What should tuple_map return?

Question

I want to implement a generic tuple_map function that takes a functor and an std::tuple, applies the functor to every element of this tuple and returns an std::tuple of results. The implementation is pretty straightforward, however the question arises: what type should this function return? My implementation used std::make_tuple. However, here std::forward_as_tuple was suggested.

To be more specific, the implementation (handling of empty tuples is omitted for brevity):

#include <cstddef>
#include <tuple>
#include <type_traits>
#include <utility>

template<class Fn, class Tuple, std::size_t... indices>
constexpr auto tuple_map_v(Fn fn, Tuple&& tuple, std::index_sequence<indices...>)
{
    return std::make_tuple(fn(std::get<indices>(std::forward<Tuple>(tuple)))...);
    //          ^^^
}

template<class Fn, class Tuple, std::size_t... indices>
constexpr auto tuple_map_r(Fn fn, Tuple&& tuple, std::index_sequence<indices...>)
{
    return std::forward_as_tuple(fn(std::get<indices>(std::forward<Tuple>(tuple)))...);
    //          ^^^
}

template<class Tuple, class Fn>
constexpr auto tuple_map_v(Fn fn, Tuple&& tuple)
{ 
    return tuple_map_v(fn, std::forward<Tuple>(tuple), 
        std::make_index_sequence<std::tuple_size_v<std::remove_reference_t<Tuple>>>{});
}

template<class Tuple, class Fn>
constexpr auto tuple_map_r(Fn fn, Tuple&& tuple)
{ 
    return tuple_map_r(fn, std::forward<Tuple>(tuple), 
        std::make_index_sequence<std::tuple_size_v<std::remove_reference_t<Tuple>>>{});
}

In the case 1 we use std::make_tuple which decays type of each argument (_v for value), and in the case 2 we use std::forward_as_tuple which preserves references (_r for reference). Both cases have their pros and cons.

1. Dangling references.

   auto copy = [](auto x) { return x; };
   auto const_id = [](const auto& x) -> decltype(auto) { return x; };
   
   auto r1 = tuple_map_v(copy, std::make_tuple(1));
   // OK, type of r1 is std::tuple<int>
   
   auto r2 = tuple_map_r(copy, std::make_tuple(1));
   // UB, type of r2 is std::tuple<int&&>
   
   std::tuple<int> r3 = tuple_map_r(copy, std::make_tuple(1));
   // Still UB
   
   std::tuple<int> r4 = tuple_map_r(const_id, std::make_tuple(1));
   // OK now
   

2. Tuple of references.

   auto id = [](auto& x) -> decltype(auto) { return x; };
   
   int a = 0, b = 0;
   auto r1 = tuple_map_v(id, std::forward_as_tuple(a, b));
   // Type of r1 is std::tuple<int, int>
   ++std::get<0>(r1);
   // Increments a copy, a is still zero
   
   auto r2 = tuple_map_r(id, std::forward_as_tuple(a, b));
   // Type of r2 is std::tuple<int&, int&>
   ++std::get<0>(r2);
   // OK, now a = 1
   

3. Move-only types.

   NonCopyable nc;
   auto r1 = tuple_map_v(id, std::forward_as_tuple(nc));
   // Does not compile without a copy constructor 
   
   auto r2 = tuple_map_r(id, std::forward_as_tuple(nc));
   // OK, type of r2 is std::tuple<NonCopyable&>
   

4. References with std::make_tuple.

   auto id_ref = [](auto& x) { return std::reference_wrapper(x); };
   
   NonCopyable nc;
   auto r1 = tuple_map_v(id_ref, std::forward_as_tuple(nc));
   // OK now, type of r1 is std::tuple<NonCopyable&>
   
   auto r2 = tuple_map_v(id_ref, std::forward_as_tuple(a, b));
   // OK, type of r2 is std::tuple<int&, int&>
   

(Probably, I got something wrong or missed something important.)

It seems that make_tuple is the way to go: it doesn't produce dangling references and still can be forced to deduce a reference type. How would you implement tuple_map (and what would be the pitfalls associated with it)?

Answer 1

The problem you highlighted in your question is that using std::forward_as_tuple on a functor that returns by value will leave you with an rvalue reference in the resulting tuple.

By using make_tuple you cannot keep lvalue-refs, however by using forward_as_tuple, you cannot keep plain values. You can instead rely on std::invoke_result to find out what are the types your result tuple must hold and use the appropriate std::tuple constructor.

template<class Fn, class Tuple, std::size_t... indices>
constexpr auto tuple_map_r(Fn fn, Tuple&& tuple, std::index_sequence<indices...>) {
    using tuple_type = std::tuple<
        typename std::invoke_result<
            Fn, decltype(std::get<indices>(std::forward<Tuple>(tuple)))
        >::type...
    >;
    return tuple_type(fn(std::get<indices>(std::forward<Tuple>(tuple)))...);
}

This way you preserve the value category of the result of the fn call. Live demo on Coliru