Question

I am attempting to write a dynamic dispatcher for a function that’s templated on integer values (not on types). While I could either write a code generator or use a big macro chain to create the dispatcher source, it seems that a templated solution would be more elegant.

I’ve stripped down my dispatcher to a simple form (which doesn’t actually do any dispatching):

// works fine with full template specialization template <int N> struct TestDispatcher1D {   int f(int n) {     if (n == N) return n; // replace me with actual dispatch     TestDispatcher1D<N-1> t;     return t.f(n);   } };  template<> struct TestDispatcher1D<-1> {   int f(int n) { return -1; } };  // partial template specialization is problematic template <int M, int N> struct TestDispatcher2D {   int f(int m, int n); };  template<int M> struct TestDispatcher2D<M,-1> {   int f(int m, int n) { return -1; } };  template<int N> struct TestDispatcher2D<-1,N> {   int f(int m, int n) { return -1; } };  template<> struct TestDispatcher2D<-1,-1> {   int f(int m, int n) { return -1; } };  template <int M, int N> int TestDispatcher2D<M,N>::f(int m, int n) {   if ((n == N) && (m == M)) return n + m; // replace me with actual dispatch   if (m < M) {     if (n < N) {       TestDispatcher2D<M-1,N-1> t;       return t(m,n);     } else {       TestDispatcher2D<M-1,N> t;       return t(m,n);     }   } else {     TestDispatcher2D<M,N-1> t;     return t(m,n);   } }  // test code void testIt() {   {      TestDispatcher1D<16> t;      t.f(16);    }   {     TestDispatcher1D<16>t;      t.f(0);   }   {     TestDispatcher2D<16,16>t;      t.f(8,8);   } } 

When compiling this on gcc 4.1.1, I get the following errors:

 t.cpp: In member function 'int TestDispatcher2D::f(int, int) [with int M = 16, int N = 16]': t.cpp:63:   instantiated from here t.cpp:40: error: no match for call to '(TestDispatcher2D) (int&, int&)' t.cpp:63:   instantiated from here t.cpp:43: error: no match for call to '(TestDispatcher2D) (int&, int&)' t.cpp:63:   instantiated from here t.cpp:47: error: no match for call to '(TestDispatcher2D) (int&, int&)' 

Apparently, when I try to create the recursive objects, the compiler isn’t treating this as a request to instantiate a new template.

Any suggestions?

Answer 1

You’re simply not calling the f() function in your recursive call, you’re trying to ‘call the object’:

You write:

TestDispatcher2D<...> t; return t(m,n); 

But you want:

TestDispatcher2D<...> t; return t.f(m,n);