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Arbitrary Transformations of Arguments and Results

I have a class like this:

template <typename T>
struct operation {
    typedef T result_type;
    typedef ::std::shared_ptr<operation<T> > ptr_t;
};

I have a functor that will match this type ::std::function:

::std::function<int(double, ::std::string)>

I want to create a functor that has a signature something like this:

operation<int>::ptr_t a_func(operation<double>::ptr_t, operation< ::std::string>::ptr_t);

I want to do this in an automatic way to create a similar functor for any type ::std::function.

Finally, I would like to put this wrinkle. It:

::std::function<int(operation<double>::ptr_t, ::std::string)>

should result in the following:

operation<int>::ptr_t a_func(operation<double>::ptr_t, operation< ::std::string>::ptr_t);

Because if the functor already accepts operation<T>::ptr_t, which means that he understands what he is and wants to deal with their asynchronous nature.

How should I do it? I have a naive and partially working attempt:

template <typename argtype>
struct transform_type {
   typedef typename operation<argtype>::ptr_t type;
};

template <typename ResultType, typename... ArgTypes>
::std::function<typename transform_type<ResultType>::type(typename transform_type<ArgTypes...>::type)>
make_function(::std::function<ResultType(ArgTypes...)>)
{
   return nullptr;
}

It does not detect arguments that already have a type std::shared_ptr<operation<T> >. And this transform_type specialization will not compile:

template <typename argtype>
struct transform_type<typename operation<argtype>::ptr_t>
{
   typedef typename stub_op<argtype>::ptr_t type;
};
+3
2

, R. Martinho Fernandez:

template <typename T>
struct is_op_ptr {
 private:
   // Returns false_type, which has a ::value that is false.
   template <class AT>
   static constexpr std::false_type is_it_a_ptr(...);

   // Returns true_type (if enable_if allows it to exist).
   template <class AT>
   static constexpr typename ::std::enable_if<
      ::std::is_same<
         AT,
         typename operation<typename AT::element_type::result_type>::ptr_t>::value,
      std::true_type>::type  // note the true_type return
   is_it_a_ptr(int); // no definition needed

 public:
   // do everything unevaluated
   static constexpr bool value = decltype(is_it_a_ptr<T>(0))::value;
};

template <typename T>
struct transform_type
   : ::std::conditional< is_op_ptr<T>::value, T, typename operation<T>::ptr_t>
{
};

, .

0
template<template<typename...> class F, typename Sig>
struct transform;

template<template<typename...> class F, typename R, typename... A>
struct transform<F, R(A...)> {
    using type = typename F<R>::ptr_t(typename F<A>::ptr_t...);
};

:

template<typename Sig>
void foo(std::function<Sig> f)
{
    using transformed_type = typename transform<operation, Sig>::type;
    std::function<transformed_type> g;
}

, , :

template<typename T>
struct operation<std::shared_ptr<T>> {
    using ptr_t = std::shared_ptr<T>;
    using result_type = ptr_t; // Or perhaps this needs to be T, you haven't said
};
+2

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