NetNews Usenet Archive 1993 #1/NN_1993_1.iso/spool/comp/lang/cplus/18818

Internet Message Format | 1993-01-07 | 1.7 KB

Path: sparky!uunet!spool.mu.edu!sgiblab!adagio.panasonic.com!nntp-server.caltech.edu!willow!roy
From: Roy_Williams
Newsgroups: comp.lang.c++
Subject: parsing
Date: 8 Jan 1993 00:34:23 GMT
Organization: CCSF Caltech, Pasadena, CA
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Distribution: world
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Suppose I have some class, for example vectors, with lots of
operators defined so I can do things like

    a += b + 3*c + a*b;

meaning in normal language

    for(int i=0; i<vector_length; i++)
        a[i] += b[i] + 3*c[i] + a[i]*b[i];

If the vectors are small and 'new' is cheap, we can define
each operator by creating a new one to hold the result,
evaluating, then returning the whole vector. They get destroyed
by the system.

When the vectors get a bit larger, we want to return a pointer
to the new vector, using the magic of reference counting
to do the garbage collection.

Now suppose we get stingy about either the time taken for
memory allocation, or we are working at the limits of the
machine's available memory: if I were computing the above
operation with a hand-held calculator and pencil, I wouldn't
write down any temporaries, so I want my implementation of
vectors to do as well.

I'd like a vector to be either a set of numbers, as before,
or an operator type and pointer(s) to vector(s). Nothing would
actually get computed until we get to the assignment (or even
until the thing is written out!).

Is there any code or book or reference or ftpable class library
so I don't have to do this myself? How about optimizing the
parse tree before evaluation so a*b + a*c becomes a*(b + c)?

Roy Williams
Caltech Concurrent Supercomputing Facilities
Pasadena, CA