Created
February 28, 2015 02:32
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Empty tuples
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#include <type_traits> | |
#include <cassert> | |
template<int i, class Whence, class T, bool = std::is_empty<T>::value> | |
struct tuple_leaf | |
{ | |
tuple_leaf(T x) : val(x) {} | |
T& get() { return val; } | |
T val; | |
}; | |
template<int i, class Whence, class T> | |
struct tuple_leaf<i,Whence,T,true> : private T | |
{ | |
tuple_leaf(T x) : T(x) {} | |
T& get() { return *this; } | |
}; | |
template<int i, class T1, class T2> | |
struct type_at | |
{ | |
using type = T1; | |
}; | |
template<class T1, class T2> | |
struct type_at<1,T1,T2> | |
{ | |
using type = T2; | |
}; | |
template<class T1, class T2> | |
struct tuple_base : tuple_leaf<0,tuple_base<T1,T2>,T1>, tuple_leaf<1,tuple_base<T1,T2>,T2> | |
{ | |
tuple_base(T1 a, T2 b) : tuple_leaf<0,tuple_base<T1,T2>,T1>(a), tuple_leaf<1,tuple_base<T1,T2>,T2>(b) {} | |
template<int i> | |
tuple_leaf<i,tuple_base,typename type_at<i,T1,T2>::type> get_leaf() | |
{ | |
// XXX how to disambiguate this conversion? | |
return *this; | |
} | |
}; | |
// XXX deriving from tuple_base rather than creating | |
// making tuple_base a member is the root of the issue | |
template<class T1, class T2> | |
struct my_tuple : tuple_base<T1,T2> | |
{ | |
my_tuple(T1 a, T2 b) : tuple_base<T1,T2>(a, b) {} | |
}; | |
template<int i, class T1, class T2> | |
typename type_at<i,T1,T2>::type& get(my_tuple<T1,T2>& t) | |
{ | |
return (t.template get_leaf<i>()).get(); | |
} | |
template<class T1,class T2> | |
my_tuple<T1,T2> make_tuple(T1 a, T2 b) | |
{ | |
return my_tuple<T1,T2>(a,b); | |
} | |
struct empty {}; | |
int main() | |
{ | |
auto tuple = make_tuple(empty(), make_tuple(empty(),empty())); | |
assert(std::is_empty<decltype(tuple)>::value == true); | |
get<0>(tuple); | |
return 0; | |
} |
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