Number of Template Parameters in Trait Types?
 

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? I know that it is not legal, but can be done in trick.

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;
}

Re: Number of Template Parameters in Trait Types?
 

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

Re: Number of Template Parameters in Trait Types?
 

On Friday, February 15, 2013 2:38:13 AM UTC-6, SG wrote:
> 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>
>
> {
>
> };

Yes, that is what I mean. Why didn't Visual C++ compile and show errors?
The code you posted here is not supported. Do GCC Compiler support?

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