[Boost-users] mpl help

From: Zeljko Vrba (zvrba_at_[hidden])
Date: 2005-07-04 13:40:40

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Hi!

I would like to use mpl to generate code along the following lines (I'm
trying to build a reusable library for data processing in a 'pipelined'
fashion):

// Arg1 and Arg2 are instances of 'stages' providing results to this
// stage. F is the function taking as arguments const Arg1::result_type&
// and const Arg2::result_type&.
template<typename Result, typename F, typename Arg1, typename Arg2>
struct stage_2 {
~  typedef Result result_type;

~  Result current_;
~  F      f_;
~  Arg1   input1_;
~  Arg2   input2_;

~  stage_2(F f, Arg1 a1, Arg2 a2) : f_(f), input1_(a1), input2_(a2) { }

~  const Result &operator() {
~    current_ = f_(input1_(), input2_());
~    return current_;
~  }
};

The question: is it possible not to write manually separate stage_0,
stage_1, stage_2, ... for stages taking as input results from the
previous 0, 1, 2, ... stages?

== Actually, what I'm trying to design:

S1 -> S2 -> S3 -> .. -> Sk -> ..-> Sn
~       \_______________/

This is just a rough sketch; in the bottom line the arrow goes from S2
to Sk. Each stage (S'es in the diagram) provides input to its
'children'. This is akin to UNIX shell pipes, but more powerful in that
each stage can have multiple inputs. The catch: data transformation at
any stage can produce data type on output different from the one
received on input.

The stages should (obviously?) be parametrized by the stage result, the
types of input data and the data transformation to be performed.

My last resort is writing a python script to generate the boiler-plate
C++ code from high-level description :)

Maybe I'm on the wrong way trying to use mpl? any help is appreciated!

Thanks.
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