I have a function template that takes an arbitrarily nested list and returns an array:
#include <array>
#include <initializer_list>
template<size_t N, typename List>
std::array<size_t,N> some_function (const List& list)
{
// N is the number of times the list is nested.
std::array<size_t,N> arr;
return arr;
}
When I use this function for some nested std::initializer_list
, like this:
int main () {
using List = std::initializer_list<std::initializer_list<double>>;
List list = {{1.,2.,3.},{4.,5.,6.}};
std::array<size_t,2> arr;
arr = some_function (list);
return 0;
}
I receive the error that type N can not be deduced
couldn't deduce template parameter āNā
std::initializer_list
for this case?You can write two overloaded constexpr
function templates to calculate the nested times, with the help of std::enable_if
and SFINAE.
// types have not member type value_type
template <typename T, typename = void>
struct has_value_type: std::false_type {};
// types have member type value_type
template <typename T>
struct has_value_type<T, std::void_t<typename T::value_type>> : std::true_type {};
// return nested times as 0 for types without member type value_type
template<typename T>
constexpr std::enable_if_t<!has_value_type<T>::value, size_t> get_nested_times() {
return 0;
}
// return nested times as 1 plus times got on the nested type recursively
template<typename T>
constexpr std::enable_if_t<has_value_type<T>::value, size_t> get_nested_times() {
return 1 + get_nested_times<typename T::value_type>();
}
then you can get the nested times at compile-time as
template<typename List>
auto some_function (const List& list)
{
// N is the number of times the list is nested.
constexpr auto N = get_nested_times<List>();
std::array<size_t, N> arr;
return arr;
}