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Newsgroups: comp.lang.ada
From: porton@ex-code.com (Victor Porton)
Date: Wed, 22 Jan 2003 06:13:47 +0500
Organization: Extreme Code Software (http://ex-code.com)
Subject: How to do placement new in Ada
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Xref: archiver1.google.com comp.lang.ada:33328
Date: 2003-01-22T01:14:37+00:00
List-Id: <comp.lang.ada>

I want something like C++'ish placement new in Ada:

generic

  type Object is limited private;
  -- To simplify the task you can exclude "limited".
  
procedure Placement_New(Addr: System.Address);
-- This should initialize the memory region 
-- Addr..Addr+Object'Storage_Size-1
-- the same way as if a variable would be declared so:
-- Var: Object; for Var'Address use Addr;
-- but with the difference that it would be not finalized
-- when goes out-of-scope.

I tried to do this with:

1. "for XXX'Address use ...;" This does not work as it is
deallocated when goes out-of-scope.

2. With special allocator and 'Storage_Pool attribute.
This does not work as for every allocation we need separate allocator 
instance (because addresses are different) and consequencely a
separate access type which is local. So we fall into the same pit:
When goes out-of-scope the allocated storage is automatically
reclaimed (e.g. errant Finalize is called).

Well, it is possible using a global variable of a special storage
pool type  and setting the address at which such the  allocator
would allocate before every allocation (using a global field
in the allocator), but we know that global variables are bad.

P.S. Now I have funny idea of using special allocators which
themselves are dynamically allocated by a special completely
fake (not allocating anything at all in fact) allocator...