[Boost-users] compile-time mapping: a[1] --> a1 ?

7 Sep 2005

      I would like to do a compile-time mapping of an array accessor to a list of
scalars as in the following class:

class MyArray1 {
  public:
  double& operator[](int n) { ??? }

  private:
  double a0, a1, a2, a3 ;
} ;

int main() {
  MyArray1 x ;

  x[1] = 2. ;
}

I've left off all the extraneous stuff like constructors.  I don't know how to
write the MyArray1::operator[] so that it will evaluate to one of the data
members at compile time; an if-test would correctly evaluate but might not be
optimized away and run slowly.  Is there some way to do the compile-time
evaluation using the MPL?  I looked thru the tutorial but did not find anything
that looked like it would work.

One possibility that I considered was the C preprocessor concatenation operator
##,

#define PICKMEMBER(n)	a##n

double& operator[](int n) { PICKMEMBER(n) ; }

Unfortunately, this will not work for expressions like x[i+1].  It probably has
additional problems other than being quite distasteful.

Obviously, this would all be easy if I just implemented the member data with an
array,

class MyArray2 {
  public:
  double& operator[](int n) { return data[n] ; }

  private:
  double data[4] ;
} ;

In fact, I did do so and everything works fine.  The reason that I am trying to
implement the list of scalars method is to see if there are any CPU timing
differences for my application, which amounts to matrix-vector operations for
small, dense vectors whose sizes are known at compile time.

I have tried uBlas but was disappointed with the timing results (and yes, I did
compile with -DNDEBUG).  I've also looked at other packages such as tvmet (it
did worse).  I finally tried rolling my own vector and matrix classes with lazy
evaluation of binary operators via Eckel's "Thinking in C++, Vol 2"; I also
studied source for uBlas and tvmet.  So far, I am able to beat uBlas on my
choice of compilers/platforms, but I'm still not getting the consistent
performance that I was hoping for.

Why am I so interested in performance?  Two reasons.  First, if I can get
consistently good performance (i.e. performance that is close to raw C or
Fortran) for fairly straightforward operations, then I can be more confident
that as I increase abstraction, performance will not deteriorate substantially. 
Second, I am evaluating C++ for applications that have been traditionally
written in Fortran -- in my case, CFD codes for airflow simulation.  Issues of
modularity, extensibility, maintenance, and the fact that very few students are
learning Fortran anymore may not be enough to convince my peers.  I think I will
also need to show that we will not experience a substantial performance hit with
C++.

Thanks in advance for any help.

Steven Allmaras
Boeing Commercial Airplanes