How to store variadic template parameters in c++ ?

Question

I want to pass the variadic template parameters (a set of integers) of an derived class to another templated class. I can not modify the second class because its part of an libary.

I already figured out how I can store this parameters at compile time (e.g. constant array or integer_sequence) but I have no clue how to pass these structures to the library class. The solution might be obvious but I'm currently lost in all the possibilities to handle this variadic stuff.

I tried to build a simple example to explain my problem better:

// Example program
#include <iostream>
#include <array>
#include <utility>


// this class has some compile time paramters
template <int... N>
class BaseClass
{
  public:
  BaseClass(){};
  ~BaseClass(){};

   //one idea to store the parameters using a compile time array 
   static constexpr std::array<int, sizeof...(N)> _N = {{N...}};

   //another idea using a integer sequence type
   using _Ni = std::integer_sequence<int, N...>;
};

// this special case of BaseClass hast the parameter 5,6,7,8
class SpecialClass:public BaseClass<5,6,7,8>
{
  public:
  SpecialClass(){};
  ~SpecialClass(){};
};


// this class is fixed and can not be modified because it's part of an libary
template <int... N>
class Printer
{
    public:
    Printer(){};
    ~Printer(){};

    // it can (for example) print its template parameters
    void print()
    {
        int dummy[sizeof...(N)] = { (std::cout << N, 0)... };
    }
};


int main()
{

  // this obviously works
  Printer <1,2,3,4> TestPrinter;
  TestPrinter.print();

  // this works not
  Printer <SpecialClass::_N> TestPrinterSpecialArray;
  TestPrinterSpecialArray.print();

  // this also works not
  Printer <SpecialClass::_Ni> TestPrinterSpecialSequence;
  TestPrinterSpecialSequence.print();

  return 0;
}

Answer 1

You can write a helper function that unpacks std::integer_sequence,

template<int... is>
auto make_printer_impl(std::integer_sequence<int, is...>)
{
    Printer<is...> printer;
    return printer;
}

template<class T>
auto make_printer()
{
    return make_printer_impl(typename T::_Ni{});
}

and then use it like this:

auto TestPrinterSpecialSequence = make_printer<SpecialClass>();
TestPrinterSpecialSequence.print();

You can do the similar thing for std::array member:

template<class T, std::size_t... is>
auto make_printer_impl(std::index_sequence<is...>)
{
    Printer<T::_N[is]...> printer;
    return printer;
}

template<class T>
auto make_printer()
{
    return make_printer_impl<T>(std::make_index_sequence<T::_N.size()>{});
}

Also note that identifiers that begin with an underscore followed by an uppercase letter are reserved. Their usage leads to the undefined behavior. Don't use them.

Answer 2

You can create helper to do that:

template <typename T, template <std::size_t...> class Other> struct remap;

template <template <std::size_t...> class Orig,
          std::size_t... Is,
          template <std::size_t...> class Other>
struct remap<Orig<Is...>, Other>
{
    using type = Other<Is...>;
};

template <typename T, template <std::size_t...> class Other>
using remap_t = typename remap<T, Other>::type;

And then

using SpecialClass = BaseClass<5,6,7,8>;

remap_t<SpecialClass, Printer> TestPrinterSpecialSequence; // Printer <5, 6, 7, 8>
TestPrinterSpecialSequence.print();