Sum the components of a tuple up by using std::get, std::tuple_size, std::tuple_element

问题

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 so you just have to specialize these functions to use my algorithms. Let's call the concept that requires these function specializations Tuple.

Now I am trying to sum up the components of a Tuple. The function declaration should be something like this:

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.

回答1:

This is very easy in c++17.

template<class Tuple>
decltype(auto) sum_components(Tuple const& tuple) {
  auto sum_them = [](auto const&... e)->decltype(auto) {
    return (e+...);
  };
  return std::apply( sum_them, tuple );
};

or (...+e) for the opposite fold direction.

In previous versions, the right approach would be to write your own apply rather than writing a bespoke implementation. When your compiler updates, you can then delete code.

In c++14, I might do this:

// namespace for utility code:
namespace utility {
  template<std::size_t...Is>
  auto index_over( std::index_sequence<Is...> ) {
    return [](auto&&f)->decltype(auto){
      return decltype(f)(f)( std::integral_constant<std::size_t,Is>{}... );
    };
  }
  template<std::size_t N>
  auto index_upto() {
    return index_over( std::make_index_sequence<N>{} );
  }
}
// namespace for semantic-equivalent replacements of `std` code:
namespace notstd {
  template<class F, class Tuple>
  decltype(auto) apply( F&& f, Tuple&& tuple ) {
    using dTuple = std::decay_t<Tuple>;
    auto index = ::utility::index_upto< std::tuple_size<dTuple>{} >();
    return index( [&](auto...Is)->decltype(auto){
      auto target=std::ref(f);
      return target( std::get<Is>( std::forward<Tuple>(tuple) )... );
    } ); 
  }
}

which is pretty close to std::apply in c++14. (I abuse std::ref to get INVOKE semantics). (It does not work perfectly with rvalue invokers, but that is very corner case).

In c++11, I would advise upgrading your compiler at this point. In c++03 I'd advise upgrading your job at this point.

---

All of the above do right or left folds. In some cases, a binary tree fold might be better. This is trickier.

If your + does expression templates, the above code won't work well due to lifetime issues. You may have to add another template type for "afterwards, cast-to" to cause the temporary expression tree to evaluate in some cases.

回答2:

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>{});
}

demo

---

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>{});
}

demo

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));
}

demo

And the C++14 isn't much worse, but there are more changes:

template<typename T, typename... Us>
auto sum(T&& t, Us&&... us)
{
    return sum(std::forward<Us>(us)...) + std::forward<T>(t);
}

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)...);
}

demo

来源：https://stackoverflow.com/questions/40700181/sum-the-components-of-a-tuple-up-by-using-stdget-stdtuple-size-stdtuple