On Wed, Nov 19, 2008 at 5:32 PM, Yechezkel Mett <ymett.on.boost@gmail.com> wrote:
On Wed, Nov 19, 2008 at 5:08 PM, Vicente Botet <vicente.botet@wanadoo.fr> wrote:
Comming back to my example, I need to store the tasks on an array. I do it now like this:
template <> struct task_array<3> { task arr_[3]; template <typename F_1, typename F_2, typename F_3> task_array(F_1 f1, F_2 f2, F_3 f3) { arr_[0]=task(f1); arr_[1]=task(f2); arr_[2]=task(f3); } };
I don't see a simple way to do it with variadic templates other than recursion.
Is there a simple way to achieve this something like
template <std::size N> struct task_array { task arr_[N]; template <typename... F> task_array(F... f) { static_assert(sizeof...(T) == N, "Constructor must provide N parameters") { arr_={task(f)...}; } };
Recursion is the way to do anything with variadic templates :-) But the following should work in theory:
template <std::size N> struct task_array { task arr_[N]; template <typename... F> task_array(F... f) : arr_({task(f)...}) { static_assert(sizeof...(T) == N, "Constructor must provide N parameters") } };
if you can find a compiler that supports the new initialisation syntax, that is.
On a current compiler: template <std::size N> struct task_array { task arr_[N]; template <typename... F> task_array(F... f) { static_assert(sizeof...(T) == N, "Constructor must provide N parameters") boost::array<task, N> arr = { task(f)... }; std::copy(arr.begin(), arr.end(), arr_); } }; if you don't mind the multiple copies -- perhaps better to move rather than copy. Recursion isn't too bad either: namespace detail { template<typename T, typename Head, typename... Tail> void fill_array(T* arr, Head h, Tail... t) { *arr = T(h); fill_array(arr + 1, t...); } template<typename T> void fill_array(T* arr) {} } template <std::size N> struct task_array { task arr_[N]; template <typename... F> task_array(F... f) { static_assert(sizeof...(T) == N, "Constructor must provide N parameters") detail::fill_array(arr_, f...); } }; Yechezkel Mett