Deducing types from member function template

Oskar Linde oskar.lindeREM at OVEgmail.com
Mon Dec 4 05:07:24 PST 2006


Li Jie wrote:
> I want to write a Matrix class, a c++ version looks like:
> 
> [code]
> template <class T, int R, int C>
> class Matrix
> {
> };
> 
> template <class T, int R, int C, int C1>
> Matrix<T,R,C1> operator* (const Matrix<T,R,C>& lhs, const Matrix<T,C,C1>& rhs)
> {
>     return Matrix<T,R,C1>();
> }
> 
> int main()
> {
>     Matrix<int,3,4> m1;
>     Matrix<int,4,3> m2;
>     Matrix<int,3,3> m3 = m1 * m2;
>     Matrix<int,2,3> m4;
>     // m1 * m4;  <== MUST compile fail
> 
>     return 0;
> }
> [/code]
> 
> It can be compiled. D version:
> 
> [code]
> class Matrix(T, int R, int C){
>     static assert(R > 0 && C > 0);
> 
>     Matrix!(T,R,C1) opMul(int C1)(Matrix!(T,C,C1) rhs){
>         return null;
>     }
> }
> 
> 
> int main()
> {
>     Matrix!(int,3,4) m1;
>     Matrix!(int,4,3) m2;
>     // auto m3 = m1 * m2;  <== NEED compile success
>     Matrix!(int,3,3) m3 = m1.opMul!(3)(m2);
>     Matrix!(int,2,3) m4;
>     // m1 * m4;  <== MUST compile fail
> 
>     return 0;
> }
> [/code]
> 
> "m1 * m2" can't be compiled:
> 
> matrix.d(14): template matrix.Matrix!(int,3,4).Matrix.opMul(int C1) does not
> match any template declaration
> matrix.d(14): template matrix.Matrix!(int,3,4).Matrix.opMul(int C1) cannot
> deduce template function from argument types (Matrix)
> matrix.d(14): template matrix.Matrix!(int,3,4).Matrix.opMul(int C1) does not
> match any template declaration
> matrix.d(14): template matrix.Matrix!(int,3,4).Matrix.opMul(int C1) cannot
> deduce template function from argument types (Matrix)
> 
> 
> I must write "m1.opMul!(3)(m2)". Can you help me?

This is indeed a limitation of D's implicit template instantiation 
deduction. Try this instead:

class Matrix(T, int R, int C){
     static assert(R > 0 && C > 0);
     const r = R;
     const c = C;

     Matrix!(T,R,M.c) opMul(M)(M rhs){
         static assert(C == M.r,
                       "Matrix inner dimensions must agree for *");
         return null;
     }
}

/Oskar



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