Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!wasatch!gr.utah.edu!mecklen
From: mecklen%gr.utah.edu@wasatch.utah.edu (Robert Mecklenburg)
Newsgroups: comp.lang.c++
Subject: Re: Generating temporaries
Message-ID: <1531@wasatch.utah.edu>
Date: 5 Apr 89 14:48:50 GMT
References: <7.UUL1.2#261@persoft.UUCP> <SCP.89Apr4162957@raven.lanl.gov>
Sender: news@wasatch.utah.edu
Reply-To: mecklen%gr.utah.edu.UUCP@wasatch.utah.edu (Robert Mecklenburg)
Organization: University of Utah CS Dept
Lines: 56

>In article <7.UUL1.2#261@persoft.UUCP> ericf@persoft.UUCP (Eric R. Feigenson) writes:
>
>   [I'm rather new to C++, so this may be a rather naive query.  Please bear
>    with me]
>
>   I have a class M for which I want to define operator+.  My question(s) have
>   to do with how I generate the temporary that contains the result, and how
>   such a temporary gets destroyed.

We have a points package (xyz coordinates) which I have just converted
to use '+', '-', and '*' operators.  The basic method I used for
generating temporaries was to allocate a permanent pool of points and
treat it as a circular list.  Points are declared as

	struct point_type { 
	    real_type xyz[3];
	    point_type &operator+( point_type &p );
	 };

And the pool looks like

	extern const int MAX_POINT_POOL;
	extern point_type point_pool[];
	extern point_type *pt_ptr;

When I need another temporary I call the allocation function

	inline point_type *
	pt_nextpt()
	{
	    if ( ++pt_ptr >= &point_pool[MAX_POINT_POOL] )
		pt_ptr = point_pool;
	    return pt_ptr;
	}

So the following code allocates two temporaries and performs one
structure copy at the end

	point_type p;
	p = a + b + c;

The advantages of this are:

	1.  easy to implement and understand
	2.  efficient (overhead of an increment and test-and-branch)

The disadvantages are:

	1.  breaks easily if programmer holds on to a temporary too
	    long, the circular queue will eventually trash his
	    temporary with no warning;
	2.  to be reasonably safe the pool must be reasonably large
	    (we use MAX_POINT_POOL = 64).

Comments?
Robert Mecklenburg