Templated struct operator overloading

[Philippe Sigaud]

On Fri, Aug 6, 2010 at 21:54, Peter Alexander
<peter.alexander.au at gmail.com>wrote:

> Is this a bug, or a limitation of how operator overloads handle generic
> operands?
>
>  struct Vec(int N) {}
>
>  struct Mat(int M, int N)
>  {
>      Vec!(M) opBinary(string op)(in Vec!(N) rhs)
>      {
>          return Vec!(M)();
>      }
>  }
>
>  void main()
>  {
>      Vec!(3) v;
>      Mat!(3, 3) m;
>
>      Vec!(3) x = m * v; // Error: incompatible types for m * v
>      Vec!(3) y = m.opBinary("*")(v); // Works
>  }
>
> Note that if you change the operator signature to:
>
>  Vec!(3) opBinary(string op)(in Vec!(3) rhs)
>
> then the first statement also compiles, but of course this makes the
> program incorrect.
>
> Is this a bug or a language limitation?
>

I don't know how to classify it. You can help the compiler by defining
aliases to types:

struct Mat(int M, int N)
 {
     alias Vec!N ColumnType;
     alias Vec!M RowType;

     RowType opBinary(string op)(ColumnType rhs)
     {
         return RowType();
     }
 }

And then it works!
A nice side-effect is that, given an M, you now have M.ColumType and
M.RowType exposed, to do what you want.

Maybe another possibility is to use templates constraints?

Philippe
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