Handle FormatSpec!char in the virtual toString() method of a class.

[H. S. Teoh]

On Wed, May 13, 2020 at 12:26:21PM +0000, realhet via Digitalmars-d-learn wrote:
> Hello,
> 
> Is there a way to the following thing in a class instead of a struct?
> 
> --------------------------------------
> static struct Point
> {
>     int x, y;
> 
>     void toString(W)(ref W writer, scope const ref FormatSpec!char f)
>     if (isOutputRange!(W, char))
>     {
>         // std.range.primitives.put
>         put(writer, "(");
>         formatValue(writer, x, f);
>         put(writer, ",");
>         formatValue(writer, y, f);
>         put(writer, ")");
>     }
> }
> --------------------------------------
> 
> I think the problem is that the toString() in a class is virtual, and
> don't let me to have multiple overloaded methods.
> 
> Is there a way to let format() deal with classes too?
[...]

The main issue is that template functions cannot be virtual. So to make
it work for classes, you need to forward toString to a virtual function
implemented by subclasses.  Here's one way to do it:

	class Base {
		// virtual method implmented by subclasses
		abstract void toStringImpl(scope void delegate(const(char)[]) sg,
				scope const ref FormatSpec!char f);

		void toString(W)(ref W writer, scope const ref FormatSpec!char f) {
			// forward to virtual method
			toStringImpl((s) => formattedWrite(writer, s), f);
		}
	}


T

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