Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!microsoft!jimad
From: jimad@microsoft.UUCP (Jim ADCOCK)
Newsgroups: comp.std.c++
Subject: Re: Creeping feature #769: `new auto'
Keywords: dynamic stack allocation, alloca
Message-ID: <72915@microsoft.UUCP>
Date: 14 Jun 91 19:29:44 GMT
References: <1991May2.074928.25507@kestrel.edu> <12759@exodus.Eng.Sun.COM> <1991May27.233808.14632@hal.com>
Reply-To: jimad@microsoft.UUCP (Jim ADCOCK)
Distribution: comp.std.c++
Organization: Microsoft Corp., Redmond WA
Lines: 134

In article <1991May27.233808.14632@hal.com> shap@hal.com (Jonathan Shapiro) writes:
>Until recently, I was inclined to agree with you. Since then, I have
>run into at least one case where the need to use malloc/free becomes
>impossibly cumbersome: functions that do error recovery or error-based
>abandonment.
>
>Consider a function that does the following:
>
>	allocate something
>
>	conditionally return failure
>
>	allocate something else
>
>	conditionally return failure
>
>	allocate something else
>
>	deallocate everything
>	return success
>
>In the absence of alloca(), the management of deallocation becomes a
>very likely source of programmer error, especially as the function is
>maintained by third parties.
>
>In my view, this is a compelling reason to have alloca.

I disagree.  Its easy to solve the kinds of problems that alloca can
solve from within C++ -- see the following simple-minded example.
Alloca simply allows one to create variable sized objects of known
lifetime [namely the lifetime of the surrounding routine] and this
is not a hard problem anyway.

Rather, the hard problem is managing the deallocation of objects of
indefinite lifetime, which in general is not a solvable problem excepting
the use of garbage collection.  To which I propose:  Garbage Collection.
Given garbage collection, any advantages of alloca go away anyway,
since GC schemes exist where allocation simply becomes an inline ptr
addition -- which is as efficient or better than alloca anyway.

----



extern "C"
{
#include <stdio.h>
#include <stdlib.h>
#include <malloc.h>
}

void* traced_malloc(size_t size)
{
	void* p = malloc(size);
	printf("allocate %d bytes at $%lX\n", size, (long)p);
	return p;
}

void traced_free(void* p)
{
	printf("free at $%lX\n", (long)p);
}

void* operator new(size_t size) { return malloc(size); }
void operator delete(void* p) { free(p); }

#define malloc(x) traced_malloc(x)
#define free(x) traced_free(x)

int failed() // simulate unpredictable failures
{
	return !(3 & rand());
}

class Tracker
{
public:
	void* p;
	operator void*() { return p; }
	Tracker(void* pT) : p(pT) {}
	Tracker() : p(0) {}
	~Tracker() { if (p != 0) free(p); }
};

#define FAILURE 1
#define SUCCESS 0

int troublesome_routine()
{
	// allocate something:

	Tracker p1 = malloc(100);

	// conditionally return failure:

	if (failed())
		return FAILURE;

	// allocate something else:

	Tracker p2 = malloc(200);

	// conditionally return failure:

	if (failed())
		return FAILURE;

	// allocate something else:

	Tracker p3 = malloc(300);

	// conditionally return failure:

	if (failed())
		return FAILURE;

	// trackers will automatically deallocate everything, so just:

	return SUCCESS;
}


// lets try these "troublesome" routine a few times and see what happens:

main()
{
	for (int tries=0; tries<10; ++tries)
	{
		printf("----------------------\n");
		troublesome_routine();
	}
}

// ---- its no trouble at all!