Overloading + on points

[Eduardo Cavazos]

(Discussion originally on digitalmars-d-learn. Moving to digitalmars-d 
list.)

Eduardo Cavazos <wayo.cavazos at gmail.com> wrote:

 > Here's a short program which creates a type for points and overloads 
'+'
 > to do element wise addition as well as addition to floats, however it
 > produces an error:
 >
 > ----------------------------------------------------------------------
 > import std.stdio ;
 >
 > struct Pt
 > {
 >    float x , y ;
 >
 >    Pt opBinary ( string op ) ( Pt a ) if ( op == "+" )
 >      { return Pt ( x + a.x , y + a.y ) ; }
 >
 >    Pt opBinary ( string op ) ( float a ) if ( op == "+" )
 >      { return Pt ( x + a , y + a ) ; }
 > }
 >
 > void main () { Pt ( 1.0 , 2.0 ) + Pt ( 3.0 , 4.0 ) ; }
 > ----------------------------------------------------------------------
 >
 > The error:
 >
 > pt_overload_test_a.d(15): Error: template instance opBinary!("+")
 > matches more than one template declaration,
 > pt_overload_test_a.d(8):opBinary(string op) if (op == "+") and
 > pt_overload_test_a.d(11):opBinary(string op) if (op == "+")
 >
 > So, how should I go about this? :-)

Simen kjaeraas wrote:

> That is indeed a perplexing error. It is caused by dmd not knowing
> how to overload template functions based on both normal and template
> parameters.
>
> As for the solution:
>
> Pt opBinary( string op : "+", T : Pt )( T a ) {...}
> Pt opBinary( string op : "+", T : float )( T a ) {...}
>
> should work. More explicitly:
>
> Pt opBinary( string op, T )( T a ) if ( ( op == "+" ) && is( T == Pt ) )
> {...}
> Pt opBinary( string op, T )( T a ) if ( ( op == "+" ) && is( T == float )
> ) {...}

Thanks for the help Simen!

Is it a bug that D doesn't handle the orignal code?

Ed