“Partial application” for template parameters

Question

I have the following “main” template:

template <
        template <typename> class S
    > struct TT { /*...*/ };

and the template I want to use with TT:

template <int N, typename T> struct S1 {};

In particular, I want to use something like

TT< S1<5> > t2; // "Invalid template arguments" here

It is a kind of partial application for templates. I know that Boost.MPL involves this kind of stuff. The problem is that I already have some code using TT and templates like

template <typename T> struct S2 {}; // S3, S4…

which are fed to TT.

So the question is: how can I use S1 with TT with smallest modifications to existing code. If it is mandatory to use Boost.MPL please show me most suitable solution.

Answer 1

Define a class template deriving from S1 as:

template <typename T> 
struct S11 : S1<5,T>
{
};

And then use S11, instead of S1 as:

TT< S11> t2;  //it is as if TT< S1<5> > t2

Working code : http://ideone.com/y2s7n

---

Reading your comment, it seems you need this:

template<int N>
struct Magic
{
   template <typename T> 
   struct S11 : S1<N,T>
   {
   };
};

//Usage
TT<Magic<5>::S11> t2;

Magic Demo : http://ideone.com/4yxvK