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

From: Ovanes Markarian (om_boost_at_[hidden])
Date: 2008-01-28 15:27:06

Hi,

besides the more complex possibility to use boost::tuple and generate
recursive Tree<X> List : derived from Tree<X-1> ...: derived from Tree<0>
specialization, why not using a boost::array instead?

Here an example:

template<class T, size N>
struct Tree
{
    typedef boost::array<T, N>        parameters_types;

    T sum(parameters_type const& p)
     {
         return std::accumulate(p.begin(), p.end(), 0);
     }

};

This is type safe and fast ;) and your users can initialize boost array with
an initializer list like:

parameters_type x = { 1, 2, 3, 4, 5... };

Regards,
Ovanes

On Jan 28, 2008 8:26 PM, Hicham Mouline <hicham_at_[hidden]> wrote:

> 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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