Re: Number of Template Parameters in Trait Types?

From:

SG <s.gesemann@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Fri, 15 Feb 2013 00:38:13 -0800 (PST)

Message-ID:

<f7923a43-9b45-4084-820c-5e4ea820c1b6@r13g2000yqg.googlegroups.com>

On Feb 15, 4:16 am, Nephi Immortal wrote:

I can create trait types with the fixed number of template parameters.
How can I define different number of parameters in the same class
name?

What problem are you trying to solve like that?

enum E
{
    eA, eB
};

template< E e, typename T >
struct A
{
};

template< E e, typename T, typename T2 >
struct A
{
};

int main()
{
    A< eA, long > a;
    A< eB, short, long > a2;
    return 0;
}

C++11 allows something like that via variadic templates:

  template<E e, typename T1, typename...More >
  struct A

Here, "More" is a template parameter pack containing zero or more
parameters. It can be expanded using the ellipsis on the right-hand
side of More:

  {
    std::tuple<T1,More...> stuff;
  };

Alternativly, you can do partial specialization:

  template<E e, typename T1, typename...More >
  struct A;

  template<E e, typename T1>
  struct A<e,T1>
  {
  };

  template<E e, typename T1, typename T2>
  struct A<e,T1,T2>
  {
  };

But I would try to avoid that and define the class template
generically if possible.

Cheers!
SG