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Subject: Re: True faiths ( was Re: The true faith )
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Date: 2002-01-24T08:14:38+00:00
List-Id: <comp.lang.ada>

Kenny Tilton wrote:
> Perhaps I misunderstood. The problem of /copying/ a structure is
> clear--how deep is something only the application can decide. Fine. Now
> what is all this about assignment having to worry about copying?

It was in the problem spec. Anyway, the (now) canonical way to do this
ib C++, thanks to Herb Sutter's "Exceptional C++", is to make objects
have a non-throwing swap() method, and base assignment on copying:

	struct s
	{
		void swap(s &other) throw();	// non-throwing swap
		s(const s &other);		// copy constructor
		s &operator=(const s &other)
		{
			s(other).swap(*this);
			return *this;
		}
	};

> The story I saw seemed to think you needed both a copy and assignment
> method in C++, and that the assignment method also had to worry about
> what to copy (and blowing up during a copy).

Assignment exists in C++ not least because it exists in C. Like Ada, the
default is to do a memberwise copy. If you choose to allow assignment in
your class, you have to get it right.

> Why confuse the two? I just pass pointers (if you will) around.
> assignment is always by reference. Automatic GC gets to clean up after
> me. Life is good. When I /copy/ a structure...well, I'll have to get
> back to you if it ever comes up.