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From: herwin@gmu.edu (Harry Erwin)
Subject: Safety of the Booch Ada 95 Components
Date: 1999/12/10
Message-ID: <1e2lds4.7trgj21rgj9i0N%herwin@gmu.edu>#1/1
X-Deja-AN: 559023172
X-Server-Date: 10 Dec 1999 15:32:47 GMT
Followup-To: comp.lang.ada
Organization: HDE Associates
Keywords: Exception, safety, neutrality, self-assignment
User-Agent: MacSOUP/2.4.1
Reply-To: herwin@gmu.edu
Newsgroups: comp.lang.ada,comp.lang.c++
Date: 1999-12-10T15:32:47+00:00
List-Id: <comp.lang.ada>

In C++, the equate operation for a container class C<T> is preferably
defined as follows (Sutter, Exceptional C++, Addison-Wesley, 2000):

template <typename T>
C<T>& operator=(const C<T>& c)
{
    if(&c == this) return *this; // not required; done for efficiency
    C temp(c); // uses the copy constructor, may throw
    swap(temp, *this); // swaps the guts of this container with
                       // temp. May not throw.
    return *this; // May not throw.
} // the destructor for temp releases the memory originally in this
  // container, and destructors may not throw

This is strongly exception-safe and exception-neutral, since only the
copy constructor can throw, and that occurs before the guts are swapped.
Hence the container remains useable, and no objects are lost from it.
Now I've been looking at the Booch components for Ada 95, and have
noticed that the Copy function typically starts by clearing the To
container. That immediately implies that they are not exception-safe,
but I'm concerned that they may not be safe under self-assignment as
well. Is there something about the Ada 95 standard that guarantees that
there will be no aliasing of container args?

-- 
Harry Erwin, PhD, <http://mason.gmu.edu/~herwin>