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From: mhyman@hsfmsh.uucp (Marco S. Hyman)
Newsgroups: comp.lang.c++
Subject: Questions about new and array definitions
Message-ID: <2473@hsfmsh.UUCP>
Date: 7 Mar 90 20:19:14 GMT
Sender: mhyman@hsfmsh.UUCP
Reply-To: marc@dumbcat.UUCP (Marco S. Hyman)
Organization: SoftCom, Inc.  San Francisco
Lines: 47

A co-worker came up with a problem that brings up an interesting C++
question.  He wanted to create a pointer to an array of void pointers.
The definition he used was:

	void * (*a)[];

In the constructor of the class `a' was defined in he tried to allocate
memory using

	a = new (void *)[size];  // size was passed to the constructor

Cfront complained with the error:

	error: bad assignment type: void *(*)[] = void *

When the problem came to me I first tried some things to see if I could
get memory allocated for `a' as defined.  Things tried were

	a = new void *(*)[size];
	error: syntax error

	a = new (void *(*)[size]);
	error: bad assignment type: void *(*)[] = void *(**)[size]

	a = new void **[size];
	error: bad assignment type: void *(*)[] = void **[size]

After several tries I came up with something cfront liked

	a = new (void *(*)[])[size];

The problem is that the code generated was not what was expected.  Memory
was allocated for one void pointer; size was not part of the allocation
equation.  The generated code looked like (cleaned up for easier reading):

	this->a = (((char *(**)[]) __nw__FUi(sizeof(char *(*)[]))))[size];

The solution was, of course, to get the [] out of the definition and use

	void ***a;

	a = new void **[size];

What is void * (*)[] then?  Is there a way to allocate memory for it using
new?

// marc