Max - 3 months ago 17

C++ Question

I've got a custom class that has a tuple-like interface. Because I want my code to be as generic as possible, I thought that it would be a good idea to base my algorithms on the functions

`std::get`

`std::tuple_size`

`std::tuple_element`

`Tuple`

Now I am trying to sum up the components of a

`Tuple`

`template <class Tuple>`

int sum_components(const Tuple& t);

I guess that there is a lot of template programming involved but I just can't figure out how to do it.

For the addition I would just use an overload of the global

`+ operator`

I am using c++1z.

Answer

With C++1z it's pretty simple with fold expressions. First, forward the tuple to an `_impl`

function and provide it with index sequence to access all tuple elements, then sum:

```
template<typename T, size_t... Is>
auto sum_components_impl(T const& t, std::index_sequence<Is...>)
{
return (std::get<Is>(t) + ...);
}
template <class Tuple>
int sum_components(const Tuple& t)
{
constexpr auto size = std::tuple_size<Tuple>{};
return sum_components_impl(t, std::make_index_sequence<size>{});
}
```

A C++14 approach would be to recursively sum a variadic pack:

```
int sum()
{
return 0;
}
template<typename T, typename... Us>
auto sum(T&& t, Us&&... us)
{
return std::forward<T>(t) + sum(std::forward<Us>(us)...);
}
template<typename T, size_t... Is>
auto sum_components_impl(T const& t, std::index_sequence<Is...>)
{
return sum(std::get<Is>(t)...);
}
template <class Tuple>
int sum_components(const Tuple& t)
{
constexpr auto size = std::tuple_size<Tuple>{};
return sum_components_impl(t, std::make_index_sequence<size>{});
}
```

A C++11 approach would be the C++14 approach with custom implementation of `index_sequence`

. For example from here.

As @ildjarn pointed out in the comments, the above examples are both employing right folds, while many programmers expect left folds in their code. The C++1z version is trivially changeable:

```
template<typename T, size_t... Is>
auto sum_components_impl(T const& t, std::index_sequence<Is...>)
{
return (... + std::get<Is>(t));
}
```

And the C++14 isn't much worse:

```
template<typename T, size_t... Is>
auto sum_components_impl(T const& t, std::index_sequence<Is...>)
{
constexpr auto last_index = sizeof...(Is) - 1;
return sum(std::get<last_index - Is>(t)...);
}
```