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

28 Jan 2008

      ...
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
Hi, thanks for your answers,

Because I had tested just with :

1.      static double sum(double d1, double d2, double d3)
     {
         return d1+d2+d3;
     }

vs

2.      static double sum(double d[3])
     {
         return d[0]+d[1]+d[2];

     }

with calls to sum() in a tight loop.

I suspect boost::array is equivalent to 2.

was some 40% faster than 2 with advanced intel optimization.

Could you elaborate on the recursive generation?

Rds,

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

};

From: boost-users-bounces@lists.boost.org
[mailto:boost-users-bounces@lists.boost.org] On Behalf Of Ovanes Markarian
Sent: 28 January 2008 21:27
To: boost-users@lists.boost.org
Subject: Re: [Boost-users] Template instantiation function arguments and
Boost.Preprocessor

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@mouline.org> 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@lists.boost.org
[mailto:boost-users-bounces@lists.boost.org] On Behalf Of Steven Watanabe
Sent: 28 January 2008 16:42
To: boost-users@lists.boost.org
Subject: Re: [Boost-users] Template instantiation function arguments and
Boost.Preprocessor

AMDG

Hicham Mouline wrote:
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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