How do I transform all elements of a varadic std::tuple using numerical indices?

user_5096557 0
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Currently, I have an implementation like this in order to transform all values of a tuple using a function bar() that takes in each element of the tuple.

template<typename ... Args>
void foo(const std::tuple<Args...>& a)
{
    std::tuple<Args...> transformedTuple = std::make_tuple(bar(std::get<Args>(a))...);
}

The problem with this is that this will no longer work if Args contains duplicate types. Hence, I would like to change the std::get<> call to use numerical indices into the tuple instead of using types. Given that my development environment is stuck on C++14, is there any way of getting this to work? Thanks!

c++14 variadic-templates templates c++
2 Answers

You can use a helper function that takes a std::integer_sequence to do this. Add a helper function that takes an integer_sequence like

template<typename Tuple, std::size_t... I>
auto foo_helper(const Tuple& a, std::integer_sequence<std::size_t, I...>)
{
    return std::make_tuple(bar(std::get<I>(a))...);
}

And then changing foo to call the helper like

template<typename ... Args>
auto foo(const std::tuple<Args...>& a)
{
    return foo_helper(a, std::make_index_sequence<sizeof...(Args)>{});
}

Live demo

user_43424980

In C++17, you might do

template<typename ... Args>
void foo(const std::tuple<Args...>& t)
{
    auto transformedTuple =
        std::apply([](const auto&... args){ return std::make_tuple(bar(args)...); }, t);
}
user_26845390
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