• Peter Alexander <peter.alexander.au gmail.com> Aug 06 2010
• Philippe Sigaud <philippe.sigaud gmail.com> Aug 06 2010

Peter Alexander <peter.alexander.au gmail.com> writes:

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?

Philippe Sigaud <philippe.sigaud gmail.com> writes:

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On Fri, Aug 6, 2010 at 21:54, Peter Alexander
<peter.alexander.au 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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<div class=3D"gmail_quote">On Fri, Aug 6, 2010 at 21:54, Peter Alexander <s=
pan dir=3D"ltr">&lt;<a href=3D"http://peter.alexander.au">peter.alexander.a=
u</a> <a href=3D"http://gmail.com">gmail.com</a>&gt;</span> wrote:<br><bloc=
kquote class=3D"gmail_quote" style=3D"margin: 0pt 0pt 0pt 0.8ex; border-lef=
t: 1px solid rgb(204, 204, 204); padding-left: 1ex;">
Is this a bug, or a limitation of how operator overloads handle generic ope=
rands?<br>
<br>
 =A0struct Vec(int N) {}<br>
<br>
 =A0struct Mat(int M, int N)<br>
 =A0{<br>
 =A0 =A0 =A0Vec!(M) opBinary(string op)(in Vec!(N) rhs)<br>
 =A0 =A0 =A0{<br>
 =A0 =A0 =A0 =A0 =A0return Vec!(M)();<br>
 =A0 =A0 =A0}<br>
 =A0}<br>
<br>
 =A0void main()<br>
 =A0{<br>
 =A0 =A0 =A0Vec!(3) v;<br>
 =A0 =A0 =A0Mat!(3, 3) m;<br>
<br>
 =A0 =A0 =A0Vec!(3) x =3D m * v; // Error: incompatible types for m * v<br>
 =A0 =A0 =A0Vec!(3) y =3D m.opBinary(&quot;*&quot;)(v); // Works<br>
 =A0}<br>
<br>
Note that if you change the operator signature to:<br>
<br>
 =A0Vec!(3) opBinary(string op)(in Vec!(3) rhs)<br>
<br>
then the first statement also compiles, but of course this makes the progra=
m incorrect.<br>
<br>
Is this a bug or a language limitation?<br>
</blockquote></div><br>I don&#39;t know how to classify it. You can help th=
e compiler by defining aliases to types:<br><br>struct Mat(int M, int N)<br=
=A0{<br>=A0=A0=A0=A0 alias Vec!N ColumnType;<br>=A0=A0=A0=A0 alias Vec!M R=


=A0=A0=A0=A0 RowType opBinary(string op)(ColumnType rhs)<br>=A0=A0=A0=A0 {<=
br>=A0=A0=A0=A0=A0=A0=A0=A0 return RowType();<br>=A0=A0=A0=A0 }<br>=A0}<br>=
<br>And then it works!<br>A nice side-effect is that, given an M, you now h=
ave M.ColumType and M.RowType exposed, to do what you want.<br>
<br>Maybe another possibility is to use templates constraints?<br><br>Phili=
ppe<br><br><br>

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