Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!rutgers!bellcore!faline!thumper!ulysses!andante!alice!shopiro
From: shopiro@alice.UUCP (Jonathan Shopiro)
Newsgroups: comp.lang.c++
Subject: Re: Calling constructors on already allocated data
Summary: Use a new allocation/construction mechanism -- placement
Message-ID: <9298@alice.UUCP>
Date: 4 May 89 13:59:12 GMT
References: <13096@pasteur.Berkeley.EDU>
Organization: AT&T Bell Laboratories, Murray Hill NJ
Lines: 42

In article <13096@pasteur.Berkeley.EDU>, faustus@dogwood.Berkeley.EDU (Wayne A. Christopher) writes:
> Is it possible to call a constructor on data that has already been
> allocated?  I have a buffer that I want to put some objects into,
> but I don't know what objects and their order until runtime.  I can
> think of a few ways to do this -- cast the contents of the pointer
> into the type and let the casting function do the work, or create an
> auto variable, then assign it to the contents of the buffer.  Is there
> a better way?
> 
> 	Wayne



I assume you mean ``call a constructor on _storage_ that has already been
allocated.''  This can be easily done with the new placement syntax.

First, put the following line in your header file.

	inline void*	operator new(long, void* p) { return p; }

[This syntax can be overloaded and generalized, but the above is appropriate
for your needs and most others.  Also, eventually you will have to replace
``long'' in the above with ``size_t'', an ANSI-ism.]

Then if p is a pointer to the storage you want to initialize as an
object of type X, you can write

	X*	xp = new (p) X(args);

where ``args'' is the argument list for the constructor.  You can invoke
the destructor on the object without de-allocating its storage with

	xp->X::~X();

Note that you are explicitly taking control of storage allocation here,
and abandoning the protection against improperly overlapping objects, etc.,
that is provided by the usual C++ mechanisms.  Therefore you should be
even more careful than usual when using this technique.
-- 
		Jonathan Shopiro
		AT&T Bell Laboratories, Warren, NJ  07060-0908
		research!shopiro   (201) 580-4229