Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!cs.utexas.edu!uunet!taumet!steve
From: steve@taumet.com (Stephen Clamage)
Newsgroups: comp.lang.c++
Subject: Re: call a constructor
Message-ID: <381@taumet.com>
Date: 4 Aug 90 20:49:06 GMT
References: <1990Aug1.182945.23905@dg-rtp.dg.com> <9008031721.AA18726@zardoz.coral.com>
Organization: Taumetric Corporation, San Diego
Lines: 45

taylor@dg-rtp.dg.com (William Taylor) writes:

> 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 (and its base and member classes, etc.) to initialize 
> the memory?

You cannot call a constructor directly.  In version 2.0 and later, you
can use a user-defined function "new" to get what you want.  Here is a
simple example:

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

class T { ... };
class U { ... };

void my_func()
{
    T *t = ...	// assign space for a T object
    U *u = ...	// assign space for a U object
    ...
    new (t) T;	// T constructor called
    new (u) U;	// U constructor called
    ...
}

The "new" function takes a size_t parameter, and a pointer.  Operator
new takes an optional "placement" parameter in parens, shown here as
(t) and (u).  The placement is an address at which the object is already
located.  The effect is to bypass the storage allocation, and do
everything else done by operator new -- such as call appropriate
contructors.  In this case, the constructor for type T (and for
any base classes, etc) is called.

The "new" function should probably static (here inline) to avoid
conflict with other uses of various types and use of "new".  It could
also be a member function of the class.
-- 

Steve Clamage, TauMetric Corp, steve@taumet.com