Abstract functions in child classes

[Timon Gehr]

On 12/02/2011 08:10 PM, Adam wrote:
>
>> A second possible use case:
>>
>> class C(T): T{
>>       // some declarations
>> }
>
>> Now you really want that template to be instantiable with T being
> either
>> an abstract or a concrete class. Anything else is bound to become
>> extremely annoying.
>
> Could you expand on this case a bit? I'm not sure I follow the point
> one way or another.

This is an useful pattern. I don't have a very useful example at hand, 
but this one should do. It does similar things that can be achieved with 
traits in Scala for example.


import std.stdio;
abstract class Cell(T){
	abstract void set(T value);
	abstract const(T) get();
private:
	T field;
}

class AddSetter(C: Cell!T,T): C{
	override void set(T value){field = value;}
}
class AddGetter(C: Cell!T,T): C{
	override const(T) get(){return field;}
}

class DoubleCell(C: Cell!T,T): C{
	override void set(T value){super.set(2*value);}
}

class OneUpCell(C: Cell!T,T): C{
	override void set(T value){super.set(value+1);}	
}

class SetterLogger(C:Cell!T,T): C{
	override void set(T value){
		super.set(value);
		writeln("cell has been set to '",value,"'!");
	}
}

class GetterLogger(C:Cell!T,T): C{
	override const(T) get(){
		auto value = super.get();
		writeln("'",value,"' has been retrieved!");
		return value;
	}
}

class ConcreteCell(T): AddGetter!(AddSetter!(Cell!T)){}
class OneUpDoubleSetter(T): OneUpCell!(DoubleCell!(AddSetter!(Cell!T))){}
class DoubleOneUpSetter(T): DoubleCell!(OneUpCell!(AddSetter!(Cell!T))){}
void main(){
	Cell!string x;
	x = new ConcreteCell!string;
	x.set("hello");
	writeln(x.get());

	Cell!int y;
	y = new SetterLogger!(ConcreteCell!int);
	y.set(123); // prints: "cell has been set to '123'!
	
	y = new GetterLogger!(DoubleCell!(ConcreteCell!int));
	y.set(1234);
	y.get(); // prints "'2468' has been retrieved!"

	y = new AddGetter!(OneUpDoubleSetter!int);
	y.set(100);
	writeln(y.get()); // prints "202"

	y = new AddGetter!(DoubleOneUpSetter!int);
	y.set(100);
	writeln(y.get()); // prints "201"

	// ...
}