Re: [Boost-users] Template instantiation function arguments and Boost.Preprocessor

From: Hicham Mouline (hicham_at_[hidden])
Date: 2008-01-28 14:26:12

After some though, here is more precisely what I'd like to have...
I apologize that it is quite different from the initial problem:

template<int n> class Tree {
  static double sum();  // 
};

If the user instantiates tree<2>, he should get:

template<> class Tree<2> {
  static double sum(double d1, double d2);
};

template<> class Tree<3> {
  static double sum(double d1, double d2, double d3);
};
etc etc...

so that in user code, for e.g.:

  double d= Tree<4>::sum(d1, d2, d3, d4);

should compile.

Is it possible for me to just define the template Tree for the n-case
without the 2- and 3- specializations?

Rds,

-----Original Message-----
From: boost-users-bounces_at_[hidden]
[mailto:boost-users-bounces_at_[hidden]] On Behalf Of Steven Watanabe
Sent: 28 January 2008 16:42
To: boost-users_at_[hidden]
Subject: Re: [Boost-users] Template instantiation function arguments and
Boost.Preprocessor

AMDG

Hicham Mouline wrote:
> hi,
>
> i have a template function, the arguments of which i wish to have depend
on a non-type int template argument:
>
> template<int n>
> double f<n>( double arg0, double arg1, ..., double argn )   // ... is not
the variadic notation for var number of args
> {
>  // for(int i=0; i<n; i++)
>  // code
>  return sum;
> }
>
> int main()
> {
>  return x = f<3>( 1.0, 2.0, 3.0 );
> }
>
> BOOST_PP_REPEAT can't seem to do the job as the template argument n needs
to be known
> at the preprocessing stage, which happens before the template
instantiation stage (
> this is part of compilation)
>   

I don't understand why you want n to be passed explicitly.  Can't it be 
deduced from the
number of arguments (warning untested):

#define SUM_IMPL(z, n, data) + arg ## n
#define F_DEF(z, n, data) double f(BOOST_PP_ENUM_PARAMS_Z(z, n, double 
arg)) { return(0.0 BOOST_PP_REPEAT_ ## z(n, SUM_IMPL, ~)); }

BOOST_PP_REPEAT(MAX_ARITY, F_DEF, ~)

In Christ,
Steven Watanabe

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