Question

I have the following code defining a Functor template and the function
running_op, which takes an array, it’s length and a functor to apply
to the list:

  template <class Type>
  struct SumFunctor {
    Type sum;
    SumFunctor() : sum(0) {};
    Type operator()(Type next) {
      return sum += next;
    }
  };

  template <class Container, class Functor>
  inline Container running_op(Container& container, Functor functor) {
    transform(container.begin(), container.end(), container.begin(), functor);
    return container;  
  }

This is used in the following way:

  list<float> a({1,1,1,1});
  running_op(a, SumFunctor<float>());

What I would like to be able to do to avoid having type the name of
the container in the instantiated functor is use it as so:

  list<float> a({1,1,1,1});
  running_op(a, SumFunctor);

Since the contained type of a can be found in the running_op template
using Container::value_type I would like to do something as follows
(which does not work) to instantiate the functor of appropriate type:

  template <class Container, class Functor>
  inline Container running_op(Container& container, Functor functor) {
    typedef typename Container::value_type ContainerType;
    transform(container.begin(), container.end(), container.begin(), functor<ContainerType>());
    return container;
  }

Is there any way that I can pass an
uninstantiated template to another template for later instantiation?
Is there a special keyword I should use other than class in the
template parameter list (template did not work in this case)? Really
I just want to pass in a symbol which is the functor template name; is
that possible?

Answer 1

You can use template templates.

  template <template <class> class Functor, class Container>
  inline Container running_op(Container& container) {
    typedef typename Container::value_type ContainerType;
    transform(container.begin(), container.end(), container.begin(), functor<ContainerType>());
    return container;
  }


  running_op<sum_function>(a);

If you want that certain syntax you can use macros, although I don’t recommend it.

#define running_op(a, b) running_op_<b>(a)