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Date: Fri, 10 Jan 2003 04:26:21 -0500
From: Hyman Rosen <hyrosen@mail.com>
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Subject: Re: Why is Ada NOT a good choice for a beginner to programming?
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Date: 2003-01-10T04:26:21-05:00
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Grein, Christoph wrote:
> For me, the code posted looked like Kazakov's solution with run-time handling of 
> dimension exponents. Your reply now seems to say that C++ does the checking and 
> exponent addition during compile time. Is that true?

Yes. All those exponents are part of the type, not the object.

When you multiply or divide two units, the compiler can find
only one specialization of the operator* or operator/ template
which accepts the correct parameters, and is therefore forced to
synthesize the correct return type. C++ instantiates the correct
operator for you; in Ada, you would have to request a generic
instantiation before you could use it.

Meanwhile, addition and subtraction are defined only for
two unit operands of the same type (that is, all exponents
equal), so any attempt to add disparate units will not
compile.

Unit objects don't need to carry around type information at runtime,
so they are only as large as the type used to hold their numeric
value. Any half-decent C++ compiler will also inline all the
arithmetic, so the compiled code should look just like arithmetic
on the underlying type, with no runtime overhead at all for the
units checking. This technique works only when all units are
statically present in the code, naturally.

My code is the bare skeletal outline of the approach, of course.