general enough problem to have an open discussion.

Thank you very much,
-- 
Michael D. Crawford
Oddball Enterprises		Consulting for Apple Computer Inc.
606 Modesto Avenue		escher@apple.com
Santa Cruz, CA 95060		Applelink: escher@apple.com@INTERNET#
oddball!mike@ucscc.ucsc.edu	The opinions expressed here are solely my own.

		alias make '/bin/make & rn'