nurettin - 6 months ago 102

C++ Question

The following paper is the first proposal I found for template parameter packs.

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1603.pdf

At page 16, it talks about introducing two new operators [] and <> for accessing parameter pack elements and parameter pack types.

`The suggested syntax for such an operator involves two new operators: .[] to access values and .<> to access types. For instance:`

`template<int N, typename Tuple> struct tuple_element;`

template<int N, ... Elements>

struct tuple_element<tuple<Elements...> >

{

typedef Elements.<N> type;

};

template<int N, ... Elements>

Elements.<N>& get(tuple<Elements...>& t)

{ return t.[N]; }

template<int N, ... Elements>

const Elements.<N>& get(const tuple<Elements...>& t)

{ return t.[N]; }

So where are these operators? If there is none, what is their replacement?

Answer

C++11 doesn't have corresponding operators which is the reason they are proposed. With C++11 you'll need to either extract the corresponding information yourself or use a class which already does the necessary operation. The easiest approach is probably to just use `std::tuple<T...>`

which already implements the corresponding logic.

If you wonder how `std::tuple<T...>`

currently implements these operations: it is basically an exercise in functional programming using a fairly bad functional programming notation. Once you know how to get the `n`

-th type of the sequence, getting the `n`

-th element using inheritance from base classes parameterized on index and type is fairly trivial. Implementing something like `tuple_element<N, T...>`

could look something like this:

```
template <int N, typename... T>
struct tuple_element;
template <typename T0, typename... T>
struct tuple_element<0, T0, T...> {
typedef T0 type;
};
template <int N, typename T0, typename... T>
struct tuple_element<N, T0, T...> {
typedef typename tuple_element<N-1, T...>::type type;
};
```

The actual more challenging bit in implementing something like `std::tuple<T...>`

is conjuring up a list of indices so you got a parallel list of type and integers which can then be expanded, e.g., for a list of base classes using something like (how the internal details look exactly will differ but the basic idea of having a parallel parameters packs for the types and their indices will be somehow there):

```
template <typename... T, int... I>
class tuple_base<tuple_types<T...>, tuple_indices<I...>>:
public tuple_field<T, I>... {
};
```