C++ 转一个 std::tuple<char, char, char> 进入 std::string?</char,>

Question

I am writing a lightweight parser combinator library (akin to Boost::Spirit) as hobby project.

One thing I'd like to do, is to automatically be able to convert a Result<std::tuple<char>> , Result<std::tuple<char, char>> , etc. into a std::string .

And similarly, if having eg a Result<std::tuple<int, int>> I'd like to be able to convert it to a Result<std::vector<int>> or more generally for any tuple containing zero or more elements of the same type T I'd like to be able to convert it automatically to a Result<Container<T>> .

How does one tackle something like this? I attempted for instance:

  template<typename Container>
  Result<decltype(std::make_from_tuple<Container>(std::declval<T>()))> () const {
      Result{Container{std::make_from_tuple<std::initializer_list<typename std::tuple_element<0, T>::type>> (std::get<T>(*this))}};
  }

but this does not work, as it turns out that it's not possible to create an initializer list programmatically like this.

Answer 1

template <typename... Ts>
std::string tuple_to_string(const std::tuple<Ts...>& t)
{
    return std::apply([](auto... cs)
    { 
        return std::string{cs...}; 
    }, t);
}

live example on godbolt.org

---

More generic solution:

template <typename T, typename... Ts>
T tuple_to_container(const std::tuple<Ts...>& t)
{
    return std::apply([](auto... cs){ return T{cs...}; }, t);
}

Usage:

std::tuple test0{'a', 'b', 'c'};
std::tuple test1{'a', 'b', 'c', 'd', 'e', 'f'};

std::cout << tuple_to_container<std::string>(test0) << '\n'
          << tuple_to_container<std::string>(test1) << '\n';

std::tuple test2{0, 1, 2, 3};
auto converted = tuple_to_container<std::vector<int>>(test2);
assert(converted == (std::vector{0, 1, 2, 3}));