Using a range as a parameter pack

debugcn Published at Dev

User.cpp

I have a function

#include <iostream>
#include <iterator>
#include <vector>

template <typename T, typename P...> Function (T&& Sth, P&&... SthElse) {
    //Perform operations on all arguments. For example:
    std::cout << Sth << " ";
    (std::cout << ... << SthElse);
    std::cout <<"\n"; 
}

If I also have a vector

int main () {
    std::vector<int> V {1, 2, 3, 4, 5};
}

is there some way to pass the range containing my numbers to the function as a parameter pack? My desired structure would be something like

    Function(SomethingMagic(V.begin(), V.end());

where SomethingMagic turns the range into a pack to get an output in the form

1 2 3 4 5

Is there any way to convert the range in a parameter pack? Thanks in advance to anyone.

Jarod42

You cannot use runtime value for compile time one.

vector size is a runtime value, size of pack is compile time.

But, if you know size at compile time, you might do something like:

template <typename C, std::size_t ... Is>
auto to_tuple_impl(C&& c, std::index_sequence<Is...>)
{
    return std::tie(c[Is]...);
}

template <std::size_t N, typename C>
auto to_tuple(C&& c)
{
    return to_tuple_impl(std::forward<C>(c), std::make_index_sequence<N>());
}

and then

std::apply([](auto...args){Function(args...); }, to_tuple<5>(v));

Demo

switch (v.size())
{
    case 5: return std::apply([](auto...args){Function(args...); }, to_tuple<5>(v));
    case 42: return std::apply([](auto...args){Function(args...); }, to_tuple<42>(v));
    // ...
    default: return; // Nothing
}

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