CODEWITHSUNDEEP
c++templatespartial-specialization
Mr Fooz
Mr Fooz

Reputation: 111866

partial template specialization for dynamic dispatch

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?

Upvotes: 2

Views: 863

Answers (1)

sth
sth

Reputation: 229633

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);

Upvotes: 1

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