Multiple parameter packs — how?

问题

I have the following problem:

#include <vector>
#include <tuple>

using namespace std;

template< size_t... N_i, typename Ts... >
class A
{
  // ...

  private:
    std::vector<size_t> _v = { N_i... };
    std::tuple<Ts...> _t;
};

int main()
{
  A<1> a;
}

As you can see above, I try to define multiple parameter packs as template arguments of the class A.
Unfortunately, the code does not compile:

error: expected nested-name-specifier before 'Ts'

How can I define multiple parameter packs for this example?

回答1:

One way to achieve the end goal is by a using a nested template:

template< size_t... N_i> class initial_values {

public:

    template <typename Ts...>
    class A
    {
       // ...

       private:
         std::vector<size_t> _v = { N_i... };
         std::tuple<Ts...> _t;
    };
};

Then the template can be referenced as, for example:

initial_values<1,2,3>::A<int, char> a;

回答2:

Consider the error:

expected nested-name-specifier before 'Ts'

It is due to the fact that you wrote:

template< size_t... N_i, typename Ts... >

Instead of:

template< size_t... N_i, typename... Ts >

That said, even if you fix it, code won't compile.
This is because you can't mix two parameters packs the way you did.
You must refactor your code so as they can be deduced from the context somehow.

As an example, you can use std::index_sequence and a partial specialization as it follows:

#include <vector>
#include <tuple>
#include<functional>

using namespace std;

template< typename... >
class A;

template< size_t... N_i, typename... Ts >
class A<index_sequence<N_i...>, Ts...>
{
  // ...

  private:
    std::vector<size_t> _v = { N_i... };
    std::tuple<Ts...> _t;
};

int main()
{
  A<index_sequence<1>> a;
}

来源：https://stackoverflow.com/questions/40052008/multiple-parameter-packs-how

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c++

templates

c++14

variadic-templates