Operator overloading problem

[div0]

On 06/08/2010 21:08, Blonder wrote:
> Hello, I am trying to understand how operator overloading works with D. I am a C++
> programmer and I am reading the book of Andrei Alexandrescu and try to understand
> D and it's language features.
>
> My Group example don't compile, the error is:
> Error: template instance opBinary!("+") matches more than one template
> declaration, ...
>
> I know that it matches more than one, that was my intention, because I want to be
> able to write also
>
> h = g+2 for example.
>
> Is this possible in D?
> Can you help me?
> Thanks,
> Andreas.


You need to add a second template parameter for the function arguments 
and add a template constrait like so:

struct Group {
     int i1;

     Group opBinary(string op, U) (U x)
		if(op == "+" && is(U: int))
	{
         // do somehting
         return this;
     }

	Group opBinary(string op, U) (U rhs)
		if(op == "+" && is(U: Group))
	{
         // do something
         return this;
     }
}


void main() {
	Group g, h;
	g.i1 = 1;

	h = g+g;
}

Personally, I'm with you and I would expect that the compiler should 
example the function parameters after the template string parameter but 
it doesn't.

It's important to note: you must use the template parameter U, you can 
not explitictly use the type (int/Group) like you can in C++.

ie you can't do:

struct Group {
     int i1;

     // DOES NOT WORK
     Group opBinary(string op, U) (int x)
		if(op == "+" && is(U: int))
	{
         // do somehting
         return this;
     }

     // DOES NOT WORK
     Group opBinary(string op, U) (Group rhs)
		if(op == "+" && is(U: Group))
	{
         // do something
         return this;
     }
}

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