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From: Dave Steffen (steffend_at_[hidden])
Date: 2001-08-19 12:37:21

Ralf W. Grosse-Kunstleve writes:
> I am thinking about implementing a "small matrix" library for
> 2-dimensional arrays. (This would be similar to boost::array,
> i.e. the small matrices will usually live on the stack to avoid
> the dynamic memory allocation overhead.)

 Yeah, I've had some thoughts about this too. Eventually this
 will/can/would/should be part of the Boost matrix library (which
 AFAIK will be == MTL version 3).

> One requirement is to have operators for *heterogeneous*
> arithmetic operations, e.g. multiplication. E.g. something
> like:
> template <class LR, class L, class R>
> matrix<LR> operator*(const matrix<L>& lhs, const matrix<R>& rhs)
> {
> // return the product
> }
> Is there a mechanism in C++ to define a table for the result type LR
> depending on the types of L and R?

 Yes, use traits.
> For example, if L=int and R=int, the result type LR=int, but for
> L=double and R=int the result type should be LR=double, etc.

 Quick example:

 template <class L, class R> struct MatrixOpTraits
 { ... possible default result? ... };

 template <> struct MatrixOpTraits<int,int>
 { typedef int Result; };

 template <> struct MatrixOpTraits<double,int>
 { typedef double Result; };

 Then, e.g.

 template <class R, class L> inline
 MatrixOpTraits<R,L>::Result // there's the return type
 operator * (const Matrix<R>& A, const Matrix<L>& B)
 { ... }

 On another subject, lots of cool things can be done if the size of
 the vectors/matrices is known at run time. Template metaprogramming
 techniques can be used to effectively unroll the loops at
 compile-time, so that (for example) given 3-vectors a,b, and c, the

 c = a + b;

 expands (via template metaprogramming magic) to

 c[0] = a[0] + b[0];
 c[1] = a[1] + b[1];
 c[2] = a[2] + b[2];

 which is typically something the compiler's optimizer can go crazy

Dave Steffen Wave after wave will flow with the tide
Numerica, Inc. And bury the world as it does
steffend_at_[hidden] Tide after tide will flow and recede
                                    Leaving life to go on as it was...
                                                        - Peart / RUSH
"The reason that our people suffer in this way....
is that our ancestors failed to rule wisely". -General Choi, Hong Hi

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