Operations on C-style arrays

[rabauke]

Operations on C-style arrays

[rabauke]

The type that is returned by the reduction operator must match the type of the 2nd argument to allreduce. Thus you have to replace float by the array type of your example above. This, however, is change not sufficient because C arrays are not normal data types with a proper value semantics (array to pointer decay, C arrays can not be used as function return values). Therefore,
mpl::plus, which is defined as

template<typename T>
  struct plus {
    T operator()(const T &x, const T &y) const { return x + y; }
  };

cannot be applied to arrays. If you would like to avoid using mpl::contiguous_layout (which is certainly a good thing) you have to wrap the array into a struct or class type, which has value semantics and can be used as as return type. See collective.cc in the examples directory for an example.