tuple parameter fwd

[Ali Çehreli]

On 08/05/2013 11:45 AM, kdmult wrote:

 > Hi,
 >
 > I would like to use a tuple template parameter in the default value of
 > another template parameter in the same class declaration as follows.
 >
 > class A(P...) {}
 >
 > class B(R = A!P, P...) {}
 >
 > P... should be rightmost, so how can I use it in the preceding parameter?
 >
 > Thanks.

Do you mean that when P[0] is already an instance of A, then R should be 
P[0]. Otherwise, R should be A!P? If so, the following works:

class A(P...)
{
     pragma(msg, "\nA: " ~ P.stringof);
}

class BImpl(R, P...)
{
     pragma(msg, "R: " ~ R.stringof ~ ", P: " ~ P.stringof);
}

template B(P...)
{
     static if (is (P[0] == A!U, U)) {
         /* The first type is already an A instance. Accept the types as 
is. */
         alias B = BImpl!P;

     } else {
         /* The first type is not an A instance. Inject one. */
         alias B = BImpl!(A!P, P);
     }
}

void main()
{
     auto b0 = new B!(int, double, char);
     auto b1 = new B!(A!long, int, double);
}

Ali