[comp.lang.c++] call a constructor

don@zardoz.coral.com (Don Dewar) (08/04/90)

) Return-Path: <info-g++-request@prep.ai.mit.edu>
) Date: 1 Aug 90 18:29:45 GMT
) From: uunet!mcnc.org!rti!dg-rtp!farm!taylor  (William Taylor)
) Organization: Data General Corp., Research Triangle Park, NC
) Subject: call a constructor
) Sender: uunet!prep.ai.mit.edu!info-g++-request
) To: info-g++@prep.ai.mit.edu
) 
) Using g++:
) 
) 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?
) 
) I have found out how to do it with cfront.  You can specify a second
) argument to "new" telling it where to construct the class - just what I wanted.
) 
) Is there any way to do this with g++?
) 
) William Taylor 				taylor@dg-rtp.dg.com
) Data General Corporation		{world}!mcnc!rti!dg-rtp!taylor
) Research Triangle Park, NC  27709	(919) 248-5801
) 
) 

I have also been wondering this same thing.  Something I used to do
with CFront all the time was call one constructor from another as a
convenient way to build up overloaded constructors.  Now, to do the
same thing I have to separate the code out of the constructor and call
call it from each constructor.  Is there any chance this will change?

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steve@taumet.com (Stephen Clamage) (08/05/90)

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