Re: 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"

        // ...
}