Path: utzoo!attcan!uunet!decwrl!sdd.hp.com!ucsd!rutgers!mcnc!rti!dg-rtp!farm!taylor
From: taylor@farm.rtp.dg.com (William Taylor)
Newsgroups: comp.lang.c++
Subject: calling a constructor
Keywords: shared memory
Message-ID: <1990Aug1.145814.16140@dg-rtp.dg.com>
Date: 1 Aug 90 14:58:14 GMT
Sender: usenet@dg-rtp.dg.com (Usenet Administration)
Reply-To: taylor@dg-rtp.dg.com
Organization: Data General Corp., Research Triangle Park, NC
Lines: 60

I would like to call a constructor directly on a piece of memory that
I have allocated.

I have a pointer to some memory (malloc'ed, shared memory, ...)
and I would like to put a class there.  How can I call the constructor
for that class to initialize the memory?

If I try the obvious:
--

class foo {
public:
	foo();

} *foobar_p;

foobar_p = (foo *)malloc(sizeof(foo));

foobar_p->foo();

--

g++ 1.37.2.1 will not complain but calls foo() with "this" set to
something other than "foobar_p".
(It seems to be a temporary class on the stack).

cfront will not let me do this.  Instead I must make a new member function,
"init()" to call the constructor.

-- 

foo::init() {
	foo();
}

--

However, this does not work either. The construct "foo()" means to allocate
an object of type "foo" and return it as the value of the expression.
Not what I want.

One method that works is to have "init()" pass "this" to the constructor
"foo()" as an argument and have "foo()" set "this" to the argument.

foo::init() {
	foo(this);
}

foo::foo(foo *ptr) {
	this = ptr;
}

But, this is pretty ugly.  Any other ideas on how to call a constructor
for some piece of memory. (We are using shared memory.)

Thanks in advance,

William Taylor 				taylor@dg-rtp.dg.com
Data General Corporation		{world}!mcnc!rti!dg-rtp!taylor
Research Triangle Park, NC  27709	(919) 248-5801