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Path: utzoo!mnetor!seismo!munnari!otc!mikem
From: mikem@otc.OZ (Michael Mowbray)
Newsgroups: comp.lang.c++
Subject: Re: Address of constructor?
Message-ID: <54@otc.OZ>
Date: Tue, 9-Dec-86 03:36:51 EST
Article-I.D.: otc.54
Posted: Tue Dec  9 03:36:51 1986
Date-Received: Tue, 9-Dec-86 11:14:33 EST
References: <165700002@uiucdcsb>
Organization: O.T.C. Systems Development, Australia
Lines: 44

In article <165700002@uiucdcsb>, kenny@uiucdcsb.cs.uiuc.edu writes:

> I discovered that I couldn't find a syntax that cfront would accept for
> taking the address of a constructor.  I want to do this because I am in
> a situation where I have allocated memory by another means that I want
> to initialize as an instance of a class, so I want to pass it to the
> constructor as ``this''.

Sounds contrary to C++ philosophy to me. There have been times when
I would have liked to do this, but I eventually realised that it just
reflected bad design. (Of course, I don't know the constraints applying
in this particular case.)

> The reason why is complicated; suffice it to say that I am
> dealing with a situation where I can't rewrite the constructor and
> have to use a memory allocator other than the standard _new operator.

You could possibly do the following, but I'd try to re-structure the design
in preference.


class Given {  // The class you're interested in.
	int i;
    public:
        Given(int ii); // - can't be re-written, say.
};
 
struct Aux : public Given { // Auxiliary class
        Aux(void* buf, int ii) : (ii)  // buf must have correct size & alignment
        {          
            this = (this==0) ? (Aux*)buf : this;         
        }
};
 
main()   
{
    int buffer[sizeof(Given)/sizeof(int)]; // Or use space which the
					   // non-standard allocator gave.
 
    Given *g = (Given*)(new Aux(buffer,1));  // we want a Given with i=1, say.
}


- not really recommended.