CODEWITHSUNDEEP
c++templatestuplesvariadic-templates
David Bermejo
David Bermejo

Reputation: 125

Generate tuple of class templates given variadic integer list

I need to generate a tuple of class templates given an integer list as a variadic argument. The result I want to achieve is presented in the example below, but doesn´t compile:

template <unsigned int First, unsigned int... Is>
using remove_first = std::integer_sequence<unsigned int, Is...>;

template <unsigned int... Is, unsigned int Last> // Invalid
using remove_last = std::integer_sequence<unsigned int, Is...>;

template <unsigned int N_prev, unsigned int N>
class A
{};

template <unsigned int... Is>
class B
{
    std::tuple<A<remove_last<Is>, remove_first<Is>>...> object;
};

int main()
{
    B<3, 4, 6, 2> b; // <<= b.object is of type: std::tuple<A<3,4>, A<4,6>, A<6,2>>
}

Is it possible to achieve in a similar way?

Upvotes: 2

Views: 70

Answers (1)

PiotrNycz
PiotrNycz

Reputation: 24372

First of all - variadic pack, in general, must be last - so this will not compile:

template <unsigned int... Is, unsigned int Last> // Invalid
using remove_last = std::integer_sequence<unsigned int, Is...>;

But you do not need neither remove_first or remove_last. You need to store these pairs A<_, _> one by one in internal template of helper template. I invented(of course, I have no idea I am the first one) this technique when I wanted to implement flat union in this toy project.

So here the code with comments:

#include <tuple>
#include <type_traits>

template <unsigned int N_prev, unsigned int N>
class A
{};

// The helper type
template <unsigned int... Is>
struct DefineAPairs;

template <unsigned int I1, unsigned I2, unsigned int... Is>
struct DefineAPairs<I1, I2, Is...>
{
    // in the following line pair A<I1, I2> is added at the proper
    // position in nested DefineAPairs
    template <typename ...FirstPairs>
    using type = typename DefineAPairs<I2, Is...>
          ::template type<FirstPairs..., A<I1, I2>>; 
    //                              here ^^^^^^^^^
  
};

template <unsigned int I1, unsigned I2>
struct DefineAPairs<I1, I2>
{
    // this is sentinel implementation
    template <typename ...FirstPairs>
    using type = std::tuple<FirstPairs..., A<I1, I2>>; 
};


template <unsigned int... Is>
class B
{
public:
    using type = typename DefineAPairs<Is...>::template type<>; 
    type object;
};


int main()
{
    B<3, 4, 6, 2> b; // <<= b.object is of type: std::tuple<A<3,4>, A<4,6>, A<6,2>>
    // the proof it works
    static_assert(std::is_same<std::tuple<A<3,4>, A<4,6>, A<6,2>>,
                               decltype(b.object)>::value);
}

And demo.

Upvotes: 1

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