[Boost-users] [concept check][mpl] (Conditional) Concept checking for functions

Subject: [Boost-users] [concept check][mpl] (Conditional) Concept checking for functions
From: Jesse Perla (jesseperla_at_[hidden])
Date: 2009-03-29 16:17:19

I have a variety of user defined function objects that I will need to call.
 Take the following examples:template<int N1, int N2 = 0>
struct Func1{ double operator()(boost::array<double, N1>&
x1, boost::array<double, N2>& x2){...}};

template<int N1>
struct Func2{ double operator()(boost::array<double, N1>& x){...}};

With a function such as

template<int N1,
               int N2 = 0,
               class F = boost::function<double (boost::array<double, N1>&
x1, boost::array<double, N2>& x2)>
class DPP{ void solve(){...... uses F...to be discussed} };

I have a few questions:
1) Is there a relatively easy way to use boost concept check to verify that
F follows the first function signature so that I can detect compile time the
problem?  I can't figure out how to do it from the concept check docs, and
they say they are out of date.
Usage I want to check in the instantiation of DPP:
      F f;
      f(boost::array<double, N1>& x1, boost::array<double, N2>& x2)

2) I want the user to be able to pass in a simplified function type if N2 =
0

I assume that to call the actual function I would have some kind of impl
function with partial specialization such as:
template<bool use_simplified, int N1, int N2>
struct use_f_impl();

template<int N1, int N2>
struct use_f_impl<true, N1, N2>{
   static double use_f()
   {
      STATIC_ASSERT(N2 == 0);
       F f;
      boost::array<double, N1> x;
       return f(x);
   }
};

template<int N1, int N2>
struct use_f_impl<false, N1, N2>{
   static double use_f()
   {
       F f;
      boost::array<double, N1> x1;
      boost::array<double, N2> x2;
       return f(x1, x2);
   }
};

And then in my solve() function I go:
  double val = use_f<(N2 == 0, N1, N2)>::use_f();

Is this the right idiom, pattern, etc?  Do I need to use the struct with a
static function since function templates don't have partial specialization?

3) Now what if I want to do a concept check on the F passed into DPP?  It
has to be conditional on the N2 value now.  How would I do this?

4) Lets say that I don't want to force them to use the simplified version of
the function signature if they don't want to.
Can I automatically detect the signature and put it into a flag integer in
the class?
i.e. inside of the DPP class:
static bool use_simple = IsSimple<F>::value;
Then pass in the use_simple value when calling use_f_impl.
If so, what would the IsSimple metafunction look like and can concept check
help?

Thanks,
Jesse