simply_fanin.hpp

/**
 * @file fanin_visitor_impl.hpp
 * @author Marcus Edel
 * @author Kris Singh
 *
 * Implementation of the fain layer abstraction.
 *
 * mlpack is free software; you may redistribute it and/or modify it under the
 * terms of the 3-clause BSD license.  You should have received a copy of the
 * 3-clause BSD license along with mlpack.  If not, see
 * http://www.opensource.org/licenses/BSD-3-Clause for more information.
 */
#ifndef MLPACK_METHODS_ANN_VISITOR_FANIN_IMPL_VISITOR_IMPL_HPP
#define MLPACK_METHODS_ANN_VISITOR_FANIN_IMPL_VISITOR_IMPL_HPP

// In case it hasn't been included yet.
#include "fanin_visitor.hpp"

namespace mlpack {
namespace ann {

//! FanIN Visitor visitor class.
template<typename LayerType>
size_t FanInVisitor::operator()(LayerType* layer) const
  {
    return LayerFanIn(layer, layer->OutputParameter());
  }

template<typename T, typename P>
inline typename std::enable_if<
      !HasParametersCheck<T, P&(T::*)()>::value && 
      !HasInputWidth<T, size_t&(T::*)()>::value, size_t>::type 
FanInVisitor::LayerFanIn(T* /* layer */, P& /* output */) const
{
  return 0;
}

template<typename T, typename P>
inline typename std::enable_if<
       !HasInputWidth<T, size_t&(T::*)()>::value && 
       HasParametersCheck<T, P&(T::*)()>::value, size_t>::type 
FanInVisitor::LayerFanIn(T* layer, P& /* output */) const
{
 return layer->Parameters().n_rows;
}

template<typename T, typename P>
inline typename std::enable_if<
      !HasParametersCheck<T, P&(T::*)()>::value && 
      HasInputWidth<T, size_t&(T::*)()>::value, size_t>::type 
FanInVisitor::LayerFanIn(T* layer, P& /* output */) const
{
  return layer->InputWidth();
}

template<typename T, typename P>
inline typename std::enable_if<
      HasParametersCheck<T, P&(T::*)()>::value && 
      HasInputWidth<T, size_t&(T::*)()>::value, size_t>::type 
FanInVisitor::LayerFanIn(T* layer, P& /* output */) const
{
  //Todo
  return 0;
}


} // namespace ann
} // namespace mlpack

#endif

We have to pass layer->OutputParameter() to deduce the type of P, as you already pointed out.