Overloading + on points

[Eduardo Cavazos]

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 )
> ) {...}

Is either one of those solutions preferrable? Or are they equivalent? 
I'd like to get these right now since there are a bunch more like this 
(for all the various operators and combinations).

For this experiment, I was going off of the examples in TDPL and I 
hadn't noticed yet one which covers a case like this; perhaps for 2nd 
edition. ;-)

Ed