Remove reference from std::tuple members

问题:

I'm implementing save/restore functionality for some variables with the help of stl tuples as follows:

double a = 1, b = 2;
int c = 3;
auto tupleRef = std::make_tuple(std::ref(a), std::ref(b), std::ref(c));

// here I'm saving current state of a, b, c
std::tuple<double, double, int> saved = tupleRef;

//here goes block of code, where a, b, and c get spoiled
......................
//

//now I'm restoring initial state of a, b, c
tupleRef = savedTuple;

This code works well. But instead of explicit specifying tuple members types in

std::tuple<double, double, int> saved = tupleRef;

I'd like to rather remove references from all tupleRef members, like in the following

auto saved = remove_ref_from_tuple_members(tupleRef);

I believe that it is possible to write "remove_ref_from_tuple_members" template to that end.

Thanks for answers.

回答1:

A simple type alias can be used to apply std::remove_reference to all types in a tuple.

template <typename... T>
using tuple_with_removed_refs = std::tuple<typename std::remove_reference<T>::type...>;

Armed with this you can now write the function template:

template <typename... T>
tuple_with_removed_refs remove_ref_from_tuple_members(std::tuple<T...> const& t) {
    return tuple_with_removed_refs { t };
}

回答2:

Thanks to R. Martinho Fernandes, whose code I was able to modify to compile in Visual Studio where tuple is hard-coded as a template with 10 types with unused types being empty structs.

#define _RR_(x) typename std::remove_reference<x>::type
#define _no_ref_tuple_  std::tuple<_RR_(T0), _RR_(T1), _RR_(T2), _RR_(T3), _RR_(T4), _RR_(T5), _RR_(T6), _RR_(T7), _RR_(T8), _RR_(T9)>

template <typename T0, typename T1, typename T2, typename T3, typename T4, typename T5, typename T6, typename T7, typename T8, typename T9>
_no_ref_tuple_ map_remove_ref(std::tuple<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9> const& t) 
{
    return _no_ref_tuple_(t);
}

I also think that binding of the refs into tuple like in

auto tupleRef = std::make_tuple(std::ref(x_0), ..., std::ref(x_n));

can be made less verbose:

auto tupleRef = std::forward_as_tuple(x_0, ..., x_n);

but this again will not work in VS, as there is no std::forward_as_tuple.